The correct answer is Option (1) → (A), (B) and (D) only
Let's analyze each statement regarding amino acids:
(A) Amino acids behave like salts
- Correct. Amino acids contain both an acidic carboxyl group ($\text{—COOH}$) and a basic amino group ($\text{—NH}_2$). In the solid state or as crystalline solids, these groups exist in their ionized form ($\text{—COO}^-$ and $\text{—NH}_3^+$), forming an internal salt known as a zwitterion. Due to their ionic nature, they have high melting points and behave similarly to salts.
(B) In aqueous solution they form zwitterion.
- Correct. When dissolved in water (or at a $\text{pH}$ near their isoelectric point), the proton from the carboxyl group is transferred to the amino group, resulting in a neutral molecule that carries both a positive and a negative charge (a dipolar ion), which is the zwitterion.
(C) Most naturally occurring amino acids exist in $\text{D}$ configuration
- Incorrect. With very few exceptions found in nature, most naturally occurring amino acids (especially those found in proteins) exist in the $\mathbf{L}$ configuration. $\text{L}$ and $\text{D}$ configurations are determined by relating the arrangement of groups around the chiral $\alpha$-carbon to $\text{L}$ and $\text{D}$-glyceraldehyde.
(D) Tyrosine is a non-essential amino acid
- Correct. Tyrosine is classified as a non-essential amino acid because the human body can synthesize it from another essential amino acid, Phenylalanine. However, Tyrosine is sometimes considered conditionally essential, as its synthesis relies on Phenylalanine.
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