Any compound that contains both a carboxyl and an amino group is amino acid . However, more often this term is used to refer to carboxylic acids whose amino group is in the a-position to the carboxyl group.
Amino acids, as a rule, are part of polymers - proteins. Over 70 amino acids occur in nature, but only 20 play an important role in living organisms. Indispensable are called amino acids that cannot be synthesized by the body from substances supplied with food in quantities sufficient to satisfy the physiological needs of the body. Essential amino acids are given in table. 1. For patients with phenylketonuria, an essential amino acid is also tyrosine (see Table 1).
Table 1
Essential amino acids R-CHNH2 COOH
Amino acids are usually named as substitutes for the corresponding carboxylic acids, denoting the position of the amino group with the letters of the Greek alphabet. For the simplest amino acids, trivial names are usually used (glycine, alanine, isoleucine, etc.). Amino acid isomerism is associated with the arrangement of functional groups and the structure of the hydrocarbon skeleton. An amino acid molecule may contain one or more carboxyl groups and, accordingly, amino acids vary in basicity. Also, an amino acid molecule can contain a different number of amino groups.
METHODS OF OBTAINING AMINO ACIDS
1. About 25 amino acids can be obtained by hydrolysis of proteins, but the resulting mixture is difficult to separate. Usually one or two acids are obtained in much larger quantities than the others, and these acids can be isolated quite easily - using ion exchange resins.
2. From halogenated acids. One of the most common synthesis methods a-amino acids involve ammonolysis a-halogenated acid, which is usually obtained by the Gel-Volhard-Zelinsky reaction:
This method can be modified to produce a-bromo acid via malonic ester:
An amino group can be introduced into the ester of an a-halogenated acid using potassium phthalimide ( Gabriel synthesis):
3. From carbonyl compounds ( Strecker synthesis). The Strecker synthesis of a-amino acids consists of the reaction of a carbonyl compound with a mixture of ammonium chloride and sodium cyanide (this improvement of the method was proposed by N.D. Zelinsky and G.L. Stadnikov).
Addition-elimination reactions involving ammonia and a carbonyl compound produce an imine, which reacts with hydrogen cyanide to form a-aminonitrile. As a result of its hydrolysis, an a-amino acid is formed.
Chemical properties of amino acids
All a-amino acids, except glycine, contain a chiral a-carbon atom and can occur in the form enantiomers :
It has been proven that almost all naturally occurring a-amino acids have the same relative configuration at the a-carbon atom. The a-carbon atom of (-)-serine was conventionally assigned L-configuration, and the a-carbon atom of (+)-serine - D-configuration. Moreover, if the Fischer projection of an a-amino acid is written so that the carboxyl group is located at the top and R at the bottom, then L-amino acids, the amino group will be on the left, and D- amino acids - on the right. Fischer's scheme for determining amino acid configuration applies to all a-amino acids that have a chiral a-carbon atom.
From the figure it is clear that L-amino acid can be dextrorotatory (+) or levorotatory (-) depending on the nature of the radical. The vast majority of a-amino acids found in nature are classified as L-row. Their enantiomorphs, i.e. D-amino acids are synthesized only by microorganisms and are called "unnatural" amino acids .
According to (R,S) nomenclature, most "natural" or L-amino acids have the S configuration.
L-Isoleucine and L-threonine, each containing two chiral centers per molecule, can be any member of a pair of diastereomers depending on the configuration at the b-carbon atom. The correct absolute configurations of these amino acids are given below.
ACID-BASE PROPERTIES OF AMINO ACIDS
Amino acids are amphoteric substances that can exist in the form of cations or anions. This property is explained by the presence of both acidic ( -COUN), and main ( - N.H. 2 ) groups in the same molecule. In very acidic solutions N.H. 2 The acid group is protonated and the acid becomes a cation. In strongly alkaline solutions, the carboxyl group of the amino acid is deprotonated and the acid is converted into an anion.
In the solid state, amino acids exist in the form zwitterions (bipolar ions, internal salts). In zwitterions, a proton is transferred from the carboxyl group to the amino group:
If you place an amino acid in a conductive medium and lower a pair of electrodes there, then in acidic solutions the amino acid will migrate to the cathode, and in alkaline solutions - to the anode. At a certain pH value characteristic of a given amino acid, it will not move either to the anode or to the cathode, since each molecule is in the form of a zwitterion (carries both a positive and negative charge). This pH value is called isoelectric point(pI) of a given amino acid.
