The physicochemical and biological properties of proteins are determined by their amino acid composition. Amino acids are amino derivatives of the class of carboxylic acids. Amino acids are not only found in proteins. Many of them perform special functions. Therefore, in living organisms, amino acids are distinguished between proteinogenic (genetically encoded) and non-proteinogenic (not genetically encoded) amino acids. There are 20 proteinogenic amino acids. 19 of them are a-amino acids. This means that the amino group is attached to the a-carbon atom of the carboxylic acid, of which they are a derivative. The general formula of these amino acids is as follows:

Only one amino acid - proline does not correspond to this general formula. It is classified as an imino acid.

The a-carbon atom of amino acids is asymmetric (the exception is the amino derivative of acetic acid - glycine). This means that each amino acid has at least 2 optically active antipodes. Nature chose the L-form to create proteins. That's why natural proteins built from L-a-amino acids.

In all cases when in a molecule of an organic compound a carbon atom is bonded to 4 different atoms or functional groups, this atom is asymmetrical because it can exist in two isomeric forms called enantiomers or optical (stereo-) isomers. Compounds with asymmetric "C" atoms occur in two forms (chiral compounds) - left-handed and right-handed, depending on the direction of rotation of the plane of polarization of plane-polarized light. All standard amino acids except one (glycine) contain an asymmetric carbon atom in the a-position, to which 4 substituent groups are associated. Therefore, they have optical activity, that is, they are able to rotate the plane

polarization of light in one direction or another.

However, the naming system for stereoisomers is based not on the rotation of the plane of polarization of light, but on the absolute configuration of the stereoisomer molecule. To determine the configuration of optically active amino acids, they are compared with glyceraldehyde, the simplest three-carbon carbohydrate that contains an asymmetric carbon atom. Stereoisomers of all chiral compounds, regardless of the direction of rotation of the plane of polarization of plane-polarized light, corresponding in configuration to L-glyceraldehyde are designated by the letter L, and those corresponding to D-glyceraldehyde by the letter D. Thus, the letters L and D refer to the absolute configuration of 4 substituent groups at the chiral atom "C", and not to the direction of rotation of the plane of polarization.

Amino acids are classified according to the structure of their radical. There are different approaches to classification. Most amino acids are aliphatic compounds. 2 amino acids are representatives of the aromatic series and 2 are heterocyclic.

Amino acids can be divided, according to their properties, into basic, neutral and acidic. They differ in the number of amino and carboxyl groups in the molecule. Neutral - contain one amino and one carboxyl group (monoaminomonocarboxylic). Acidic ones have 2 carboxyl and one amino group (monoaminodicarboxylic), basic - 2 amino groups and one carboxyl (diaminomonocarboxylic).

1. Actually, 5 amino acids can be called aliphatic. Glycine or glycocol (Gly),

when working with a computer - (G), - a-aminoacetic acid. It is the only optically inactive amino acid. Glycine is used not only for protein synthesis. Its atoms are part of nucleotides, heme, and it is part of an important tripeptide - glutathione.

Alanine (Ala), when working with a computer - (A) - a-aminopropionic acid. Alanine is often used in the body to synthesize glucose.

In structure, all amino acids, with the exception of glycine, can be considered as derivatives of alanine, in which one or more hydrogen atoms in the radical are replaced by various functional groups.

Valine (Val), when working with a computer (V) - aminoisovaleric acid. Leucine (Leu, L) is an aminoisocaproic acid. Isoleucine (Ile, I) - a-amino-b-ethyl-b-methylpropionic acid. These three amino acids, having pronounced hydrophobic properties, play an important role in the formation of the spatial structure of the protein molecule.

2. Hydroxyamino acids. Serine (Ser, S) - a-amino-b-hydroxypropionic acid and threonine (Tre, T) - a-amino-b-hydroxybutyric acid play an important role in the processes of covalent modification of protein structure. Their hydroxyl group easily interacts with phosphoric acid, which is necessary to change the functional activity of proteins.

3. Sulfur-containing amino acids. Cysteine ​​(Cys, C) - a-amino-b-thiopropionic acid. A special property of cysteine ​​is the ability to oxidize (in the presence of oxygen) and interact with another cysteine ​​molecule to form a disulfide bond and a new compound - cystine. Thanks to the active -SH group, this amino acid easily enters into redox reactions, protecting the cell from the action of oxidizing agents, participates in the formation of disulfide bridges that stabilize the structure of proteins, and is part of the active center of enzymes.

