The ability of conventional electron correlation (MP2 and QCISD) and density functional theory (B3LYP and B3P86) methods to provide accurate and reliable optimized structures, and homolytic S-N bond dissociation energies (BDEs), for a range of S-nitrosothiols (RSNOs) has been investigated. It is found that, in general, for any given method the 6-311+G(2df,p) or larger basis set must be used to obtain reliable structures. With a suitably large basis set, the different methods generally give optimized structures in close agreement with each other. However, the B3LYP method consistently overestimates the RS-NO bond length. The trends observed are found to be due in part to the fact that the RS-NO bond does not possess considerable double-bond character as previously suggested, but rather is a long single S-N bond, with the -NO moiety possessing considerable multiple-bond character. The B3P86/6-311+G(2df,p) method consistently gives BDEs in best agreement with values obtained with higher accuracy methods, e.g., CBS-Q, while the B3LYP method increasingly underestimates BDEs with increasing RSNO size. In contrast, for all RSNOs, the QCISD method significantly underestimates BDEs by as much as 55 kJ mol -1 . Overall, the B3P86/6-311+G(2df,p) method is found to perform the best of the methods considered for obtaining optimized structures and homolytic S-N BDEs of S-nitrosothiols.

Density functional theory methods have been used to investigate the role and effects of Cu+ binding to the S and N centers of the -SNO functional group within S-nitrosothiols (RSNOs), on the lability of the NO group. The binding of Cu+ to the S center is found to weaken the S-N bond, while the N-O bond is concomitantly strengthened, consistent with the notion that Cu+ binding catalyzes NO radical release. In contrast, however, the binding of Cu+ to the N center is found to dramatically shorten and strengthen the S-N bond with a concomitant lengthening of the N-O bond, suggesting stabilization of the RSNOs against NO release. Upon solvation, complexes with Cu+ bound to the N center are stabilized relative to the corresponding S-bound complexes, though remaining slightly higher in energy. The barriers to interconversion between corresponding isomers were also investigated. Implications for biochemical regulation of NO release from RSNOs are discussed.

The interaction of the nitric oxide ions NO+ and NO- with benzene (C6H6) and the aromatic R-groups of the amino acids phenylalanine (Phe), tyrosine (Tyr), histidine (His), and tryptophan (Trp) have been examined using the DFT method B3LYP and the conventional electron correlation method MP2. In particular, the structures and complexation energies of the resulting half-sandwich Ar...NO+/- and sandwich [Ar...NO...Ar]+/- complexes have been considered. For the Ar...NO+ complexes, the presence of an electron rich heteroatom within or attached to the ring is found to not preclude the cation...pi bound complex from being the most stable. Furthermore, unlike the anionic complexes, the pi...cation...pi ([Ar...NO...Ar]+) complexes do not correspond to a "doubling" of the parent half-sandwich.

The density functional theory (DFT) method B3P86/6-311+G(2df,p) has been employed to investigate the complexes formed upon interaction of Cu(+) with nitrosylated cysteine (CysNO) and its decarboxylated (H(2)NCH(2)CH(2)SNO) and deaminated (HOOCCH(2)CH(2)SNO) derivatives. Optimized structures, relative enthalpies and relative free energies have been calculated and compared. In addition, the effects of binding an H(2)O molecule to the Cu(+) centre in the resulting complexes have also been considered. It is found that the most stable complexes are formed when Cu(+) coordinates to the S-nitrosothiol via S of the SNO group. This results in dramatic lengthenings of the SN bond with concomitant shortening of the NO bond. In contrast, when Cu(+) coordinates via the nitrogen of the SNO group, a shortening of the SN bond with lengthening of the NO bond is observed. These effects are tempered by the electron donating ability of other functional groups also coordinated with the Cu(+) centre in the complexes and on the coordination state of the Cu(+) ion.