# Do carboxyl groups behave like bases and amino groups behave like acids when peptide bonds are created?

Peptide bonds are synthesized when the carboxyl group of one amino acid molecule reacts with the amino group of the other amino acid molecule, causing the release of a molecule of water.

Water molecules consist of a hydroxyl from a carboxyl group and a hydrogen from an amino group.

And that seems a bit strange to me, since carboxyl is an acid and should give a hydrogen (instead of hydroxyl) and amino groups are bases and should get proton (and not give a hydrogen).

So for me it looks like in creating this bond, the carboxyl group behaves like a base and amino group behaves like an acid. Is that correct?

• To paraphrase what Jan said, you should really look at the reaction mechanism to understand why things happen the way you described. Jun 8, 2015 at 4:41
• This paper examines the reaction mechanism for Gly peptide bond formation in the gas phase: pubs.acs.org/doi/abs/10.1021/jp504924c Jun 8, 2015 at 15:19

But these are only one type of reactions both groups are capable of performing. In the case of the amino group, as long as it is not protonated (i.e. still has an unbonded electron pair) it can attack nucleophilicly. Maybe you have heard of $\mathrm{S_N}2$ type reactions, that are rather similar.