The dipeptide, Gly. Ala has structure -

Options

A. `H_3overset(+)NCH_2overset(O)overset(||)CNHunderset(CH_3)underset(|)CHCoverset(-)(O_2)`
B. `H_3overset(+)Nunderset(CH_3)underset(|)CHoverset(O)overset(||)CNHCH_2Coverset(-)(O_2)`
C. `overset(-)(O_2)C underset(CH_3)underset(|)CH-NH-overset(O)overset(||)C-CH_2 overset(+)(N)H_3`
D. `overset(-)(OO)C CH_2 NH-overset(O)overset(||)C underset(CH_3)underset(|)C Hoverset(+)(N)H_3`

Explanation

To determine the structure of the dipeptide Gly-Ala (Glycine-Alanine), we will follow these steps:

### Step 1: Identify the Amino Acids
- Glycine (Gly) has the structure:
- NH2-CH2-COOH
- Alanine (Ala) has the structure:
- NH2-CH(CH3)-COOH

### Step 2: Understand the Peptide Bond Formation
- A dipeptide is formed when two amino acids undergo a condensation reaction, which results in the formation of a peptide bond.
- During this reaction, the carboxyl group of one amino acid reacts with the amino group of another, releasing a molecule of water (H2O).

### Step 3: Write the Structures of Glycine and Alanine
- Glycine:
- NH2-CH2-COOH
- Alanine:
- NH2-CH(CH3)-COOH

### Step 4: Form the Dipeptide
- The reaction between Glycine and Alanine involves the amino group of Glycine and the carboxyl group of Alanine:
- Glycine (NH2-CH2-COOH) + Alanine (NH2-CH(CH3)-COOH) → Gly-Ala (Dipeptide) + H2O

### Step 5: Combine the Structures
- The resulting structure of the dipeptide Gly-Ala can be represented as:
- NH2-CH2-CO-NH-CH(CH3)-COOH
- Here, the peptide bond (–CO–NH–) is formed between the carbonyl carbon of Glycine and the nitrogen of Alanine.

### Step 6: Adjust for Physiological pH
- At physiological pH, amino acids exist as zwitterions. Therefore, the terminal amino group will be protonated (NH3+) and the terminal carboxyl group will be deprotonated (COO-).
- The final structure of the dipeptide Gly-Ala will be:
- NH3+-CH2-CO-NH-CH(CH3)-COO-

### Step 7: Verify the Structure
- The final structure should be checked against the options provided in the question to identify the correct representation of the dipeptide.

### Final Structure of Gly-Ala
- The correct structure of the dipeptide Gly-Ala is:
- NH3+-CH2-CO-NH-CH(CH3)-COO-