Properties of amino acids can be divided into two groups: chemical and physical.
Chemical properties of amino acids
Depending on the compounds, amino acids can exhibit different properties.
Amino acid interactions:
Amino acids, as amphoteric compounds, form salts with both acids and alkalis.
As carboxylic acids, amino acids form functional derivatives: salts, esters, amides.
Interaction and properties of amino acids with reasons:
Salts are formed:
NH 2 -CH 2 -COOH + NaOH NH 2 -CH 2 -COONa + H2O
Sodium salt + 2-aminoacetic acid Sodium salt of aminoacetic acid (glycine) + water
Interaction with alcohols:
Amino acids can react with alcohols in the presence of hydrogen chloride gas, turning into ester. Amino acid esters do not have a bipolar structure and are volatile compounds.
NH 2 -CH 2 -COOH + CH 3 OH NH 2 -CH 2 -COOCH 3 + H 2 O.
Methyl ester / 2-aminoacetic acid /
Interaction ammonia:
Amides are formed:
NH 2 -CH(R)-COOH + H-NH 2 = NH 2 -CH(R)-CONH 2 + H 2 O
Interaction of amino acids with strong acids:
We get salts:
HOOC-CH 2 -NH 2 + HCl → Cl (or HOOC-CH 2 -NH 2 *HCl)
These are the basic chemical properties of amino acids.
Physical properties of amino acids
Let us list the physical properties of amino acids:
- Colorless
- Have a crystalline form
- Most amino acids have a sweet taste, but depending on the radical (R), they can be bitter or tasteless
- Easily soluble in water, but poorly soluble in many organic solvents
- Amino acids have the property of optical activity
- Melts with decomposition at temperatures above 200°C
- Non-volatile
- Aqueous solutions of amino acids in acidic and alkaline environments conduct electric current
1.Amino acids exhibit amphoteric properties and acids and amines, as well as specific properties due to the joint presence of these groups. In aqueous solutions, AMK exist in the form of internal salts (bipolar ions). Aqueous solutions of monoaminomonocarboxylic acids are neutral to litmus, because their molecules contain an equal number of -NH 2 - and -COOH groups. These groups interact with each other to form internal salts:
Such a molecule has opposite charges in two places: positive NH 3 + and negative on the carboxyl –COO -. In this regard, the internal salt of AMK is called a bipolar ion or Zwitter ion (Zwitter - hybrid).
A bipolar ion in an acidic environment behaves like a cation, since the dissociation of the carboxyl group is suppressed; in an alkaline environment - as an anion. There are pH values specific for each amino acid, in which the number of anionic forms in solution is equal to the number of cationic forms. The pH value at which the total charge of the AMK molecule is 0 is called the isoelectric point of AMK (pI AA).
Aqueous solutions of monoaminodicarboxylic acids have an acidic reaction:
HOOC-CH 2 -CH-COOH « - OOC-CH 2 -CH–COO - + H +
The isoelectric point of monoaminodicarboxylic acids is in an acidic environment and such AMAs are called acidic.
Diaminomonocarboxylic acids have basic properties in aqueous solutions (the participation of water in the dissociation process must be shown):
NH 2 -(CH 2) 4 -CH-COOH + H 2 O « NH 3 + -(CH 2) 4 -CH–COO - + OH -
The isoelectric point of diaminomonocarboxylic acids is at pH>7 and such AMAs are called basic.
Being bipolar ions, amino acids exhibit amphoteric properties: they are able to form salts with both acids and bases:
Interaction with hydrochloric acid HCl leads to the formation of salt:
R-CH-COOH + HCl ® R-CH-COOH
NH 2 NH 3 + Cl -
Interaction with a base leads to the formation of a salt:
R-CH(NH 2)-COOH + NaOH ® R-CH(NH 2)-COONa + H 2 O
2. Formation of complexes with metals– chelate complex. The structure of the copper salt of glycol (glycine) can be represented by the following formula:
Almost all of the copper available in the human body (100 mg) is bound to proteins (amino acids) in the form of these stable claw-shaped compounds.
3. Similar to other acids amino acids form esters, halogen anhydrides, amides.
4. Decarboxylation reactions occur in the body with the participation of special decarboxylase enzymes: the resulting amines (tryptamine, histamine, serotinine) are called biogenic amines and are regulators of a number of physiological functions of the human body.