REACTIONS OF AMINO ACIDS
Most of the reactions that amino acids undergo in the laboratory ( in vitro), characteristic of all amines or carboxylic acids.
1. formation of amides at the carboxyl group. When the carbonyl group of an amino acid reacts with the amino group of an amine, a polycondensation reaction of the amino acid occurs in parallel, leading to the formation of amides. To prevent polymerization, the amino group of the acid is blocked so that only the amino group of the amine reacts. For this purpose, carbobenzoxychloride (carbobenzyloxychloride, benzyl chloroformate) is used. rubs-butoxycarboxazid, etc. To react with an amine, the carboxyl group is activated by treating it with ethyl chloroformate. Protecting group then removed by catalytic hydrogenolysis or by the action of a cold solution of hydrogen bromide in acetic acid.
2. formation of amides at the amino group. When the amino group of an a-amino acid is acylated, an amide is formed.
The reaction proceeds better in the basic medium, since this ensures a high concentration of free amine.
3. formation of esters. The carboxyl group of an amino acid is easily esterified by conventional methods. For example, methyl esters are prepared by passing dry hydrogen chloride gas through a solution of the amino acid in methanol:
Amino acids are capable of polycondensation, resulting in the formation of polyamide. Polyamides consisting of a-amino acids are called peptides or polypeptides . The amide bond in such polymers is called peptide communication. Polypeptides with a molecular weight of at least 5000 are called proteins . Proteins contain about 25 different amino acids. When a given protein is hydrolyzed, all of these amino acids or some of them can be formed in certain proportions characteristic of an individual protein.
The unique sequence of amino acid residues in the chain inherent in a given protein is called primary protein structure . Features of twisting chains of protein molecules (mutual arrangement of fragments in space) are called secondary structure of proteins . Polypeptide chains of proteins can be connected to each other to form amide, disulfide, hydrogen and other bonds due to amino acid side chains. As a result, the spiral twists into a ball. This structural feature is called protein tertiary structure . To exhibit biological activity, some proteins must first form a macrocomplex ( oligoprotein), consisting of several complete protein subunits. Quaternary structure determines the degree of association of such monomers in the biologically active material.
Proteins are divided into two large groups - fibrillar (the ratio of the length of the molecule to the width is greater than 10) and globular (ratio less than 10). Fibrillar proteins include collagen , the most abundant protein in vertebrates; it accounts for almost 50% of the dry weight of cartilage and about 30% of the solid matter of bone. In most regulatory systems of plants and animals, catalysis is carried out by globular proteins, which are called enzymes .
Amino acids are heterofunctional compounds that necessarily contain two functional groups: an amino group - NH 2 and a carboxyl group - COOH, associated with a hydrocarbon radical. The general formula of the simplest amino acids can be written as follows:
Because amino acids contain two different functional groups that influence each other, the characteristic reactions differ from those of carboxylic acids and amines.
Properties of amino acids
The amino group - NH 2 determines the basic properties of amino acids, since it is capable of attaching a hydrogen cation to itself via a donor-acceptor mechanism due to the presence of a free electron pair at the nitrogen atom.
The -COOH group (carboxyl group) determines the acidic properties of these compounds. Therefore, amino acids are amphoteric organic compounds. They react with alkalis as acids:
With strong acids - like bases - amines:
In addition, the amino group in an amino acid interacts with its carboxyl group, forming an internal salt:
The ionization of amino acid molecules depends on the acidic or alkaline nature of the environment:
Since amino acids in aqueous solutions behave like typical amphoteric compounds, in living organisms they play the role of buffer substances that maintain a certain concentration of hydrogen ions.
Amino acids are colorless crystalline substances that melt and decompose at temperatures above 200 °C. They are soluble in water and insoluble in ether. Depending on the R- radical, they can be sweet, bitter or tasteless.