Methionine (Meth, M) -a-amino-b-thiomethylbutyric acid. Acts as a donor of a mobile methyl group necessary for the synthesis of biologically active compounds: choline, nucleotides, etc. It is a hydrophobic amino acid.

4. Dicarboxylic amino acids. Glutamic (Glu, E) - a-aminoglutaric acid and aspartic acid (Asp, D) - a-aminosuccinic acid. These are the most common amino acids in animal proteins. Possessing an additional carboxyl group in the radical, these amino acids promote ionic interaction and impart a charge to the protein molecule. These amino acids can form amides.

5. Amides of dicarboxylic amino acids. Glutamine (Gln, Q) and asparagine (Asn, N). These amino acids perform an important function in the neutralization and transport of ammonia in the body. The amide bond in their composition is partially double in nature. Due to this, the amide group has a partial positive charge and will not dissociate.

6. Cyclic amino acids have an aromatic or heterocyclic ring in their radical. Phenylalanine (Phen, F) - a-amino-b-phenylpropionic acid. Tyrosine (Tyr, Y) - a-amino-b-paraoxyphenylpropionic acid. These 2 amino acids form an interconnected pair that performs important functions in the body, among which it should be noted that they are used by cells for the synthesis of a number of biologically active substances (adrenaline, thyroxine).

Tryptophan (Tri, W) - a-amino-b-indolylpropionic acid. Used for the synthesis of vitamin PP, serotonin, and pineal gland hormones.

Histidine (His, H) is a-amino-b-imidazolylpropionic acid. Can be used in the formation of histamine, which regulates tissue permeability and exhibits its effect in allergies.

7. Diaminomonocarboxylic amino acids. Lysine (Lys, K) - diaminocaproic acid. Arginine (Arg, R) - a-amino-b-guanidine-valeric acid. These amino acids have an additional amino group that imparts essential properties to proteins containing many of these amino acids. The formation of arginine is part of the metabolic pathway for ammonia detoxification (urea synthesis).

8. Imino acid - proline (Pro, P). It differs from other amino acids in structure. Its radical forms a single cyclic structure with the a-amino group. Due to this feature, no rotation is possible around the bond between the a-amino group and the a-carbon atom. All other amino acids have the ability to rotate around this bond. In addition, proline contains a secondary amino group (only one hydrogen atom is bonded to the nitrogen nitrogen), which differs in its chemical characteristics from the primary amino group (-NH 2) in other amino acids. A special place is given to this amino acid in the structure of collagen, where proline, in the process of collagen synthesis, can be converted into hydroxyproline.

In parentheses are the abbreviations for amino acids, which are formed from the first three letters of their trivial name. Lately for the record primary structure Single-letter symbols are also used, which is important when using a computer to work with proteins.

Isomerism of amino acids depending on the position of the amino group

Depending on the position of the amino group relative to the 2nd carbon atom, α-, β-, γ- and other amino acids are distinguished.

α- and β-forms of alanine

For the mammalian body, α-amino acids are most characteristic.

Isomerism by absolute configuration

Based on the absolute configuration of the molecule, D- and L-forms are distinguished. The differences between isomers are due to relative position four substituent groups located at the vertices of an imaginary tetrahedron, the center of which is the carbon atom in the α-position. There are only two possible locations chemical groups around him.

The protein of any organism contains only one stereoisomer, for mammals these are L-amino acids.

L- and D-forms of alanine

However, optical isomers can undergo spontaneous non-enzymatic racemization, i.e. The L-shape changes to the D-shape.

As you know, a tetrahedron is a rather rigid structure in which it is impossible to move the vertices arbitrarily.

In the same way, for molecules built on the basis of a carbon atom, the structure of the glyceraldehyde molecule, established using X-ray diffraction analysis, is taken as the standard configuration. It is accepted that the most highly oxidized carbon atom (in the diagrams it is located on top) associated with asymmetrical carbon atom. Such an oxidized atom in a molecule glyceraldehyde the aldehyde group serves for alanine– COUN group. The hydrogen atom in the asymmetric carbon is positioned in the same way as in glyceraldehyde.

In dentin, the protein of tooth enamel, the racemization rate of L-aspartate is 0.10% per year. When forming a tooth in children, only L-aspartate is used. This feature makes it possible, if desired, to determine the age of centenarians. For fossil remains, along with the radioisotope method, the determination of racemization of amino acids in protein is also used.

Division of isomers by optical activity

According to optical activity, amino acids are divided into right- and left-handed.

