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Biomolecules - Proteins: Amino acids and peptide bonds

Grade 12ICSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Proteins are high molecular weight complex biopolymers of α\alpha-amino acids. They are essential for the growth and maintenance of body tissues.

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α\alpha-Amino acids contain both an amino (−NH2-NH_2) group and a carboxylic acid (−COOH-COOH) group attached to the same carbon atom (the α\alpha-carbon).

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In aqueous solution, amino acids exist as dipolar ions called Zwitterions. The −COOH-COOH group loses a proton and −NH2-NH_2 group accepts a proton, resulting in a net neutral charge but separate positive and negative centers: R−CH(NH3+)COO−R-CH(NH_3^+)COO^-.

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Isoelectric Point (pIpI): It is the pH at which the amino acid shows no net charge and does not migrate towards any electrode under the influence of an electric field.

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Essential Amino Acids: These cannot be synthesized by the human body and must be obtained through diet (e.g., Valine, Leucine). Non-essential amino acids are synthesized by the body (e.g., Glycine, Alanine).

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Peptide Bond: It is an amide linkage (−CO−NH−-CO-NH-) formed between the carboxyl group of one α\alpha-amino acid and the amino group of another α\alpha-amino acid by the elimination of a water molecule.

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Classification based on nature: Amino acids are acidic if they contain more −COOH-COOH groups than −NH2-NH_2 groups (e.g., Aspartic acid), basic if they contain more −NH2-NH_2 than −COOH-COOH (e.g., Lysine), and neutral if both groups are equal (e.g., Alanine).

📐Formulae

R−CH(NH2)−COOHR-CH(NH_2)-COOH

H2N−CH(R)−COOH⇌H3N+−CH(R)−COO−H_2N-CH(R)-COOH \rightleftharpoons H_3N^{+}-CH(R)-COO^{-}

nH2N−CH(R)−COOH→(−NH−CH(R)−CO−)n+nH2OnH_2N-CH(R)-COOH \rightarrow (-NH-CH(R)-CO-)_n + nH_2O

H2N−CH(R1)−COOH+H2N−CH(R2)−COOH→−H2OH2N−CH(R1)−CO−NH−CH(R2)−COOHH_2N-CH(R_1)-COOH + H_2N-CH(R_2)-COOH \xrightarrow{-H_2O} H_2N-CH(R_1)-CO-NH-CH(R_2)-COOH

💡Examples

Problem 1:

Show the formation of a dipeptide between Glycine (H2N−CH2−COOHH_2N-CH_2-COOH) and Alanine (H2N−CH(CH3)−COOHH_2N-CH(CH_3)-COOH).

Solution:

H2N−CH2−COOH+H2N−CH(CH3)−COOH→H2N−CH2−CO−NH−CH(CH3)−COOH+H2OH_2N-CH_2-COOH + H_2N-CH(CH_3)-COOH \rightarrow H_2N-CH_2-CO-NH-CH(CH_3)-COOH + H_2O

Explanation:

The carboxyl group of Glycine reacts with the amino group of Alanine. A water molecule is eliminated, resulting in the formation of the peptide linkage −CO−NH−-CO-NH-. The resulting molecule is Glycylalanine.

Problem 2:

Explain why amino acids have high melting points and are soluble in water compared to corresponding halo-acids.

Solution:

Amino acids exist as Zwitterions (H3N+−CH(R)−COO−H_3N^{+}-CH(R)-COO^{-}).

Explanation:

Due to the Zwitterionic structure, amino acids behave like ionic salts rather than simple organic molecules. Strong electrostatic forces of attraction between the dipolar ions lead to high melting points and significant solubility in polar solvents like H2OH_2O.

Problem 3:

Represent the structure of the simplest amino acid which is not optically active.

Solution:

H2N−CH2−COOHH_2N-CH_2-COOH

Explanation:

Glycine is the only naturally occurring α\alpha-amino acid that is optically inactive because the α\alpha-carbon atom is not chiral (it is bonded to two hydrogen atoms).

Proteins: Amino acids and peptide bonds Revision - Class 12 Chemistry ICSE