Amino acids are divided into natural (found in living organisms) and synthetic. Among natural amino acids (about 150), proteinogenic amino acids (about 20) are distinguished, which are part of proteins. They are L-shapes. About half of these amino acids are irreplaceable, because they are not synthesized in the human body. Essential acids are valine, leucine, isoleucine, phenylalanine, lysine, threonine, cysteine, methionine, histidine, tryptophan. These substances enter the human body with food. If their quantity in food is insufficient, the normal development and functioning of the human body is disrupted. In certain diseases, the body is unable to synthesize some other amino acids. Thus, in phenylketonuria, tyrosine is not synthesized. The most important property of amino acids is the ability to enter into molecular condensation with the release of water and the formation of the amide group -NH-CO-, for example:
The high-molecular compounds obtained as a result of this reaction contain a large number of amide fragments and are therefore called polyamides.
These, in addition to the synthetic nylon fiber mentioned above, include, for example, enant, formed during the polycondensation of aminoenanthic acid. Amino acids with amino and carboxyl groups at the ends of the molecules are suitable for producing synthetic fibers.
Alpha amino acid polyamides are called peptides. Depending on the number of amino acid residues, they are distinguished dipeptides, tripeptides, polypeptides. In such compounds, the -NH-CO- groups are called peptide groups.
Isomerism and nomenclature of amino acids
Amino acid isomerism is determined by the different structure of the carbon chain and the position of the amino group, for example:
The names of amino acids are also widespread, in which the position of the amino group is indicated by the letters of the Greek alphabet: α, β, y, etc. Thus, 2-aminobutanoic acid can also be called an α-amino acid:
Methods for obtaining amino acids
Amino acids
Any compound that contains both a carboxyl and an amino group is an amino acid. However, more often this term is used to refer to carboxylic acids whose amino group is in the a-position to the carboxyl group.
Amino acids, as a rule, are part of polymers - proteins. Over 70 amino acids occur in nature, but only 20 play an important role in living organisms. Essential amino acids are those that cannot be synthesized by the body from substances supplied with food in quantities sufficient to satisfy the physiological needs of the body. Essential amino acids are given in table. 1. For patients with phenylketonuria, tyrosine is also an essential amino acid (see Table 1).
Table 1
Essential amino acids R-CHNH 2 COOH
| Name (abbreviation) | R |
| isoleucine (ile, ileu) | CH 3 CH 2 CH(CH) 3 - |
| leucine (leu) | (CH 3) 2 CHCH 2 - |
| lysine (lys) | NH 2 CH 2 CH 2 CH 2 CH 2 - |
| methionine (met) | CH 3 SCH 2 CH 2 - |
| phenylalanine (phe) | |
| threonine (thr) | |
| tryptophan (try) | |
| valine (val) | |
| tyrosine (tyr) |
Amino acids are usually named as substitutes for the corresponding carboxylic acids, denoting the position of the amino group with the letters of the Greek alphabet. For the simplest amino acids, trivial names are usually used (glycine, alanine, isoleucine, etc.). Amino acid isomerism is associated with the arrangement of functional groups and the structure of the hydrocarbon skeleton. An amino acid molecule may contain one or more carboxyl groups and, accordingly, amino acids vary in basicity. Also, an amino acid molecule can contain a different number of amino groups.
METHODS OF OBTAINING AMINO ACIDS
1. About 25 amino acids can be obtained by hydrolysis of proteins, but the resulting mixture is difficult to separate. Usually one or two acids are obtained in much larger quantities than the others, and these acids can be isolated quite easily - using ion exchange resins.
2. From halogenated acids. One of the most common methods for the synthesis of a-amino acids is ammonolysis of an a-halogenated acid, which is usually obtained by the Gehl-Volhard-Zelinsky reaction:
This method can be modified to produce a-bromo acid via malonic ester:
An amino group can be introduced into the ester of an a-halogenated acid using potassium phthalimide (Gabriel synthesis):
3. From carbonyl compounds (Strecker synthesis). The Strecker synthesis of a-amino acids consists of the reaction of a carbonyl compound with a mixture of ammonium chloride and sodium cyanide (this improvement of the method was proposed by N.D. Zelinsky and G.L. Stadnikov).
Addition-elimination reactions involving ammonia and a carbonyl compound produce an imine, which reacts with hydrogen cyanide to form a-aminonitrile. As a result of its hydrolysis, an a-amino acid is formed.