The presence of an asymmetric α-carbon atom (chiral center) in an amino acid makes only two arrangements of chemical groups around it possible. This leads to a special difference between substances from each other, namely, a change direction of rotation of the plane of polarized light passing through the solution. The rotation angle is determined using a polarimeter. In accordance with the angle of rotation, dextrorotatory (+) and levorotatory (–) isomers are distinguished.

Almost all natural biological compounds containing a chiral center are found in only one stereoisomer form - D or L. With the exception of glycine, which does not have an asymmetric carbon atom, all amino acids that make up protein molecules are L-stereoisomers. This conclusion was drawn from numerous carefully conducted chemical studies in which the optical properties of amino acids were compared with their behavior in chemical reactions. Below we will see that some D-amino acids are also found in living nature, but they are never part of proteins.

The presence of only L-stereoisomers of amino acids in proteins is quite remarkable, since conventional chemical reactions used for the synthesis of compounds with an asymmetric carbon atom always produce optically inactive products. This occurs because in ordinary chemical reactions both D- and L-stereoisomers are formed at the same rate. The result is a racemic mixture, or racemate, an equimolar mixture of D and L isomers, which does not rotate the plane of polarization in either direction. A racemic mixture can only be separated into D- and L-isomers using very labor-intensive methods based on differences in physical properties stereoisomers The separated D- and L-isomers eventually revert back to the racemic mixture (see Exhibit 5-2).

Addendum 5-2. How to Determine a Person's Age Using Amino Acid Chemistry

Optical isomers of amino acids undergo very slow and spontaneous non-enzymatic racemization, so that over a very long period of time, a pure L or D isomer can turn into an equimolar mixture of D and L isomers. Racemization of each L-amino acid at a given temperature occurs at a certain rate. This circumstance can be used to determine the age of people and animals or fossil remains of organisms. For example, in the protein dentin, found in the hard enamel of teeth, L-aspartate spontaneously racemizes at human body temperature at a rate per year. In children, during the period of tooth formation, dentin contains only L-aspartate. It is possible to isolate dentin from just one tooth and determine its D-aspartate content. Such analyzes were carried out on the dentin of the inhabitants of the mountain villages of Ecuador, many of whom attributed too much age to themselves. Since this was in doubt in some cases, a racemization test was used for verification, which turned out to be quite accurate. Thus, for a 97-year-old woman whose age was documented, according to the test, the age was set to 99 years.

Tests performed on fossil remains of prehistoric animals - elephants, dolphins and bears - showed that the data obtained by this method are in good agreement with the results of dating based on the decay rate of radioactive isotopes.

Living cells have the unique ability to synthesize L-amino acids using stereospecific enzymes. The stereospecificity of these enzymes is due to the asymmetric nature of their active centers. Below we will see that the characteristic three-dimensional structure of proteins, due to which they exhibit the most different types biological activity occurs only if all the amino acids included in their composition belong to the same stereochemical series.

Amino acids (AA) - organic molecules, which consist of a basic amino group (-NH 2), an acidic carboxyl group (-COOH), and an organic R radical (or side chain) that is unique to each AK

Amino acid structure

Functions of amino acids in the body

Examples of biological properties of AK. Although more than 200 different AAs occur in nature, only about one tenth of them are incorporated into proteins, others perform other biological functions:

• They are the building blocks of proteins and peptides
• Precursors of many biologically important molecules derived from AK. For example, tyrosine is a precursor to the hormone thyroxine and the skin pigment melanin, and tyrosine is also a precursor to the compound DOPA (dioxyphenylalanine). It is a neurotransmitter for the transmission of impulses in nervous system. Tryptophan is a precursor to vitamin B3 - nicotinic acid
• Sources of sulfur are sulfur-containing AK.
• AAs are involved in many metabolic pathways, such as gluconeogenesis - the synthesis of glucose in the body, the synthesis of fatty acids, etc.

Depending on the position of the amino group relative to the carboxyl group, AA can be alpha, α-, beta, β- and gamma, γ.

The alpha amino group is attached to the carbon adjacent to the carboxyl group:

The beta amino group is on the 2nd carbon of the carboxyl group

Gamma - amino group on the 3rd carbon of the carboxyl group

Proteins contain only alpha-AA

General properties of alpha-AA proteins

1 - Optical activity - property of amino acids

All AAs, with the exception of glycine, exhibit optical activity, because contain at least one asymmetric carbon atom (chiral atom).

What is an asymmetric carbon atom? It is a carbon atom with four different chemical substituents attached to it. Why does glycine not exhibit optical activity? Its radical has only three different substituents, i.e. alpha carbon is not asymmetrical.

