Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Chemistry Class 12

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Class 12 : Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

The document Doc: Amino Acids and Their Classification Class 12 Notes | EduRev is a part of the Class 12 Course Chemistry Class 12.
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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

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Name

Abbreviation

Neutral amino acid

NE

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Glycine

Gly

Neutral amino acid

NE

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Alanine

Ala

Neutral amino acid

E

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Valine

Val

Neutral amino acid

E

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Leucine

Leu

Neutral amino acid

E

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Isoleucine

Ile

Acidic amino acid

NE

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Aspartic Acid

Asp

Acidic amino acid

NE

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Glutamic Acid

Glu

Basic amino acid

E

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Lysine

Lys

 

 

Basic amino acid

NE

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Arginin

Arg

Basic amino acid

NE

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Histidine

His

Neutral amino acid

E

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Methionine

Met

Neutral amino acid

NE

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Proline

Pro

Neutral amino acid

E

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Phenylalanine

Phe

Neutral amino acid

E

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Tryptophan

Trp

Neutral amino acid

NE

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Serine

Ser

Neutral amino acid

E

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Threonine

Thr

Neutral amino acid

NE

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Cysteine

Cys

Neutral amino acid

NE

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

Tyrosine

Tyr

 

= 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.

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

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 pKa for the carboxylic acid hydrogen of alanine is considerably lower (2.3) than the pKa of an ordinary carboxylic acid (e.g., propanoic acid, pKa= 4.89):

  Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

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 pKa 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.

Doc: Amino Acids and Their Classification Class 12 Notes | EduRev

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 pKa 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-),

pKa = 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.

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