Solvochromic Effects in Model Eumelanin Compounds†

Abstract

We have created an indolic compound which is ideally suited to the study of the relationship between structure and function in eumelanin formation. N-methyl-5-hydroxy-6-methoxyindole (MHMI) is stable in solid and liquid states, highly soluble in a variety of solvents and forms a dimer only through the 4-4' positions. The limited binding possibilities are due to functional groups strategically placed to inhibit chemical interactions through the 2 and 7 positions. It forms a crystal structure with a remarkable packing arrangement, with four monomers grouped in parallel pairs spaced 3.5 A apart within each unit cell. Optical spectra reveal a multi-peaked absorbance profile similar to 5,6-dihydroxyindole (DHI) and N-acetyl-tryptophanamide (NATA), and strong fluorescence emission with radiative quantum yields of 29% and 33% in benzene and acetonitrile, respectively. The quantum yield is similar to that of 5,6-dihydroxyindole-2-carboxylic acid (DHICA) and shows that solvent aromaticity by itself does not affect the yield. Solution in chloroform results in an almost complete quenching of the fluorescence but an increase in emission is observed with photoactivation. Crystallographic results shown here suggest new structural possibilities for eumelanin and the controlled binding possibilities make this an excellent model for monitoring changes in function with increasing oligomer size in eumelanin formation.