What does optical activity mean? This means that AA in solution can be present in two isomers. A dextrorotatory isomer (+), which has the ability to rotate the plane of polarized light to the right. Levorotatory isomer (-), which has the ability to rotate the plane of polarization of light to the left. Both isomers can rotate the plane of polarization of light by the same amount, but in the opposite direction.

2 - Acid-base properties

As a result of their ability to ionize, the following equilibrium of this reaction can be written:

R-COOH<------->R-C00-+H+

R-NH2<--------->R-NH 3+

Because these reactions are reversible, this means that they can act as acids (forward reaction) or as bases (reverse reaction), which explains the amphoteric properties of amino acids.

Zwitter ion - property of AK

All neutral amino acids at a physiological pH value (about 7.4) are present as zwitterions - the carboxyl group is unprotonated and the amino group is protonated (Fig. 2). In solutions more basic than the isoelectric point of the amino acid (IEP), the amino group -NH3 + in AA donates a proton. In a solution more acidic than the IET of AA, the carboxyl group -COO - in AA accepts a proton. Thus, AA sometimes behaves like an acid and other times like a base, depending on the pH of the solution.

Polarity as general property amino acids

At physiological pH, AA are present as zwitter ions. The positive charge is carried by the alpha amino group, and the negative charge is carried by the carboxylic group. Thus, two opposite charges are created at both ends of the AK molecule, the molecule has polar properties.

The presence of an isoelectric point (IEP) is a property of amino acids

The pH value at which pure electric charge amino acid is zero and therefore it cannot move in an electric field called IET.

The ability to absorb in ultraviolet light is a property of aromatic amino acids

Phenylalanine, histidine, tyrosine and tryptophan absorb at 280 nm. In Fig. The values ​​of the molar extinction coefficient (ε) of these AAs are displayed. In the visible part of the spectrum, amino acids do not absorb, therefore, they are colorless.

AAs can be present in two isomers: L-isomer and D- isomer, which are mirror reflections, and differ in the arrangement of chemical groups around the α-carbon atom.

All amino acids in proteins are in the L-configuration, L-amino acids.

Physical properties of amino acids

Amino acids are mostly water-soluble due to their polarity and the presence of charged groups. They are soluble in polar and insoluble in non-polar solvents.

AK have high temperature melting, which reflects the presence of strong bonds supporting their crystal lattice.

General The properties of AA are common to all AA and in many cases are determined by the alpha amino group and alpha carboxyl group. AK possess and specific properties, which are dictated by the unique side chain.

Introduction........................................................ ........................................................ ................3

1. Structure and properties of acidic amino acids.................................................... ..........5

1.1. Substances........................................................ ........................................................ .......5

1.2. Organic substances........................................................ ...................................5

1.3. Functional derivatives of hydrocarbons...................................................6

1.4. Amino acids........................................................ ...................................................7

1.5. Glutamic acid................................................... ......................................9

1.6 Biological properties..................................................................................11

2.Optical activity of acidic amino acids.................................................... .....12

2.1 Chiral molecule.................................................... ........................................13

2.2 Characteristics of optical rotation.................................................... .........15

2.3 Optical rotation measurement.................................................... ...................17

2.4 Known data on the optical rotation of acidic amino acids...........18

Conclusion................................................. ........................................................ ..........21

Literature................................................. ........................................................ ..........22

Introduction
The discovery of amino acids is usually associated with three discoveries:
In 1806, the first amino acid derivative, asparagine amide, was discovered.
In 1810, the first amino acid, cystine, was discovered, which was isolated from a non-protein object, urinary stones.
In 1820, the amino acid glycine was first isolated from a protein hydrolyzate and more or less thoroughly purified.

But the discovery of glutamic acid happened quite quietly. The German chemist Heinrich Ritthausen isolated it from vegetable protein, in particular from wheat gluten, in 1866. According to tradition, the name of the new substance was given by its source: das Gluten translated from German gluten.
A possible way to obtain glutamic acid, used in Europe and the USA, is the hydrolysis of proteins, for example the same gluten from which this substance was first obtained. Typically, wheat or corn gluten was used; in the USSR, beet molasses was used. The technology is quite simple: the raw materials are purified from carbohydrates, hydrolyzed with 20% hydrochloric acid, neutralize, separate humic substances, concentrate and precipitate other amino acids. The glutamic acid remaining in the solution is again concentrated and crystallized. Depending on the purpose, food or medical, additional purification and recrystallization are carried out. The yield of glutamic acid is about 5% of the weight of gluten, or 6% of the weight of the protein itself.

The purpose of this work is to study the optical activity of acidic amino acids.

