In order to study the influence of substitutions at the a and p carbon atoms on the stability of the S-NO bond, water-soluble thionitrites RSNO have been synthesized by nitrosation of cysteamine and mercaptoethanol derivatives and characterized. 'H and 13C NMR spectroscopies have proven to be excellent probes for the nitrosation of thiols. In water, at physiological pH, the compounds decomposed into nitric oxide NO and the corresponding disulfides. The rate at which NO was released was very sensitive to modifications at the a and /3 carbon atoms. Tertiary thionitrites were more stable than primary thionitrites. The /3-substituents decreased the rates of decomposition in the following order: OH > NHCOCH3 > NH3+. S-Nitrosocysteamine derivatives were greatly stabilized at low pH. The compounds described here might be convenient and useful as vehicles for spontaneous generation of nitric oxide in biological systems, at rates that can be finely tuned and controlled over a wide range. @ (3) (a) Maragos, C. M.; Morley, D.; Wink, D. A.; Dunams, T. M.; Saavedra, J. E.; Hoffman, A.; Bove, A. A.; Isaac, L.; Hrabie, J . A.; Keefer, L. K. J . Med. Chem. 1991,34,3242-3247. (b) Saavedra, J. E.; Dunams, T. M.; Flippen-Anderson, J. L.; Keefer, L. K. J. O g . Chem. 1992, 57, 6134-6138.0022-326319411959-7019$04.50/0 ing disulfide RSSR and nitric oxide.4 However, most stable S-nitroso compounds, such as tert-butyl and trityl thionitrite and other alkylthionitrites, are completely insoluble in water.5 Furthermore, water-soluble thionitrites are generally much too unstable and thus too difficult to prepare and purify. Thus, only few of these compounds, such as cysteine and glutathione derivatives, have been isolated.6 The indefinite stability of S-nitroso-N-acetyl-D,L-penicillamine (SNAP) as green crystals allows analysis by X-ray crystallography.' SNAP has actually been used as a generator of NO in biological studies. Finally, the cysteine residues of a number of proteins can be nitrosated.a The stability of S-nitrosocysteines within a polypeptide chain is remarkable compared to that of free S-nitro~ocysteine.~In the present report we describe the preparation and the characterization of new thionitrites from cysteamine and mercaptoethanol derivatives. We have studied their stabilities in aqueous solutions as a function of pH and shown that they spontaneously generate nitric oxide at physiological pH. Their properties make them convenient and useful NO generators in water.