A peptide synthesized by the reactions of one molecule each of Glycine, Leucine, Aspartic acid and Histidine will have _________ peptide linkages.

Solution

Chemistry ma’am · 4 steps

Four amino acids in a chain need only three links, just like four beads on a string.

1. One bond, one water. A peptide linkage -CO-NH⁻ forms when the -COOH of one amino acid joins the -NH₂ of the next, with loss of one H₂O. Each such bond joins exactly two amino acids.
2. Count the gaps. Glycine, leucine, aspartic acid and histidine join in a straight chain, which makes a tetrapeptide. A chain of n units has n - 1 joins between neighbours.

   Gly-Leu-Asp-His: n - 1 = 4 - 1 = 3

3. Ends stay free. The first amino acid keeps a free -NH₂ (the N-terminal) and the last keeps a free -COOH (the C-terminal), so neither end makes an extra bond. The extra -COOH on the side chain of aspartic acid is not counted either, because a peptide chain is built through the main-chain groups.
4. So. The tetrapeptide has 3 peptide linkages.

The common slip: 4. Counting one peptide linkage for each amino acid gives 4, but the two chain ends stay free, so four units need only three joins.

The idea to keep. A peptide made from n amino acids has n - 1 peptide bonds and loses n - 1 water molecules. The two ends stay free as -NH₂ and -COOH.

Answer from the NTA final answer key. Solution by PrepWiser.