5. Interaction with formaldehyde(aldehydes)
R-CH-COOH + H 2 C=O ® R-CH-COOH
Formaldehyde binds the NH 2 group, the -COOH group remains free and can be titrated with alkali. Therefore, this reaction is used for the quantitative determination of amino acids (Sørensen method).
6. Interaction with nitrous acid leads to the formation of hydroxy acids and the release of nitrogen. Based on the volume of released nitrogen N2, its quantitative content in the object under study is determined. This reaction is used for the quantitative determination of amino acids (Van-Slyke method):
R-CH-COOH + HNO 2 ® R-CH-COOH + N 2 + H 2 O
This is one of the ways to deaminate AMK outside the body
7. Acylation of amino acids. The amino group of AMK can be acylated with acid chlorides and anhydrides already at room temperature.
The product of the recorded reaction is acetyl-α-aminopropionic acid.
Acyl derivatives of AMK are widely used in studying their sequence in proteins and in the synthesis of peptides (protection of the amino group).
8.Specific properties reactions associated with the presence and mutual influence of amino and carboxyl groups - the formation of peptides. The general property of a-AMK is polycondensation process, leading to the formation of peptides. As a result of this reaction, amide bonds are formed at the site of interaction between the carboxyl group of one AMK and the amino group of another AMK. In other words, peptides are amides formed as a result of the interaction of amino groups and carboxyls of amino acids. The amide bond in such compounds is called a peptide bond (explain the structure of the peptide group and peptide bond: three-center p, p-conjugated system)
Depending on the number of amino acid residues in the molecule, di-, tri-, tetrapeptides, etc. are distinguished. up to polypeptides (up to 100 AMK residues). Oligopeptides contain from 2 to 10 AMK residues, proteins contain more than 100 AMK residues. In general, a polypeptide chain can be represented by the diagram:
H 2 N-CH-CO-NH-CH-CO-NH-CH-CO-... -NH-CH-COOH
Where R 1, R 2, ... R n are amino acid radicals.
Concept of proteins.
The most important biopolymers of amino acids are proteins - proteins. There are about 5 million in the human body. various proteins that make up the skin, muscles, blood and other tissues. Proteins (proteins) get their name from the Greek word “protos” - first, most important. Proteins perform a number of important functions in the body: 1. Construction function; 2. Transport function; 3. Protective function; 4. Catalytic function; 5. Hormonal function; 6. Nutritional function.
All natural proteins are formed from amino acid monomers. When proteins are hydrolyzed, a mixture of AMK is formed. There are 20 of these AMKs.
4. Illustrative material: presentation
5. Literature:
Main literature:
1. Bioorganic chemistry: textbook. Tyukavkina N.A., Baukov Yu.I. 2014
- Seitembetov T.S. Chemistry: textbook - Almaty: EVERO LLP, 2010. - 284 p.
- Bolysbekova S. M. Chemistry of biogenic elements: textbook - Semey, 2012. - 219 p. : silt
- Verentsova L.G. Inorganic, physical and colloidal chemistry: textbook - Almaty: Evero, 2009. - 214 p. : ill.
- Physical and colloidal chemistry / Edited by A.P. Belyaev. - M.: GEOTAR MEDIA, 2008
- Verentseva L.G. Inorganic, physical and colloidal chemistry, (verification tests) 2009
Additional literature:
- Ravich-Scherbo M.I., Novikov V.V. Physical and colloidal chemistry. M. 2003.
2. Slesarev V.I. Chemistry. Fundamentals of living chemistry. St. Petersburg: Khimizdat, 2001
3. Ershov Yu.A. General chemistry. Biophysical chemistry. Chemistry of biogenic elements. M.: VSh, 2003.
4. Asanbaeva R.D., Ilyasova M.I. Theoretical foundations of the structure and reactivity of biologically important organic compounds. Almaty, 2003.
- Guide to laboratory classes in bioorganic chemistry, ed. ON THE. Tyukavkina. M., Bustard, 2003.
- Glinka N.L. General chemistry. M., 2003.
- Ponomarev V.D. Analytical chemistry part 1, 2 2003
6. Test questions (feedback):
1. What determines the structure of the polypeptide chain as a whole?
2. What does protein denaturation lead to?
3. What is the isoelectric point called?
4. What amino acids are called essential?
5. How are proteins formed in our body?
Related information.
