How many linear tripeptides are possible with valine (Val), Glycine (Gly) and Alanine (Ala). No amino acid should be repeated?

The correct option is 6.

To find the number of linear tripeptides that can be formed using three distinct amino acids—Valine (Val), Glycine (Gly), and Alanine (Ala)—without repeating any amino acid, we can analyze the problem as a permutation of three unique items.

A tripeptide is formed by linking three amino acids together via peptide bonds. Since peptides have directionality (with a free amino group at the N-terminus, and a free carboxyl group at the C-terminus), the sequence order of the amino acids matters. For example, Val-Gly-Ala is chemically distinct from Ala-Gly-Val.

Since we have 3 distinct amino acids and we cannot repeat any of them, the number of possible linear tripeptides is given by the number of permutations of 3 distinct objects taken 3 at a time.

The number of permutations is calculated using the factorial function:
$3!$
Expanding this calculation:
$3!=3\times 2\times 1=6$

We can list all 6 unique tripeptide sequences to verify:
1. Val-Gly-Ala
2. Val-Ala-Gly
3. Gly-Val-Ala
4. Gly-Ala-Val
5. Ala-Val-Gly
6. Ala-Gly-Val

Therefore, there are exactly 6 possible linear tripeptides.