Pii: S0022-2860(98)00904-1

The results of a series of ab initio (3-21G) and semiempirical (PM3) molecular orbital (MO) calculations on neutral, zwitterionic and cationic forms of dopamine [1,2-benzenediol-4(2-aminoethyl)] are reported. In particular, optimised geometries, relative stabilities, dipole moments and electron charge distributions for the relevant conformational states of the studied molecules are presented and the conformational dependence of some relevant structural parameters is used to characterise the most important intramolecular interactions present in the studied conformers. It is shown that all the studied molecules have a considerably high degree of conformational flexibility, and may exist as a mixture of several conformers of similar energies differing by the relative orientation of the aromatic ring with respect to the alkylamine chain or of the hydroxyl groups. For both neutral dopamine and dopamine cation, the conformational ground state corresponds to a form where the meta-hydroxy group has its hydrogen atom directed towards the para-hydroxy group, the aromatic ring and the alkylamine axis are nearly perpendicular and the C–C–C–N axis assumes a gauche geometry, with the amine group in the same side of the meta-hydroxy group. In turn, the zwitterionic form of dopamine is predicted not to correspond to a minimum in the potential energy surface (PES) for the isolated molecule situation. However, in the zwitterion dimmer, the conformation assumed by the individual molecules is predicted to be similar to that previously observed in crystalline dopamine hydrochloride. q 1999 Elsevier Science B.V. All rights reserved.