>> Chemistry: Amino acids
The general formula of the simplest amino acids can be written as follows:
H2N-CH-COOH
I
R
Because amino acids contain two different functional groups that influence each other, their reactions differ from the characteristic properties of carboxylic acids and amines.
Receipt
Amino acids can be obtained from carboxylic acids by replacing the hydrogen atom in their radical with a halogen, and then with an amino group when reacting with ammonia. A mixture of amino acids is usually obtained by acid hydrolysis of proteins.
Properties
The amino group -NH2 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 amine bases.
In addition, the amino group in the amino acid molecule interacts with the carboxyl group included in its composition, forming an internal salt: 
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 composition of the R- radical, they can be sweet, bitter or tasteless.
Amino acids are optically active because they contain carbon atoms (asymmetric atoms) linked to four different substituents (the exception is amino-acetic acid - glycine). An asymmetric carbon atom is indicated by an asterisk.
As you already know, optically active substances occur in the form of pairs of antipodal isomers, the physical and chemical properties of which are the same, with the exception of one thing - the ability to rotate the plane of a polarized beam in opposite directions. The direction of rotation of the plane of polarization is indicated by the sign (+) - right rotation, (-) - left rotation.
There are D-amino acids and L-amino acids. The location of the NH2 amino group in the projection formula on the left corresponds to the L-configuration, and on the right - to the D-configuration. The sign of rotation is not related to whether the connection belongs to the L- or D-series. Thus, L-ce-rin has a rotation sign (-), and L-alanine has a rotation sign (+).
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 considered essential, as they are not synthesized in the human body. Essential amino acids are valine, leucine, isoleucine, phenylalaline, lysine, threonine, cysteine, methionine, histidine, tryptophan. These substances enter the human body with food (Table 7). 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 an amide group -NH-CO-, for example:
H2N-(CH2)5-COOH + H-NH-(CH2)5-COOH ->
aminocaproic acid
H2N-(CH2)5-CO-NH-(CH2)5-COOH + H20
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 fiber nylon 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 (think about why).
Table 7. Daily requirement of the human body for amino acids
Polyamides of a-amino acids are called peptides. Depending on the number of amino acid residues, dipeptides, tripeptides, and polypeptides are distinguished. In such compounds, the -NP-CO- groups are called peptide groups.
Isomerism and nomenclature
Amino acid isomerism is determined by the different structure of the carbon chain and the position of the amino group. The names of amino acids in which the positions of the amino group are designated by letters of the Greek alphabet are also widespread. Thus, 2-aminobutanoic acid can also be called a-aminobutyric acid: 
20 amino acids are involved in protein biosynthesis in living organisms, for which historical names are often used. These names and the Russian and Latin letter designations adopted for them are given in Table 8.
1. Write down the equations for the reactions of aminopropionic acid; you with sulfuric acid and sodium hydroxide, as well as methyl alcohol. Give all substances names according to the international nomenclature.
2. Why are amino acids heterofunctional compounds?
3. What structural features should the amino acids used for the synthesis of fibers and the amino acids involved in the biosynthesis of proteins in the cells of living organisms have?
4. How do polycondensation reactions differ from polymerization reactions? What are their similarities?
5. How are amino acids obtained? Write down the reaction equations for producing aminopropionic acid from propane.
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Amino acids(aminocarboxylic acids) - organic compounds, the molecule of which simultaneously contains carboxyl (-COOH) And amine groups (-NH 2).
The structure of amino acids can be expressed by the general formula below,
(Where R– hydrocarbon radical, which may contain various functional groups).
Amino acids can be considered as derivatives carboxylic acids, in which one or more hydrogen atoms are replaced by amine groups (-NH2).
An example is the protozoa: aminoacetic acid, or glycine, and aminopropionic acid or alanine:
Chemical properties of amino acids
Amino acids are amphoteric compounds, i.e. Depending on the conditions, they can exhibit both basic and acidic properties.
Due to the carboxyl group ( -COOH) they form salts with bases.
Due to the amino group ( -NH 2) form salts with acids.
A hydrogen ion that is eliminated upon dissociation from a carboxyl ( -HE) amino acid, can pass to its amino group with the formation of an ammonium group ( NH3+).
Thus, amino acids also exist and react in the form of bipolar ions (internal salts).