Chemical properties of amino acids
All a-amino acids, except glycine, contain a chiral a-carbon atom and can occur as enantiomers:
It has been proven that almost all naturally occurring a-amino acids have the same relative configuration at the a-carbon atom. The a-carbon atom of (-)-serine was conventionally assigned the L-configuration, and the a-carbon atom of (+)-serine was assigned the D-configuration. Moreover, if the Fischer projection of an a-amino acid is written so that the carboxyl group is located at the top and R at the bottom, the L-amino acid will have the amino group on the left, and the D-amino acid will have the amino group on the right. Fischer's scheme for determining amino acid configuration applies to all a-amino acids that have a chiral a-carbon atom.
The figure shows that an L-amino acid can be dextrorotatory (+) or levorotatory (-) depending on the nature of the radical. The vast majority of a-amino acids found in nature belong to the L-series. Their enantiomorphs, i.e. D-amino acids are synthesized only by microorganisms and are called “unnatural” amino acids.
According to (R,S) nomenclature, most "natural" or L-amino acids have the S configuration.
L-Isoleucine and L-threonine, each containing two chiral centers per molecule, can be any member of a pair of diastereomers depending on the configuration at the b-carbon atom. The correct absolute configurations of these amino acids are given below.
ACID-BASE PROPERTIES OF AMINO ACIDS
Amino acids are amphoteric substances that can exist in the form of cations or anions. This property is explained by the presence of both acidic (-COOH) and basic (-NH 2) groups in the same molecule. In very acidic solutions, the NH 2 group of the acid is protonated and the acid becomes a cation. In strongly alkaline solutions, the carboxyl group of the amino acid is deprotonated and the acid is converted into an anion.
In the solid state, amino acids exist in the form of zwitterions (bipolar ions, internal salts). In zwitterions, a proton is transferred from the carboxyl group to the amino group:
If you place an amino acid in a conductive medium and lower a pair of electrodes there, then in acidic solutions the amino acid will migrate to the cathode, and in alkaline solutions - to the anode. At a certain pH value characteristic of a given amino acid, it will not move either to the anode or to the cathode, since each molecule is in the form of a zwitterion (carries both a positive and negative charge). This pH value is called the isoelectric point (pI) of a given amino acid.
REACTIONS OF AMINO ACIDS
Most of the reactions that amino acids undergo in laboratory conditions (in vitro) are characteristic of all amines or carboxylic acids.
1. formation of amides at the carboxyl group. When the carbonyl group of an amino acid reacts with the amino group of an amine, a polycondensation reaction of the amino acid occurs in parallel, leading to the formation of amides. To prevent polymerization, the amino group of the acid is blocked so that only the amino group of the amine reacts. For this purpose, carbobenzoxychloride (carbobenzyloxychloride, benzyl chloroformate), tert-butoxycarboxazid, etc. is used. To react with an amine, the carboxyl group is activated by exposing it to ethyl chloroformate. The protecting group is then removed by catalytic hydrogenolysis or by the action of a cold solution of hydrogen bromide in acetic acid.
2. formation of amides at the amino group. When the amino group of an a-amino acid is acylated, an amide is formed.
The reaction proceeds better in the basic medium, since this ensures a high concentration of free amine.
3. formation of esters. The carboxyl group of an amino acid is easily esterified by conventional methods. For example, methyl esters are prepared by passing dry hydrogen chloride gas through a solution of the amino acid in methanol:
Amino acids are capable of polycondensation, resulting in the formation of polyamide. Polyamides consisting of a-amino acids are called peptides or polypeptides. The amide bond in such polymers is called a peptide bond. Polypeptides with a molecular weight of at least 5000 are called proteins. Proteins contain about 25 different amino acids. When a given protein is hydrolyzed, all of these amino acids or some of them can be formed in certain proportions characteristic of an individual protein.
The unique sequence of amino acid residues in the chain inherent in a given protein is called the primary structure of the protein. The peculiarities of twisting the chains of protein molecules (the relative arrangement of fragments in space) are called the secondary structure of proteins. Polypeptide chains of proteins can be connected to each other to form amide, disulfide, hydrogen and other bonds due to amino acid side chains. As a result, the spiral twists into a ball. This structural feature is called the tertiary structure of the protein. To exhibit biological activity, some proteins must first form a macrocomplex (oligoprotein) consisting of several complete protein subunits. The quaternary structure determines the degree of association of such monomers in the biologically active material.
