Thermal Stability of Primary S-Nitrosothiols:  Roles of Autocatalysis and Structural Effects on the Rate of Nitric Oxide Release

S-Nitrosothiols (RSNOs) are considered to play important roles in storing, transporting, and releasing nitric oxide (nitrogen monoxide, NO) in vivo. Although tertiary RSNOs are known to be intrinsically more stable than primary RSNOs, the correlation between the structure of primary RSNOs and the kinetics of thermal NO release in solution has not been established yet. We have characterized the kinetics of thermal NO release from three primary RSNOs: S-nitrosocysteine (CySNO), S-nitroso-N-acetylcysteine (SNAC), and S-nitrosoglutatione (GSNO) in aqueous solutions. It was found that the rates of NO release are strongly affected by the initial concentration of the solutions. Increasing the concentration of CySNO and SNAC from 1.0 × 10-1 to 61.0 mmol L-1 led to 5.7- and 14.6-fold increases in their initial rates of decomposition, respectively, whereas GSNO was much less affected (a 2-fold increase). However, a smaller increase in concentration (0.1 to 1.0 mM) led to a 4.6-fold decrease, on average, in the rates of NO release in the three cases. This result was assigned to the combination of an autocatalytic effect promoted by the secondary reaction of thyil radicals with authentic RSNO molecules, which accelerates the decomposition reaction in concentrated solutions, and a nongeminate (diffusive, outside the cage) radical pair recombination effect that leads to a reduction in the rates of reaction in dilute solutions. In the low-concentration range, GSNO and SNAC were shown to be significantly more stable than CySNO. This result is in accordance with the conclusions derived from single-point energy calculations at the MP2/6-31G(2df,p)//MP2/6-31G(d) level of theory, which have shown that the acetamido group that is present in SNAC plays a key role in increasing the S−N bond strength. These results show that comparisons of stability among different S-nitrosothiols in solution must take the concentration effect carefully into account and indicate that the half-lives of primary RSNOs found in vivo can be partially determined by their intrinsic structural properties.