The effect of electron correlation on the conformational space of melatonin.

The pineal gland hormone melatonin regulates several physiological processes including circadian rhythm and also alleviates oxidative stress-induced degenerative diseases. In spite of its important biological roles, no high level ab initio conformational study has been conducted to reveal its structural features. In this work, the conformational flexibility of melatonin was investigated using correlated ab initio calculations. Conformers, obtained previously at the Hartree-Fock level (HF/6-31G*), were fully optimized using second order Møller-Plesset perturbation theory applying the frozen core approximation (MP2(FC)/6-31G*). Furthermore, single-point MP4(SDQ,FC)/6-31G*//MP2(FC)/6-31G* computations were performed to investigate the effect of higher order perturbation terms. The HF and MP2 conformational spaces are considerably different: the initial 128 structures converged into 102 different local minima as confirmed by frequency calculations; 28 new minima appeared and 26 previous HF local minima disappeared; no "all-trans" C3 side chain conformations are seen at the MP2(FC) level. The MP2 global minimum conformation is stabilized by an aromatic-side chain interaction.