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Hint:-Amino acids are monomers of proteins. They are composed of an amino group, a carboxyl group, a hydrogen atom, and a distinctive side chain, all bonded to the alpha carbon atom. Since the amino acids have both an amino group and a carboxyl group, they are known as amphoteric compounds. Therefore, it can either donate or accept a proton, thus acting either as a base or an acid. Every amino acid exists as a dipolar ion (zwitterion) at specific values of pH called the isoelectric point (pI). Complete solution: 1. Cystine - It is a sulfur-containing amino acid as it contains disulfide bonds. Disulfide bonds are formed by oxidizing agents or in an oxidizing environment. Cystine is formed by two units of cysteine amino acids, therefore, it has two amino groups and two di-carboxylic groups. Hence, it is a diamino dicarboxylic amino acid. It is a polar, hydrophilic amino acid and contains a negative charge. 2. Lysine- It is a basic amino acid. It has two amino groups and one carboxylic group, hence it is a diamino amino acid. The two amino groups are the alpha-amino group and the epsilon amino group. Since it is a basic amino acid, it has an overall positive charge. 3. Valine- It is a hydrophobic amino acid. It contains one amino group and one carboxylic group. It has a side chain isopropyl group and is an essential amino acid in humans. 4. Aspartic acid- It is a negatively charged acidic side chain containing amino acid. It contains two carboxylic groups and one amino group. Hence, it is a dicarboxylic acid. Therefore, the correct answer is A) Cystine. Note:- Cystine vs cysteine- Cystine is formed by two cysteine molecules. It is more stable than cysteine. Cysteine is a mono-amino monocarboxylic acid. Human hair is made up of cysteine amino acids. The curliness of hair depends upon the cysteine amino acid content or degree of disulfide bonding in keratin filaments. The presence of disulfide bonds indicates curliness. Therefore people with straight hair have less cysteine and those with curly hair have more cysteine. #aminoAcid #DiaminoDicarboxylicAminoAcid #protein #peptide #polypeptide #examQuestions
