The number of possible tripeptides formed involving alanine (ala), glycine (gly) and valine (val), where no amino acid has been used more than once is:

We need to find the number of possible tripeptides using alanine (ala), glycine (gly), and valine (val), with each amino acid used at most once.

A tripeptide is a sequence of 3 amino acids. Since the order matters (N-terminal to C-terminal), and we have 3 different amino acids each used exactly once:

Number of arrangements = 3! = 3 × 2 × 1 = 6

The possible tripeptides are:

1. ala-gly-val

2. ala-val-gly

3. gly-ala-val

4. gly-val-ala

5. val-ala-gly

6. val-gly-ala

Therefore, the number of possible tripeptides is Option 4: 6.