Consider the following reaction:
Reactions 1 and 2 can be respectively characterized as:
A) peptide bond formation, nucleophilic acyl addition
B) polymerization, hydrolysis
C) hydrolysis, peptide bond formation
D) nucleophilic addition, hydrolysis
The reaction depicted shows two alanine molecules forming a peptide bond. The process of combining monomers, such as amino acids, into polymers, such as polypeptides is called polymerization. Reaction 2 shows the reverse reaction with water being used to break a peptide bond. Using water to break a bond can be described as hydrolysis. Thus (B) is the correct answer.
A: While reaction 1 is peptide bond formation, the reverse reaction is not an addition reaction. In addition reactions two molecules combine. Here, reaction 2 shows one large molecule being broken down.
C: These are the right labels, but applied backwards.
D: Reaction 2 is hydrolysis, but reaction 1 is not a nucleophilic addition; it’s a substitution reaction. The OH group is being substituted for the amino group of the amino acid on the right. Thus, this is a nucleophilic acyl substitution.