To achieve this goal, the following tasks have been set:
1. Study the properties, structure and biological significance acidic amino acids, using glutamic acid as an example, and prepare a literature review.
2. Study the optical activity in amino acids and prepare a review of the literature on their research.

Chapter 1. Structure and properties of acidic amino acids

To study amino acids, it is necessary to study the basic properties, structure and application, so in this chapter we will look at the main types of functional carbon derivatives and consider glutamic acid.

1.1. Substances

All substances are divided into simple (elementary) and complex. Simple substances consist of one element, complex ones include two or more elements.
Simple substances, in turn, are divided into metals and nonmetals or metalloids. Complex substances are divided into organic and inorganic: carbon compounds are usually called organic, all other substances are called inorganic (sometimes mineral).
Not organic matter are divided into classes either by composition (two-element, or binary, compounds and multi-element compounds; oxygen-containing, nitrogen-containing, etc.) or by chemical properties, i.e., by functions (acid-base, redox, etc. .), which these substances carry out in chemical reactions, according to their functional characteristics. Next, organic substances will be considered, since they contain amino acids.

1.2. Organic matter

Organic substances are a class of compounds that contain carbon (with the exception of carbides, carbonic acid, carbonates, carbon oxides and cyanides).

Organic compounds are usually made up of chains of carbon atoms linked together covalent bonds, and various substituents attached to these carbon atoms. For systematization and to make it convenient to name organic substances, they are divided into classes in accordance with what characteristic groups are present in the molecules. For hydrocarbons and functional derivatives of hydrocarbons. Compounds consisting only of carbon and hydrogen are called hydrocarbons.

Hydrocarbons can be aliphatic, alicyclic and aromatic.
1) Aromatic hydrocarbons are otherwise called arenes.
2) Aliphatic hydrocarbons, in turn, are divided into several narrower classes, the most important of which are:
- alkanes (carbon atoms are connected to each other only by simple covalent bonds);
- alkenes (contain a double carbon-carbon bond);

Alkynes (contain a triple bond, such as acetylene).

3) Cyclic hydrocarbons hydrocarbons with a closed carbon chain. In turn, they are divided:
-carbocyclic (the cycle consists only of carbon atoms)
- heterocyclic (the cycle consists of carbon atoms and other elements)

1.3. Functional derivatives of hydrocarbons

There are also derivatives of hydrocarbons. These are compounds consisting of carbon and hydrogen atoms. The hydrocarbon skeleton is made up of carbon atoms connected by covalent bonds; the remaining bonds of the carbon atoms are used to bind them to hydrogen atoms. Hydrocarbon skeletons are very stable because the electron pairs in carbon-carbon single and double bonds are shared equally by both adjacent carbon atoms.

One or more hydrogen atoms in hydrocarbons may be replaced by various functional groups. This creates different families organic compounds.
Typical families of organic compounds with characteristic functional groups include alcohols, whose molecules contain one or more hydroxyl groups, amines and amino acids containing amino groups; ketones containing carbonyl groups and acids with carboxyl groups.

Many physical and chemical properties hydrocarbon derivatives depend more on some group attached to the main hydrocarbon chain than on the chain itself.
Since the purpose of my coursework is to study amino acids, we will focus on it.

1.4. Amino acids

Amino acids are compounds containing both an amino and a carboxyl group:

Typically, amino acids are soluble in water and insoluble in organic solvents. In neutral aqueous solutions, amino acids exist in the form of bipolar ions and behave like amphoteric compounds, i.e. properties of both acids and bases are manifested.
There are over 150 amino acids in nature, but only about 20 essential amino acids serve as monomers for the construction of protein molecules. The order in which amino acids are incorporated into proteins is determined by the genetic code.

According to the classification, each amino acid contains at least one acidic and one basic group. Amino acids are different from each other chemical nature radical R, representing a group of atoms in an amino acid molecule associated with an α-carbon atom and not involved in the formation of a peptide bond during protein synthesis. Almost all α-amino- and α-carboxyl groups participate in the formation of peptide bonds of the protein molecule, while losing their acid-base properties specific to free amino acids. Therefore, all the variety of features of the structure and function of protein molecules is associated with the chemical nature and physicochemical properties of amino acid radicals.

By chemical structure Group R amino acids are divided into:
1) aliphatic (glycine, alanine, valine, leucine, isoleucine);

2) hydroxyl-containing (serine, threonine);

3) sulfur-containing (cysteine, methionine);

4) aromatic (phenylalanine, tyrosine, tritrophan);

5) acidic and amides (aspartic acid, asparagine, glutamic acid, glutamine);

6) basic (arginine, histidine, lysine);

7) imino acids (proline).

