Amino Acids & Their Classification - Chemistry Class 12 - NEET

Amino Acids

1. Intorudction:

Amino acids are the compounds which contain both an amino group and a carboxy group in their molecules. They constitute a particularly imortant class of difunctional compounds as they are the building blocks of proteins.

While several hundred different amino acids are known to occur naturally, 20 of them deserve special mention as they ae preesent in proteins. These amino acids are listed in Table. As given in this table, for amino acids trivial names are common. The convention to use a three letter code, as an abbreviation, for each amino acid is also given in the table. These abbreviations are particularly useful in designating the sequence of amino acids in peptides and proteins which your will study.

Nature of amino acid	E/N.E		Name	Abbreviation
Neutral amino acid	NE		Glycine	Gly
Neutral amino acid	NE		Alanine	Ala
Neutral amino acid	E		Valine	Val
Neutral amino acid	E		Leucine	Leu
Neutral amino acid	E		Isoleucine	Ile
Acidic amino acid	NE		Aspartic Acid	Asp
Acidic amino acid	NE		Glutamic Acid	Glu
Basic amino acid	E		Lysine	Lys

Basic amino acid	NE		Arginin	Arg
Basic amino acid	NE		Histidine	His
Neutral amino acid	E		Methionine	Met
Neutral amino acid	NE		Proline	Pro
Neutral amino acid	E		Phenylalanine	Phe
Neutral amino acid	E		Tryptophan	Trp
Neutral amino acid	NE		Serine	Ser
Neutral amino acid	E		Threonine	Thr
Neutral amino acid	NE		Cysteine	Cys
Neutral amino acid	NE		Tyrosine	Tyr

E = essential amino acid NE = Non essential amino acid

AMINO ACID AS DIPOLAR IONS:

Amino Acids contain both a basic group (-NH₂) and an acidic group (-COOH). In the dry solid state, amino acids exist as dipolar ions, a form in which the carboxyl group is present as a carboxylate ion, -CO₂^-, and the amino group is present as an aminium ion, -NH₃  (Dipolar ions are also called zwitter ions.) In aqueous solution, an equilibrium exists between the dipolar ion and the anionic and cationic forms of amino acids.

If alanine is dissolved in a strongly acidic solution (e.g. pH 0), it is present in mainly a net cationic form. In this state, the amine group is protonated (bears a formal 1 charge) and the carboxylic acid group is neutral (has no formal charge). As is typical of α-amino acids, the pK_a for the carboxylic acid hydrogen of alanine is considerably lower (2.3) than the pK_a of an ordinary carboxylic acid (e.g., propanoic acid, pK_a= 4.89):

The reason for this enhanced acidity of the carboxyl group in an α-amino acid is the inductive effect of the neighbouring aminium cation, which helps to stabilize the carboxylate anion formed when it loses a proton. Loss of proton from the carboxyl group in a cationic α-amino acid leaves the molecule electrically neutral (in the form of a dipolar ion). This equilibrium is shown in the red-shaded portion of the equation below.

The protonated amine group of an α-amino acid is also acidic, but less so that the carboxylic acid group. The pK_a of the animium group in alanine is 9.7. The equilibrium for loss of an aminium proton is shown in the blue-shaded portion of the equation below. The carboxylic acid proton is always lost before a proton from the aminium group in an α-amino acid.

The state of an a-amino acid at any given pH is governed by a combination of two equilibrium, as shown in the above equation for alanine. The isoelectric point (pI) of an amino acid such as alanine is the average of pKa₁ and pKa₂;

pI = ½ (2.3 9.7) = 6.0 (isoelectric point of alanine)

When a base is added to a solution of the net cationic form of alanine (initially at pH 0, for example), the first proton removed is the carboxylic acid proton, as we have said. In the case of alanine, when a pH of 2.3 is reached, the acid proton will have been removed from half of the molecules. This pH represents the pK_a of the alanine carboxylic acid proton, as can be demonstrated using the Henderson-Hasselbalch equation. The Henderson - Hasselbalch equation shows that for an acid (HA) and its conjugate base (A^-),

pK_a = pH+ log[HA]/[A^-]

When the acid is half neutralized,

b) Co-polymers are another type of polymers. These contain more than one sub-unit (or monomer).

Example:

In the above example, styrene and maleic anhydride monomers laternate. Co-polymer can be a block co-polymer.

Example:

Co-polymers can be random as well.

— B - A - A - B - A - B - B - A - B - A -B - B - A —

A and B are monomers.

6. There are many polymers in nature.

Example: Cellulose, starch, pepsin, insulin, egg albumin, rubber, DNA (Deoxyribonucleic acid) etc. These are called Biopolymers.

Man made polymers are, Nylon, Terylene, Polythene, Polystyrene, PVC (Polyvinyl chloride), Bakelite, Perspex, Polysiloxane etc.

7. The properties of a polymer solution are strikingly different from those of a true solution. For example, when polyvinyl alcohol is added to water, it swells.

a) Its shape gets distorted and after a long time it dissolves.

b) When more of polymer is added to a given solvent, saturation point is not reached. The mixture of polymer and solvent assumes a soft dough-like consistency.

8. Addition polymers and condensation polymers are two important types of polymers.

9. Polymer can be described as linear, branched and network.