Interplay of Intra- and Intermolecular H-Bonds for the Addition of a Water Molecule to the Neutral and N-Protonated Forms of Noradrenaline

The conformational flexibility of noradrenaline in its N-protonated and neutral forms was considered at the HF/6-31G* level, taking into account the orientation of the two hydroxy groups located on the catechol ring and the interconversion pathways between their stable arrangements. The difference in stability among the various forms of N-protonated noradrenaline was maintained including either more diffuse functions on the heteroatoms or MP2 correlation corrections. The embedding in aqueous solution, in the polarizable continuum model framework, of the conformers kept rigid at their in vacuo geometries favored the T conformers with respect to the G ones at the HF level (for neutral noradrenaline at the MP2 level as well). The addition of a single water molecule to noradrenaline (either N-protonated or not) caused the intramolecular H-bond within the side chain to weaken till formation of one or two intermolecular H-bonds with water. This fact, coupled to the intramolecular H-bond strain relaxation, further stabilized the G1 and T conformers. Continuum solvation of the hydrated clusters of neutral noradrenaline favored significantly the T...water adducts at the HF level, but only slightly at the MP2 level, because of their large energy gap in vacuo with respect to G1...water, even enhanced by MP2 geometry optimizations. The dependence of the potential energy profile for the NH2 group rotation on the catechol ring and side chain OH group orientation was examined in noradrenaline and compared to the trend shown by dopamine.