Desulfurization Mechanism of Cysteine in Synthesis of Polypeptides

The free-radical-based selective desulfurization of cysteine residue is an efficient protocol to achieve ligations at alanine sites in the synthesis of polypeptide and proteins. In this work, the mechanism of desulfurization process has been studied using the density functional theory methods. According to the calculation results, the desulfurization of the thiol group occurs via a three-steps mechanism： the abstraction of hydrogen atom on the thiol group with the radical initiator VA-044 （2,2＇-azobis[2-（2-imidazolin-2-yl）propane]dihydrochloride）, the removal of S atom under the reductant TCEP （tris（2-carboxyethyl）phosphine）, and the formation of RH molecule （with the regeneration of RS radical）. The second step （desulfurization step） is the rate-determining step, and the adduct t-BuSH facilitates the desulfurization of cysteine via benefiting the formation of the precursor of the desulfurization step.