Effect of alkyl group on transnitrosation of N-nitrosothiazolidine thiocarboxamides

S-Nitrosoglutathione (GSNO) relaxes vascular smooth muscles, prevents platelet aggregation, and acts as a potential in vivo nitric oxide donor. 3-Nitroso-1,3-thiazolidine-4-thiocarboxamide (1), a N-nitrosothio-proline analogue, exhibited a high GSNO formation activity. In this study, two compounds (2 and 3) based on compound 1 were newly synthesized by introducing either one or two methyl groups onto a nitrogen atom on the thioamide substituent in 1. The pseudo-first-order rate constants (k_obs) for the GSNO formation for the reaction between the compound and glutathione followed the order 1>2=3. Thus, the introduction of a methyl group(s) onto the thioamide group led to a decrease in the transnitrosation activity. On the basis of density functional theoretical calculations, the transnitrosation for the N-nitrosothiazolidine thiocarboxamides was proposed to proceed via a bridged intermediate pathway. Specifically, the protonated compound 1 forms a bridged structure between the nitrogen atom in the nitroso group and two sulfur atoms - one in the ring and the other in the substituent. The bridged intermediate gives rise to a second intermediate in which the nitroso group is bonded to the sulfur atom in the thioamide group. Finally, the nitroso group is transferred to GSH to form GSNO.