ELECTRONIC-PROPERTIES AND REDOX ACTIVITY OF NAPHTHOQUINONES

The o-naphthoquinones beta-lapachone (I), CG 8-935 (II), CG 10-248 (II I) CG 9-442 (IV), mansonone C (VI) mansonone E (VII) and mansonone F ( VIII) catalyze ascorbate, dihydrolipoamide and dithiothreitol oxidatio n by dioxygen, their catalytic action depending on the quinone structu re. Mansonone A (V), and the p-naphthoquinone, alpha-lapachone (IX) an d menadione (X) are much less effective. The oxidative process implies reduction of quinones, production of the semiquinone, transfer of the unpaired electron, to oxygen and superoxide anion production. Number and position of oxygen atoms exert a significant influence on naphthoq uinones electronic structure. The more electrophylic oxygen atoms (as compared with carbon atoms) attract pi electrons from the C=O bond, th us increasing their negative net charge (Q(a)) and decreasing that of the carbonyl carbon atom. The positive partial net charge of the carbo nyl dipole carbon atom contrast with the negative charge of the remain der carbon atoms of the naphthoquinone molecule. The electron density probability (f(o)(N)) in the molecular atomic centers of quinones vari es in accordance with the atomic partial net charge. The aromatic stru cture of the naphthalene ring is essential for the catalytic activity of naphthoquinones since quinone V, a tetrahydronaphthoquinone was sca rcely active as catalyst of substrate oxidation. Substitution of a met hyl group by ethyl or phenyl groups in the o-naphthoquinone molecules scarcely affect the calculated quantum-chemical parameters. Correlatio n of catalytic activity and atomic partial net charge values of quinon es was positive (r greater than or similar to 0,90) for C and O of the 2-carbonyl group. The vicinity of carbonyl groups in o-naphthoquinone s is essential for their redox activity, as indicated by the different effects of o-and p-naphthoquinones. Electronic asymetry and polarity of C=O bond are relevant factors for the oxidizing activity of quinone s.