Question

A tripeptide contains valine, lysine, and phenylalanine residues. (a) How many tripeptides are possible from these amino acids? (b) Draw a structural formula for a possible form of the tripeptide and name it, using the shorthand form.

Short answer

Answer: There are 27 possible tripeptides that can be formed using valine, lysine, and phenylalanine. One example of a tripeptide is Valine-Lysine-Phenylalanine, with the structural formula: H-N(Val)-C-N(Lys)-C-N(Phe)-C-OH, and the shorthand name: Val-Lys-Phe.

Step-by-step solution

Calculate possible tripeptides

To find the number of possible tripeptides, we use the permutation formula to find all the combinations using the three different amino acid residues. Since there are no restrictions and all residues can be used at every position, we can calculate the permutations with repetition as: Number of permutations with repetition = n^r where n = number of distinct amino acids (valine, lysine, and phenylalanine) r = number of positions in the tripeptide (3 in this case) So, we have: Number of permutations with repetition = 3^3 = 27 possible tripeptides.

Draw a structural formula

Now we can draw a structural formula for one possible form of the tripeptide. Let's take Valine-Lysine-Phenylalanine for this example. You can replace the amino acid order with any possible permutation to form another structure. The structure of Valine-Lysine-Phenylalanine tripeptide is as follows: H - N (Val) - C - N (Lys) - C - N (Phe) - C - OH Valine, having the abbreviation of Val, would be on the N-terminal end of the tripeptide, followed by Lysine (Lys), and then Phenylalanine (Phe) would be on the C-terminal end.

Name the tripeptide

Finally, we can name this specific tripeptide using the shorthand form which is derived from the single-letter abbreviations of the amino acids it consists of. The shorthand form for Valine-Lysine-Phenylalanine tripeptide would be: Val-Lys-Phe Now we have provided the number of possible tripeptides, drawn a structural formula for one of those possible tripeptides, and named it using the shorthand form.