This explains that solutions of amino acids containing one carboxyl and one amino group have a neutral reaction.
Alpha amino acids
From molecules amino acids molecules of protein substances are built or proteins, which, upon complete hydrolysis under the influence of mineral acids, alkalis or enzymes, decompose, forming mixtures of amino acids.
The total number of naturally occurring amino acids reaches 300, but some of them are quite rare.
Among the amino acids, there is a group of 20 most important. They are found in all proteins and are called alpha amino acids.
Alpha amino acids– crystalline substances soluble in water. Many of them have a sweet taste. This property is reflected in the name of the first homolog in the series of alpha amino acids - glycine, which was also the first alpha amino acid found in a natural material.
Below is a table with a list of alpha amino acids:
Name | Formula | Remaining name |
|---|---|---|
| Amino acids with aliphatic radicals | ||
| OH group | ||
| Ser | ||
| Thr | ||
| Amino acids with radicals containing a COOH group | ||
| Asp | ||
| Glu | ||
| Amino acids with radicals containing NH2CO-group | ||
| Asn | ||
| Gln | ||
| Amino acids with radicals containing NH 2-group | ||
| Lys | ||
| Arg | ||
| Amino acids with sulfur-containing radicals | ||
| Cys | ||
| Met | ||
| Amino acids with aromatic radicals | ||
| Phe | ||
| Tyr | ||
| Amino acids with heterocyclic radicals | ||
| Trp | ||
| His | ||
| Pro | ||
Essential amino acids
Main source alpha amino acids food proteins serve the animal body.
Many alpha amino acids are synthesized in the body, but some alpha amino acids necessary for protein synthesis are not synthesized in the body and must come from outside, with food. These amino acids are called irreplaceable. Here is their list:
Amino acid name | Name of food products |
|---|---|
grains, legumes, meat, mushrooms, dairy products, peanuts |
|
almonds, cashews, chicken, chickpeas, eggs, fish, lentils, liver, meat, rye, most seeds, soy |
|
meat, fish, lentils, nuts, most seeds, chicken, eggs, oats, brown rice |
|
fish, meat, dairy products, wheat, nuts, amaranth |
|
milk, meat, fish, eggs, beans, beans, lentils and soybeans |
|
dairy products, eggs, nuts, beans |
|
legumes, oats, bananas, dried dates, peanuts, sesame seeds, pine nuts, milk, yogurt, cottage cheese, fish, chicken, turkey, meat |
|
legumes, nuts, beef, chicken, fish, eggs, cottage cheese, milk |
|
pumpkin seeds, pork, beef, peanuts, sesame seeds, yogurt, Swiss cheese |
|
tuna, salmon, pork tenderloin, beef fillet, chicken breasts, soybeans, peanuts, lentils |
In some, often congenital, diseases, the list of essential acids expands. For example, with phenylketonuria, the human body does not synthesize another alpha amino acid - tyrosine, which in the body of healthy people is obtained by hydroxylation of phenylalanine.
Use of amino acids in medical practice
Alpha amino acids occupy a key position in nitrogen metabolism. Many of them are used in medical practice as medicines, affecting tissue metabolism.
So, glutamic acid used to treat diseases of the central nervous system, methionine And histidine– treatment and prevention of liver diseases, cysteine– eye diseases.
In an acidic environment, α-amino acids act as bases (at the amino group), and in an alkaline environment they act as acids (at the carboxyl group). The radical (R) of some amino acids can also be ionized, and therefore all amino acids can be divided into charged and uncharged (at a physiological pH value of 6.0 - 8.0) (see Table 4). Examples of the former include aspartic acid and lysine:
If the amino acid radicals are neutral, then they do not affect the dissociation of the α-carboxyl or α-amino group, and the pK values (the negative logarithm indicating the pH value at which these groups are half dissociated) remain relatively constant.
The pK values for the α-carboxyl group (pK 1) and the α-amino group (pK 2) are very different. At pH< pK 1 почти все молекулы аминокислоты протежированы и заряжены положительно. Напротив, при рН >pK 2 almost all amino acid molecules are negatively charged, since the α-carboxyl group is in a dissociated state.
Consequently, depending on the pH of the environment, amino acids have a total zero positive or negative charge. The pH value at which the total charge of the molecule is zero and it does not move in the electric field to either the cathode or the anode is called the isoelectric point and is denoted pI.
