Amino Acid Identification and Chirality

The tetrapeptide is composed of four specific amino acids obtained upon hydrolysis: Glycine (Gly), Alanine (Ala), Phenylalanine (Phe), and Leucine (Leu).

• Glycine (Gly): Achiral (alpha-carbon has two hydrogen atoms).
• Alanine (Ala): Chiral.
• Phenylalanine (Phe): Chiral.
• Leucine (Leu): Chiral.

Applying Sequence Constraints

The sequence must satisfy two main conditions:

1. The N-terminal residue is Alanine (Ala). This fixes the first amino acid in the sequence. Sequence structure: $Ala - AA2 - AA3 - AA4$.
2. Alanine is coupled to a chiral amino acid. This means the second amino acid (AA2) must be chiral.

Determining Possible Sequences

The available amino acids for positions AA2, AA3, and AA4 are {Gly, Phe, Leu}.

Considering the chiral constraint for AA2:

• The chiral amino acids available for AA2 are Phe and Leu. Gly is achiral and cannot be placed at AA2.

Case 1: AA2 is Phenylalanine (Phe)

• Sequence starts: $Ala - Phe - ...$
• Remaining amino acids for AA3 and AA4: {Gly, Leu}.
• Possible arrangements: $Gly - Leu$ or $Leu - Gly$.
• Resulting sequences: $Ala-Phe-Gly-Leu$ and $Ala-Phe-Leu-Gly$. (2 possibilities)

Case 2: AA2 is Leucine (Leu)

• Sequence starts: $Ala - Leu - ...$
• Remaining amino acids for AA3 and AA4: {Gly, Phe}.
• Possible arrangements: $Gly - Phe$ or $Phe - Gly$.
• Resulting sequences: $Ala-Leu-Gly-Phe$ and $Ala-Leu-Phe-Gly$. (2 possibilities)

Calculating Total Number of Sequences

The total number of possible sequences is the sum of possibilities from Case 1 and Case 2.

Total Sequences = (Possibilities with Phe at AA2) + (Possibilities with Leu at AA2)

Total Sequences = 2 + 2 = 4

Therefore, there are 4 possible sequences for the tetrapeptide.