Geometric and spectroscopic study of some molecules related to eumelanins.1. Monomers

We have carried out ab initio and semiempirical PM3 (parametric method 3) a nd ZINDO (Zerner's intermediate neglect of differential overlap) calculatio ns on neutral and charged 5,6-indolequinone and its reduced forms semiquino ne and hydroquinone. These molecules are believed to compose the major part of the active material of eumelanin, a biological pigment present in illum inated and nonilluminated areas in living organisms. Our results show that these molecules can behave as electron accepters and that their electronic behavior is consistent with that of the semiconductor models proposed for m elanins. The relationship between electronic behavior and biological functi ons is also addressed.