The number of tripeptides formed by three different amino acids using each amino acid once is ______.

Find the number of tripeptides formed by 3 different amino acids, each used once.

A tripeptide is a chain of 3 amino acids linked by peptide bonds. The sequence matters because each position in the chain is chemically distinct (N-terminal, middle, C-terminal).

We have 3 distinct amino acids (say A, B, C) and need to arrange all 3 in a sequence of 3 positions. This is a permutation problem:

$$ P(3,3) = 3! = 3 \times 2 \times 1 = 6 $$

The 6 tripeptides are: ABC, ACB, BAC, BCA, CAB, CBA.

The answer is 6.