A tetrapeptide "x" on complete hydrolysis produced glycine (Gly), alanine (Ala), valine (Val), leucine (Leu) in equimolar proportion each. The number of tetrapeptides (sequences) possible involving each of these amino acids is

We are asked to find how many different tetrapeptide sequences can be formed when the peptide is made of exactly four different amino acids: glycine (Gly), alanine (Ala), valine (Val) and leucine (Leu), each appearing exactly once.

Key idea: A peptide sequence is an ordered arrangement of the constituent amino acids. Hence, the problem reduces to counting the number of ways to arrange four distinct objects in a line.

For $$n$$ distinct objects, the number of possible linear arrangements (permutations) is given by the factorial formula$$n!$$.

Here $$n = 4$$, so

$$\text{Number of sequences} = 4! = 4 \times 3 \times 2 \times 1 = 24$$

Therefore, $$24$$ different tetrapeptide sequences are possible.

Hence, the correct choice is Option D.