Computational investigation of the reaction mechanisms of nitroxyl and thiols

Nitroxyl, or nitrosyl hydride, (HNO) is a pharmacologically relevant molecule whose physiological responses have been thought to result from modification of intracellular thiols. The reaction of HNO with thiols has been shown to lead to disulfides and sulfinamides. The free energies of reaction (DeltaG) and activation (DeltaG(++)) were determined for the reaction pathways of HNO and five different thiols using computational methods. The methods employed included B3LYP, MP2, and CBS-QB3, as well as IEF-PCM to approximate implicit water solvation. The five examined thiols were hydrogen sulfide, methanethiol, trifluoromethanethiol, thiophenol, and cysteine. A putative N-hydroxysulfenamide intermediate was the initial product for the reaction of HNO with a thiol. Analysis of the Wiberg bond indices indicated that the formation of the S-N bond was concerted with the proton transfers that led to the intermediate. The calculated pK(a) of protonated N-hydroxysulfenamide was approximately 13, and from the protonated N-hydroxysulfenamide intermediate, two irreversible reactions that lead to either the disulfide or sulfinamide were found. The calculated values of DeltaG(++) indicated the preferred reaction pathway would be dependent upon the hydrophobicity of the environment, the availability of a local base, and the identity of the thiol substituent. In a hydrophobic environment, the formation of the disulfide was kinetically favored. Formation of the sulfinamide product was expected to occur upon the protonation of the hydroxy group of the N-hydroxysulfenamide intermediate.