According to the polarity of the R-group:

1) Polar (glycine, serine, threonine, cysteine, tyrosine, aspartic acid, glutamic acid, asparagine, glutamine, arginine, lysine, histidine);
2) Non-polar (alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, proline).

According to the ionic properties of the R-group:

1) Acidic (aspartic acid, glutamic acid, cysteine, tyrosine);
2) Basic (arginine, lysine, histidine);

3) Neutral (glycine, alanine, valine, leucine, isoleucine, methionine, phenylalanine, serine, threonine, asparagine, glutamine, proline, tryptophan).

By nutritional value:

1) Replaceable (threonine, methionine, valine, leucine, isoleucine, phenylalanine, tryptophan, lysine, arginine, histidine);

2) Essential (glycine, alanine, serine, cysteine, proline, aspartic acid, glutamic acid, asparagine, glutamine, tyrosine).

Let's take a closer look at the properties of glutamic acid.

1.5. Glutamic acid

Glutamic acid is one of the most common in proteins; moreover, among the remaining 19 protein amino acids there is also its derivative glutamine, which differs from it only by an additional amino group.
Glutamic acid is sometimes also called glutamic acid, less often alpha-aminoglutaric acid. Very rare, although chemically correct
2-aminopentanedioic acid.
Glutamic acid is also a neurotransmitter amino acid, one of the important representatives of the class of “excitatory amino acids”.

The structure is shown in Fig. 1.

Fig.1 Structural formula glutamic acid

Physico-chemical properties

A substance in its pure form that consists of unremarkable colorless crystals, poorly soluble in water. The polarity of hydroxyl-containing amino acids is due to the presence of a large dipole moment in them and the ability of OH groups to form hydrogen bonds Therefore, glutamic acid is slightly soluble in cold water and soluble in hot water. So, per 100 g of water at 25°C, the maximum solubility is 0.89 g, and at a temperature of 75°C - 5.24 g. Practically insoluble in alcohol.

Glutamic acid and its anion glutamate are found in living organisms in free form, as well as in a number of low-molecular substances. In the body it is decarboxylated to aminobutyric acid, and through the cycle tricarboxylic acids turns into succinic acid.
A typical aliphatic α-amino acid. When heated, it forms 2-pyrrolidone-5-carboxylic acid, or pyroglutamic acid, with Cu and Zn-insoluble salts. The formation of peptide bonds involves mainly the α-carboxyl group, in some cases, for example, in the natural tripeptide glutathione, the γ-amino group. In the synthesis of peptides from the L-isomer, along with the α-NH2 group, the γ-carboxyl group is protected, for which it is esterified with benzyl alcohol or tert-butyl ether is obtained by the action of isobutylene in the presence of acids.

Chemical composition glutamic acid is presented in Table 1.

1.6 Biological properties

Glutamic acid is used in the treatment of diseases of the central nervous system: schizophrenia, psychoses (somatogenic, intoxication, involutional), reactive states occurring with symptoms of exhaustion, depression, the consequences of meningitis and encephalitis, toxic neuropathy with the use of isonicotinic acid hydrazides (in combination with thiamine and pyridoxine) , hepatic coma. In pediatrics delay mental development, cerebral palsy, consequences of intracranial birth injury, Down's disease, poliomyelitis (acute and recovery periods).Its sodium salt is used as a flavoring and preservative additive in food products. .

It has a number of contraindications, such as hypersensitivity, fever, liver and/or kidney failure, nephrotic syndrome, peptic ulcer of the stomach and duodenum, diseases of the hematopoietic organs, anemia, leukopenia, increased excitability, rapidly occurring psychotic reactions, obesity. Increased excitability, insomnia, abdominal pain, nausea, vomiting - these are the side effects of treatment. May cause diarrhea, allergic reaction, chills, short-term hyperthermia; anemia, leukopenia, irritation of the oral mucosa.

Chapter 2. Optical activity of acidic amino acids

In order to complete this task, it is necessary to consider the optical activity in detail.

Light is electromagnetic radiation, which is perceived by the human eye. Can be divided into natural and polarized. In natural light, vibrations are directed in different directions and quickly and randomly replace each other (Fig. 2.a). And light in which the directions of vibrations are somehow ordered or in one plane is called polarized (Fig. 2.b).

When polarized light passes through some substances, an interesting phenomenon occurs: the plane in which the lines of the oscillating light are located electric field, gradually rotates around the axis along which the beam runs.