For neutral α-amino acids, the pI value is found as the arithmetic mean between two pK values:
When the pH of the solution is less than pI, the amino acids are protonated and, becoming positively charged, move in the electric field to the cathode. The opposite picture is observed at pH > pI.
For amino acids containing charged (acidic or basic) radicals, the isoelectric point depends on the acidity or basicity of these radicals and their pK (pK 3). The pI value for them is found using the following formulas:
for acidic amino acids:
for basic amino acids:
In the cells and intercellular fluid of the human and animal body, the pH of the environment is close to neutral, therefore basic amino acids (lysine, arginine) have a positive charge (cations), acidic amino acids (aspartic, glutamic) have a negative charge (anions), and the rest exist in the form of bipolar zwitterion.
Stereochemistry of amino acids
An important feature of protein α-amino acids is their optical activity. With the exception of glycine, they are all built asymmetrically, and therefore, when dissolved in water or hydrochloric acid, they are capable of rotating the plane of polarization of light. Amino acids exist in the form of spatial isomers belonging to the D- or L-series. The L- or D-configuration is determined by the type of structure of the compound relative to the asymmetric carbon atom (a carbon atom bonded to four different atoms or groups of atoms). In formulas, an asymmetric carbon atom is denoted by an asterisk. Figure 3 shows the projection models of the L- and D-configurations of amino acids, which are, as it were, mirror images of each other. All 18 optically active protein amino acids belong to the L series. However, D-amino acids are found in the cells of many microorganisms and in the antibiotics produced by some of them.
Rice. 3. Configuration of L- and D-amino acids
Protein structure
Based on the results of studying the products of protein hydrolysis and those put forward by A.Ya. Danilevsky's ideas about the role of peptide bonds -CO-NH- in the construction of a protein molecule, the German scientist E. Fischer proposed the peptide theory of protein structure at the beginning of the 20th century. According to this theory, proteins are linear polymers of α-amino acids linked by peptide bonds - polypeptides:
In each peptide, one terminal amino acid residue has a free α-amino group (N-terminus) and the other has a free α-carboxyl group (C-terminus). The structure of peptides is usually depicted starting from the N-terminal amino acid. In this case, amino acid residues are designated by symbols. For example: Ala-Tyr-Leu-Ser-Tyr- - Cys. This entry denotes a peptide in which the N-terminal α-amino acid is is formed by alanine, and the C-terminal - cysteine. When reading such a record, the endings of the names of all acids, except the last ones, change to “silt”: alanyl-tyrosyl-leucyl-seryl-tyrosyl-cysteine. The length of the peptide chain in peptides and proteins found in the body ranges from two to hundreds and thousands of amino acid residues.
To determine the amino acid composition, proteins (peptides) are subjected to hydrolysis:
In a neutral environment, this reaction proceeds very slowly, but accelerates in the presence of acids or alkalis. Typically, protein hydrolysis is carried out in a sealed ampoule in a 6 M hydrochloric acid solution at 105 ° C; under such conditions, complete disintegration occurs in about a day. In some cases, the protein is hydrolyzed under milder conditions (at a temperature of 37-40 ° C) under the action of biological enzyme catalysts for several hours.
Then the amino acids of the hydrolyzate are separated by chromatography on ion exchange resins (sulfopolystyrene cation exchanger), isolating a separate fraction of each amino acid. To wash out amino acids from the ion exchange column, buffers with increasing pH values are used. Aspartate, which has an acidic side chain, is removed first; arginine with the main side chain is washed out last. The sequence of removal of amino acids from the column is determined by the leaching profile of standard amino acids. Fractionated amino acids are determined by the color formed when heated with ninhydrin:
In this reaction, colorless ninhydrin is converted; into a blue-violet product, the color intensity of which (at 570 nm) is proportional to the amount of amino acid (only proline gives a yellow color). By measuring the intensity of staining, it is possible to calculate the concentration of each amino acid in the hydrolyzate and the number of residues of each of them in the protein under study.
Currently, such an analysis is carried out using automatic devices - amino acid analyzers (see below Fig. Diagram of the device). The device displays the result of the analysis in the form of a graph of the concentrations of individual amino acids. This method has found wide application in the study of the composition of nutrients and clinical practice; with its help, in 2-3 hours you can get a complete picture of the qualitative composition of amino acids in products and biological fluids.