Peptide thioester preparation via intramolecular O-to-S acyl transfer is a recently developed method for protein chemical synthesis through Fmoc chemistry.Theoretical calculations have been carried out to study the mechanism for the formation of thioesters via O-to-S acyl transfer.It is found that the O-to-S acyl transfer occurs via an anionic stepwise mechanism in which the cleavage of the C-O bond is the rate-limiting step.The side reaction of hydrolysis also proceeds through an anionic stepwise process,and its rate-limiting step is the attack of the hydroxide ion on the carbonyl carbon.Increase of the chain length between the ester O atom and the S atom can increase the energy barrier of the O-to-S acyl transfer.On the other hand,substituents at the α-position of the ester can reduce the energy barrier. Peptide thioester preparation via intramolecular O-to-S acyl transfer is a recently developed method for protein chemical synthesis through Fmoc chemistry. These theoretical calculations have been carried out to study the mechanism for the formation of thioesters via O-to-S acyl transfer. It is found that the O-to-S acyl transfer occurs via an anionic stepwise mechanism in which the cleavage of the CO bond is the rate-limiting step. The side reaction of hydrolysis also proceeds through an anionic stepwise process, and its rate-limiting step is the attack of the hydroxide ion on the carbonyl carbon. Crease of the chain length between the ester O atom and the S atom can increase the energy barrier of the O-to-S acyl transfer. On the other hand, substituents at the α-position of the ester can reduce the energy barrier.