Lewis acidity of NO+ and NO2+ as measured by their affinity to selected bases. An ab initio background study of biological NO release

The very fact that a simple molecule such as NO, can play a key bioregulato ry function in a number of physiological responses is simply astonishing in view of the fact that most biologically active molecules are rather comple x. In order to understand better the reactivity of NO and its related speci es, we undertook the study of affinity between NO(+) and NO2(+) and differe nt inorganic as well as organic bases. For the molecules under study and th eir derived ionic species, the geometries were fully optimized at the Hartr ee-Fock level with two basis sets: 3-21G and 6-31 + G(d). Estimation of X() affinities at the two basis set used, showed that in gas phase NH3 is the most basic and H2S is the least basic of the inorganic bases. For all the three bases, protonation showed a much greater exothermicity than the inter action with NO+ or NO2+. Protonation affinities are in the order of 200 kca l/mol, suggesting a strong bond formation for these species. We observed an increase in the basicity when a methyl group replaced one hydrogen, at bot h levels of theory. Taken into consideration all the bases studied, both Br onsted and Lewis acidities show a preference for N over O or S. Inorganic s pecies derived from NO2(+) have stronger bonds than those derived from NO() with the exception of those containing S as an heteroatom. The low affini ty energy for the nitrosylated sulfur derivatives makes these molecules sui table as reservoirs for the nitrosyl group, thus release and captivation of the group is achieved easily. (C) 1999 Elsevier Science B.V. All rights re served.