The plane passing through the direction of oscillation of the light vector of a plane-polarized wave and the direction of propagation of this wave is called the plane of polarization.
Among organic compounds there are substances that can rotate the plane of polarization of light. This phenomenon is called optical activity, and the corresponding substances are called optically active.
Optically active substances occur in the form of optical pairs
antipodes - isomers, the physical and chemical properties of which are basically the same under ordinary conditions, with the exception of one thing - the direction of rotation of the plane of polarization.

2.1 Chiral molecule

All amino acids, with the exception of glycine, are optically active due to their chiral structure.

The molecule shown in Figure 3, 1-bromo-1-iodoethane, has a tetrahedral carbon atom attached to four different substituents. Therefore, the molecule does not have any symmetry elements. Such molecules are called asymmetric or chiral.

Glutamic acid has axial chirality. It arises as a result of the non-planar arrangement of substituents relative to a certain axis, the chirality axis. A chirality axis exists in asymmetrically substituted allenes. The sp-hybrid carbon atom in allene has two mutually perpendicular p-orbitals. Their overlap with the p-orbitals of neighboring carbon atoms leads to the fact that the substituents in the allene lie in mutual perpendicular planes. A similar situation is also observed in substituted biphenyls, in which rotation around the bond connecting the aromatic rings is difficult, as well as in spirocyclic compounds.

If plane-polarized light is passed through a solution of a chiral substance, the plane in which the vibrations occur begins to rotate. Substances that cause such rotation are called optically active. The angle of rotation is measured with a device called a polarimeter (Fig. 4). The ability of a substance to rotate the plane of polarization of light is characterized by specific rotation.

Let's see how optical activity is related to molecular structure substances. Below is a spatial image of a chiral molecule and its mirror image (Fig. 5).

At first glance, it may seem that these are the same molecule, depicted differently. However, if you collect models of both forms and try to combine them so that all the atoms coincide with each other, you can quickly see that this is impossible, i.e. it turns out that the molecule is incompatible with its mirror image.

Thus, two chiral molecules related to each other as an object and its mirror image are not identical. These molecules (substances) are isomers, called enantiomers. Enantiomeric forms, or optical antipodes, have different refractive indices (circular birefringence) and different molar extinction coefficients (circular dichroism) for the left- and right-hand circularly polarized components of linearly polarized light.

2.2 Characteristics of optical rotation

Optical rotation is the ability of a substance to deflect the plane of polarization when plane-polarized light passes through it.
Optical rotation occurs due to unequal refraction of light with left and right circular polarization. The rotation of a plane-polarized light beam occurs because the asymmetric molecules of the medium have different refractive indices, τ and π, for left- and right-hand circularly polarized light.
If the plane of polarization rotates to the right (clockwise) of the observer, the connection is called dextrorotatory, and the specific rotation is written with a plus sign. When rotating to the left (counterclockwise), the connection is called levorotatory, and the specific rotation is written with a minus sign.

The amount of deviation of the plane of polarization from the initial position, expressed in angular degrees, is called the angle of rotation and is denoted α.

The magnitude of the angle depends on the nature of the optically active substance, the thickness of the substance layer, temperature and wavelength of light. The rotation angle is directly proportional to the thickness of the layer. For comparative assessment capabilities various substances To rotate the plane of polarization, the so-called specific rotation is calculated. Specific rotation is the rotation of the plane of polarization caused by a layer of substance 1 dm thick when recalculated to the content of 1 g of substance per 1 ml of volume.

For liquid substances, specific rotation is determined by the formula:

For solutions of substances:

(where α is the measured angle of rotation in degrees; l is the thickness of the liquid layer, dm; c is the concentration of the solution, expressed in grams per 100 ml of solution; d is the density of the liquid)

The magnitude of the specific rotation also depends on the nature of the acidic amino acid and its concentration. In many cases, the specific rotation is constant only within a certain concentration range. In the concentration range at which the specific rotation is constant, the concentration can be calculated from the angle of rotation:

A number of optically active substances change the angle of rotation to a detectable constant value. This is explained by the presence of a mixture of stereoisomeric forms having different rotation angles. Only after some time is equilibrium established. The property of changing the angle of rotation over a period of time is called mutarotation.
The determination of the angle of rotation of the plane of polarization is carried out in instruments, as mentioned above, by so-called polarimeters (Fig. 4).

2.3 Optical rotation measurement

Determination of the angle of rotation of the plane of polarization is carried out in instruments called polarimeters. The rules for using this polarimeter model are set out in the instructions for the device. The determination is usually carried out for the sodium D line at 20 C.

