A short peptide on complete hydrolysis produces 3 moles of glycine (G), two moles of leucine (L) and two moles of valine (V) per mole of peptide. The number of peptide linkages in it are ________.

Solution

Chemistry ma’am · 3 steps

Count the amino acids first: a straight chain of n residues always has n minus 1 peptide bonds.

1. Count the residues. One mole of peptide gives 3 + 2 + 2 = 7 moles of amino acids. So each peptide molecule is a chain of 7 amino acid units.
2. Bonds join neighbours. A peptide bond (-CO-NH⁻) joins two neighbouring amino acids and releases one H₂O. In an open chain, each bond sits between two units, so there is one bond fewer than there are units.

   peptide bonds = n - 1 = 7 - 1 = 6

3. So. The heptapeptide has 6 peptide linkages, and complete hydrolysis uses 6 molecules of water to break them.

The common slip: 7. Counting one bond per amino acid gives 7, not 6. The two end residues each have only one neighbour, so a 7-unit chain has only 6 links.

The idea to keep. Hydrolysis tells you how many units are in the chain. For a linear peptide of n residues, the number of peptide bonds is n − 1. The order of the amino acids does not change this count.

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