Amino Acids: Properties and Chemical Reactions

The below mentioned article provides a study note on Amino Acids:- 1. Common Properties of Amino Acids 2. Physical Properties of Amino Acids 3. Chemical Reactions.

Common Properties of Amino Acids:

1. Pro-tonic Equilibria of Amino Acids:

(a) Amino acids have at least two ionizable groups i.e. -COOH and –NH⁺₃. The former dissociates more easily than the latter. In solution, two forms of these groups, one charged and one neutral, exist in pro-tonic equilibrium with each other.

R-COOH and R-NH₃⁺ represent the protonated or acid partners in these equilibria, R-COO⁻ and R NH₂, are the conjugated bases (i.e. proton acceptors) of the corresponding acids. Although both R-COOH and R-NH₃⁺ are weak acids, R-COOH is a several thousand times stronger acid than is R-NH₃⁻.

(b) At pH 7.4, carboxyl groups exist almost entirely as the conjugated base i.e. R-COO⁻. Most amino groups exist in the form, R-NH₃⁺.

In blood and most tissues, amino acid structures are drawn as fol­lows:

The following structure cannot exist at any pH but is frequently used as a con­venience when the chemistry of amino acids is discussed.

(c) P_k of an acid is simply the negative log of the dissociation constant

P_k = -logK.

P_k values for α-amino groups of free amino acids is about 9.8.

(d) The isoelectric pH (P^I) of an amino acid is that pH at which it has no net charge and hence does not move in an electric field.

Addition of acid or alkali depresses one type of ionisation so that the amino acid behaves as a base or an acid.

The ion at the isoelectric point which carries + and – charges internally neutralized is called “Zwitterion”. The three types of ions are represented in Fig. 5.5.

In an acid solution, the amino acid acts as a base yielding cations. When current is allowed to pass through the solution, the amino acid migrates to the cathode or positive pool.

In an alkaline solu­tion, it behaves as an acid forming anions. In the electric field, the amino acid migrates to the anode or negative pool.

On account of these opposite re­actions depending on the acidity or alkalinity of the solution the amino acids are called ampholytes.

Since Pk₁(RCOOH) = 2.35 and Pk₂ (RNH₃⁺) = 9.69, the isoelectric pH (P¹) of alanine is

Thus P¹ of lysine and arginine is 9.7 and 10.8 respectively. The ability to perform calculations of this type is of significant value in the clinical laboratory to assess the mobility of known compounds in elec­tric fields and to select appropriate buffers for sepa­ration of one from another.

Zwitterion:

i. The ion at isoelectric point which carries + and – charges is called zwitterion.

ii. In acid solution zwitterion combines with H⁺ ions to form base yielding cation, when the current is passed, the amino acid mi­grates towards cathode,

iii. In alkali solution, zwitterion combines with OH^– to form acid yielding anions and migrates towards anode during the change of current.

iv. Zwitterion is the ampholyte i.e. it is both a proton donor and proton acceptor.

v. Generally, the acidic and basic strength of the zwitterion are different and a solu­tion of pure amino acid in water is not neutral.

vi. Proteins like the amino acids, contain 6 acidic and basic groups exist in solution as zwitterion.

Isoelectric pH:

i. The isoelectric pH of an amino acid is that pH at which it has no net charge and does not move in an electric field.

ii. It can be denoted as follows:

P₁ = Pk₁ + Pk /2 [^._.^. Pk₁ = RCOO, Pk₂ = RN]

iii. Isoelectric pH (P^I) of

Aspartic acid → 2.9

Lysine → 9.7

Arginine → 10.8

iv. It has a significant value in the clinical laboratory to determine the mobility of known compounds in electric field.

v. It selects appropriate buffers for separa­tion of one from another.

vi. Proteins like amino acids have isoelectric pH (P^I) at which they are least soluble and migrate least in an electric field.

vii. Above P^I they (proteins) act as acids and form negative protein ions, whereas be­low P^I they act as bases and form positive protein ion.

Ampholyte:

i. In acid solution, the amino acid (Zwitterion) acts as base yielding cation, when current → cathode.

ii. In alkali solution → acid yielding anion, when current anode.

iii. On the basis of these opposite reactions depending upon the acidity and alkalin­ity of the solution, the amino acids are called ampholyte.

2. Structures of Amino Acids:

For many purposes, it is convenient to subdi­vide the amino acids in proteins into 7 classes as in the following table. In addition to their common names, systematic chemical names are also included in this table.

3. Optical Isomers of Amino Acids:

Except glycine, each amino acid has at least one asymmetric carbon atom and hence is optically active. Although D-amino acids occur in cells and even in polypeptides, they are not present in pro­teins.

Various other amino acids—Homocysteine, Homoserine, Ornithine, Citrulline, Arginosuccinic acid, Dopa, 3-monoiodotryrosine, 3, 5-Diiodotyro- sine, 3,5,3′-triiodotyrosine, Thyroxine, β-Alanine, Taurine etc.—in free or combined states fulfil important functions in metabolic processes other than as constituents of proteins. Many additional amino acids occur in plants or in antibiotics. Over 20 D- amino acids occur naturally.

Physical Properties of Amino Acids:

1. Amino acids are soluble in polar solvents such as water and ethanol but they are in­soluble in nonpolar solvents such as ben­zene or ether.

2. Their melting point is above 200°C.

3. The aromatic amino acids tryptophan, ty­rosine, histidine and phenylalanine absorb ultraviolet light.

Chemical Reactions of Amino Acids:

1. Ninhydrin Test:

Ninhydrin is a powerful oxidizing agent which causes oxidative decarboxylation of α-amino acids yield­ing CO₂, NH₃ and an aldehyde. The re­duced ninhydrin then reacts with the lib­erated ammonia forming a blue complex. Proline and hydroxyproline produce a yellow rather than a purple colour with ninhydrin.

2. A variety of colour reactions specific for particular functional groups in amino ac­ids are known which are useful in both qualitative and quantitative identification of particular amino acids. These are given below.

3. Formation of Peptide Bonds:

Peptide bond formation involves removal of one mole of water between the α-amino group of one amino acid and the α-carboxyl group of a second amino acid.