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A decapeptide has a molecular weight of 796. On complete hydrolysis it yields glycine (molecular weight 75), alanine, and phenylalanine as the only products. If glycine contributes 47.0% of the total weight of all hydrolysis products, how many glycine units are present in the decapeptide?

1. 3
2. 4
3. 5
4. 6

Correct answer: 6

Solution

Complete hydrolysis of a decapeptide (9 peptide bonds) adds 9*18 = 162 mass units, giving total product weight = 796 + 162 = 958. Glycine fraction: n*75 / 958 = 0.47 => n = 0.47*958 / 75 = 450.26/75 ≈ 6. Hence 6 glycine units.

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