Kinetics and Mechanism of S-Nitrosothiol Add-Catalyzed Hydrolysis: Sulfur Activation Promotes Facile NO+ Release

The denitrosation of three primary S-nitrosothiols (RSNO; S-nitrosocysteine, S-nitroso-N-acetylcysteine, and S-nitrosoglutathione) and two tertiary RSNOs (S-nitrosopenicillamine and S-nitroso-N-acetylpenicillamine) was investigated in 3.75 M H2SO4 to probe the mechanism of acid-catalyzed RSNO hydrolysis and its dependence on RSNO structure. This reversible reaction was forced to proceed in the denitrosatioin direction by trapping the nitrosating agent with HN3. The primary RSNOs exhibited hydrolysis k(obs) values of similar to 2 x 10(-4) s(-1), and the tertiary RSNO k(obs) values were an order of magnitude higher. Product analysis by HPLC revealed that the parent thiols (RSHs) were formed in 90-100% yield on 79-99% RSNO denitrosation. Possible hydrolysis mechanisms were studied computationally at the CBS-QB3 level using S-nitrosomethanethiol (MeSNO) as a model RSNO. Consideration of RSNOs as a combination; of conventional R-S-N=O, zwitterionic R-S+=N-O-, and RS-/NO+ ion-pair resonance structures was key in understanding the mechanistic details of acid-catalyzed hydrolysis. Protonation of the S-nitroso oxygen or nitrogen activates the sulfur and nucleophilic attack by H2O at this atom leads to the formation of the sulfoxide-protonated N-hydroxysulfinamide, MeS+(OH)NHOH, with barriers of 19 and 29 kcal/mol, respectively. Proton loss and reprotonation at the nitrogen lead to secondary hydrolysis that produces the sulfinic acid MeS(=O)OH and NH2OH. Notably, no low-energy RSNO hydrolysis pathway for HNO release was found in the computational analysis. Protonation of the S-nitroso sulfur gives rise to NO+ release with a low activation barrier (Delta H-calc(double dagger) approximate to 6 kcal/mol) and the formation of MeSH in agreement with experiment. The experimental k(obs) can be expressed as K(a)k(1), where K-a is the acid dissociation constant for protonation of the S-nitroso sulfur and k(1) the pseudo-first-order hydrolysis rate constant. Given the low Delta H-calc double dagger for denitrosation of the S-protonated isomer, the observed slow rates of acid-catalyzed RSNO hydrolysis must be controlled by the magnitude of K-a. The 10-fold higher K-a calculated for Me3CS(H+)NO (similar to 10(-15)) compared to MeS(H+)NO (10(-16)) is consistent with the order of magnitude larger k(obs) reported here for the tertiary vs primary RSNOs.