Lewis acidity of NO~+ and NO_2~+ as measured by their affinity to selected bases. An ab initio background study of biological NO release

摘要

The very fact that a simple molecule such as NO, can play a key bioregulatory function in a number of physiological responses is simply astonishing in view of the fact that most biologically active molecules are rather complex. In order to understand better the reactivity of NO and its related species, we undertook the study of affinity between NO~((+)) and NO_2~((+)) and different inorganic as well as organic bases. For the molecules under study and their derived ionic species, the geometries were fully optimized at the Hartree-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 NH_3 is the most basic and H_2S is the least basic of the inorganic bases. For all the three bases, protonation showed a much greater exothermicity than the interaction with NO~+ or NO_2~+. Protonation affinities are in the order of 200 kcal/mol, suggesting a strong bond formation for these species. We observed an increase in the basicity when a methyl group replaced one hydrogen, at both levels of theory. Taken into consideration all the bases studied, both Bronsted and Lewis acidities show a preference for N over O or S. Inorganic species derived from NO_2~((+)) have stronger bonds than those derived from NO~((+)) with the exception of those containing S as an heteroatom. The low affinity energy for the nitrosylated sulfur derivatives makes these molecules suitable as reservoirs for the nitrosyl group, thus release and captivation of the group is achieved easily.