General principle The design and operation of polarimeters is as follows. The beam from the light source is directed through a yellow filter into a polarizing prism. Passing through a Nicolas prism, a beam of light is polarized and vibrates only in one plane. Plane-polarized light is passed through a cuvette containing a solution of an optically active substance. In this case, the deviation of the plane of polarization of light is determined using a second, rotating Nicolas prism (analyzer), which is rigidly connected to a graduated scale. The significant field observed through the eyepiece, divided into two or three parts of different brightness, should be made evenly illuminated by turning the analyzer. The amount of rotation is read from the scale. To check the zero point of the device, similar measurements are carried out without the test solution. The direction of the plane of polarization is usually determined by the direction of rotation of the analyzer. The design of domestic polarimeters is such that if, in order to obtain a homogeneous illuminated field of view, it is necessary to turn the analyzer to the right, i.e., clockwise, then the substance under study was dextrorotatory, which is indicated by the + (plus) or d sign. When turning the analyzer counterclockwise, we obtain left rotation, indicated by the sign - (minus) or I.

In other instruments, the exact direction of rotation is determined by repeated measurements, which are carried out either with half the thickness of the liquid layer or with half the concentration. If this results in an angle of rotation or, then we can assume that the substance is dextrorotatory. If new angle rotation is 90 - or 180 -, then the substance has left rotation. Specific rotation does not depend very much on temperature, however, for accurate measurements, temperature control of the cuvette is necessary. For data on optical rotation, it is necessary to indicate the solvent used and the concentration of the substance in the solution, for example [α]о = 27.3 in water (C = 0.15 g/ml).

Polarimetric determinations are used both to establish the quantitative content of optically active substances in solutions and to check their purity.

2.4 Known data on the optical rotation of acidic amino acids
Based on general rule Since connections with the same configuration exhibit the same changes in rotation under the same influences, a number of more specific rules have been created for individual groups of connections. One of these rules applies to amino acids and it states that the optical rotation of all natural amino acids (L-series) in acidic solutions shifts to the right. Let us remind you once again: this rule should not be understood to mean that there is necessarily an increase in right rotation: a “shift to the right” can also mean a decrease in left rotation. Data on the rotations of some amino acids in acidic solutions are given below in the table. 2.

In a study of optical rotation, it was found that when a molecule transitions from the gas phase to a solution, the wavelengths of the transitions change significantly (on average ~ 5 nm), but in the solutions under study they do not differ significantly (~ 0.5 nm). It has been shown that with a decrease in the change in the dipole moment of isomer molecules in solutions, the shift in the wavelengths of the main electronic transition decreases, and with an increase in polarizability it increases. The rotational forces of transitions of isomer molecules into various solutions. It has been shown that the values ​​of the rotational forces of the transitions change greatly when going from an isolated molecule to a solution. The spectral dependences of the specific rotation of the plane of polarization in various solutions were plotted. Also, in the range of 100-300 nm, resonances are observed when the wavelengths of the transitions coincide with the wavelengths of the radiation. The specific rotation of the plane of polarization of radiation in solutions of the L isomer decreases with increasing wavelength from ~ 50 deg*m2/kg at 240 nm to 1 deg*m/kg at 650 nm, and in solutions of the D isomer from ~ 5 deg*m2/kg at 360 nm and up to ~ 2 deg*m2/kg at 650 nm. It was confirmed that the rotation angle increases linearly with increasing concentration of solutions. It has been shown that with increasing polarizabilities of solvent molecules, the specific rotation of the plane of polarization increases, and with increasing changes in the polarizabilities of molecules in solutions of both isomers they decrease.

In a study of the optical rotation of L and DL isomers of glutamic acid, it was shown that in the range from 4000 to 5000 the angle of rotation of the plane of polarization of incoherent radiation is maximum at a wavelength of 4280 and decreases with increasing wavelength of the radiation. Also, the angle of rotation of the polarization plane of laser radiation increases to -5° at a concentration of 1.6% for radiation with wavelength A = 650 nm and to -9° for X = 532 nm at the same concentration. It was found that optical activity is maximum in a neutral (pH = 7) solution of glutamic acid and decreases with increasing acidity and alkalinity of solutions. Demonstrated lack of rotational ability in aqueous solutions racemic form of glutamic acid.

Conclusion

In the course of the work, a literature review was prepared on the properties of acidic amino acids, on the mechanisms and characteristics of the optical rotation of glutamic acid.
Thus, the goal set course work fully achieved.

Literature

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