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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

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Name

Abbreviation

Neutral amino acid

NE

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Glycine

Gly

Neutral amino acid

NE

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Alanine

Ala

Neutral amino acid

E

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Valine

Val

Neutral amino acid

E

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Leucine

Leu

Neutral amino acid

E

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Isoleucine

Ile

Acidic amino acid

NE

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Aspartic Acid

Asp

Acidic amino acid

NE

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Glutamic Acid

Glu

Basic amino acid

E

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Lysine

Lys

 

 

Basic amino acid

NE

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Arginin

Arg

Basic amino acid

NE

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Histidine

His

Neutral amino acid

E

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Methionine

Met

Neutral amino acid

NE

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Proline

Pro

Neutral amino acid

E

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Phenylalanine

Phe

Neutral amino acid

E

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Tryptophan

Trp

Neutral amino acid

NE

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Serine

Ser

Neutral amino acid

E

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Threonine

Thr

Neutral amino acid

NE

Amino Acids & Their Classification | Chemistry Class 12 - NEET

Cysteine

Cys

Neutral amino acid

NE

Amino Acids & Their Classification | Chemistry Class 12 - NEET

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.

Amino Acids & Their Classification | Chemistry Class 12 - NEET

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):

  Amino Acids & Their Classification | Chemistry Class 12 - NEET

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.

Amino Acids & Their Classification | Chemistry Class 12 - NEET

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.

The document Amino Acids & Their Classification | Chemistry Class 12 - NEET is a part of the NEET Course Chemistry Class 12.
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FAQs on Amino Acids & Their Classification - Chemistry Class 12 - NEET

1. What are amino acids and why are they important?
Ans. Amino acids are organic compounds that are the building blocks of proteins. They play a crucial role in various biological processes such as protein synthesis, enzyme production, and cell signaling. Amino acids are essential for the growth, repair, and maintenance of tissues in the body.
2. How are amino acids classified?
Ans. Amino acids are classified into two main groups based on their chemical properties: essential and non-essential amino acids. Essential amino acids cannot be produced by the body and must be obtained from the diet, while non-essential amino acids can be synthesized by the body itself.
3. What are the differences between essential and non-essential amino acids?
Ans. Essential amino acids are those that cannot be synthesized by the body and must be obtained from external sources such as food. There are nine essential amino acids. On the other hand, non-essential amino acids can be produced by the body itself through various metabolic processes. There are a total of 11 non-essential amino acids.
4. Can amino acids be obtained from plant-based sources?
Ans. Yes, amino acids can be obtained from plant-based sources. Although animal-based products like meat, eggs, and dairy are considered complete protein sources as they contain all essential amino acids, plant-based sources such as legumes, grains, nuts, and seeds can also provide a variety of amino acids. By combining different plant-based protein sources, individuals can ensure they get all the essential amino acids.
5. How do amino acid imbalances affect the body?
Ans. Amino acid imbalances can have various effects on the body. Deficiency in essential amino acids can lead to impaired growth, muscle wasting, weakened immune system, and decreased production of important molecules such as enzymes and hormones. Excessive intake of certain amino acids, especially those from supplements, can also have negative effects on health and may lead to toxicity or interfere with the absorption of other nutrients. It is important to maintain a balanced intake of amino acids to support overall health and wellbeing.
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