Proteins are divided into two large groups - fibrillar (the ratio of molecular length to width is greater than 10) and globular (the ratio is less than 10). Fibrillar proteins include collagen, the most abundant protein in vertebrates; it accounts for almost 50% of the dry weight of cartilage and about 30% of the solid matter of bone. In most regulatory systems of plants and animals, catalysis is carried out by globular proteins, which are called enzymes.
A persistent substance containing a lot of sulfur. Proteins are used to make plastics and glue. Below we provide a table with some information about amino acids and proteins (on the next page). Aminoacyl transfer RNA tRNA with an aminoacyl group attached to the 2" or 3" hydroxyl group of the terminal adenosine residue. The aminoacyl group migrates quickly between 2-...
They can. Such combined food products, which contain complementary proteins, are part of the traditional cuisine of all peoples of the world. CHAPTER 3. ECOLOGICAL FEATURES OF STUDYING THE TOPIC “AMINO ACIDS” The human body cannot store proteins, therefore a person needs a balanced protein diet every day. An adult weighing 82 kg requires 79 g...
Types of animals. Regional differences in methionine concentrations are small. The effect of diet on methionine concentrations in the brain is also insignificant due to competition with neutral amino acids for transport systems. Methionine in the pool of free amino acids is utilized by 80% for protein synthesis. The metabolism of free methionine to cysteine begins with the formation of S-adenosylmethionine, ...
Amino acids, proteins and peptides are examples of the compounds described below. Many biologically active molecules contain several chemically different functional groups that can interact with each other and with each other's functional groups.
Amino acids.
Amino acids- organic bifunctional compounds, which include a carboxyl group - UNS, and the amino group is N.H. 2 .
Separate α And β - amino acids:
Mostly found in nature α -acids. Proteins contain 19 amino acids and one imino acid ( C 5 H 9NO 2 ):
The simplest amino acid- glycine. The remaining amino acids can be divided into the following main groups:
1) homologues of glycine - alanine, valine, leucine, isoleucine.
Obtaining amino acids.
Chemical properties of amino acids.
Amino acids- these are amphoteric compounds, because contain 2 opposite functional groups - an amino group and a hydroxyl group. Therefore, they react with both acids and alkalis:
Acid-base transformation can be represented as:
Nomenclature of amino acids
According to systematic nomenclature, the names of amino acids are formed from the names of the corresponding acids by adding the prefix amino and indicating the location of the amino group in relation to the carboxyl group.
For example:
Another method of constructing the names of amino acids is also often used, according to which the prefix is added to the trivial name of the carboxylic acid amino indicating the position of the amino group by a letter of the Greek alphabet. Example:
For a-amino acids, which play an extremely important role in the life processes of animals and plants, trivial names are used.
If an amino acid molecule contains two amino groups, then the prefix is used in its name diamino, three NH2 groups – triamino etc.
The presence of two or three carboxyl groups is reflected in the name by the suffix –diovy or -triic acid:
Trivial names:
- Optical isomerism
All a-amino acids, except glycine H 2 N-CH 2 -COOH, contain an asymmetric carbon atom (a-atom) and can exist in the form of mirror antipodes.
Optical isomerism of natural a-amino acids plays an important role in the processes of protein biosynthesis.
Properties of amino acids
Physical properties. Amino acids are solid crystalline substances with a high melting point; they decompose when melted. Highly soluble in water, aqueous solutions are electrically conductive. These properties are explained by the fact that amino acid molecules exist in the form of internal salts, which are formed due to the transfer of a proton from the carboxyl to the amino group
Chemical properties
Amino acids exhibit the properties of bases due to the amino group and the properties of acids due to the carboxyl group, i.e. they are amphoteric compounds. Like amines, they react with acids to form ammonium salts:
H2 N–CH2 –COOH + HCl= Cl — +
As carboxylic acids they form functional derivatives:
H2 N–CH2 –COOH + NaOH= H2 N–CH2 –COO — Na+ +H2 O
b) esters
H2 N–CH2 –COOH + C2 H5 OH= H2 N–CH2 –COOC2 H5 +H2 O
H2 N–R–COOH + NH3 = H2 N–R–CONH2 +H2 O
In addition, the interaction of amino and carboxyl groups is possible both within one molecule (intramolecular reaction for g-, d-e-, etc. amino acids) and belonging to different molecules (intermolecular reaction).
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