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.

Polymerization effects and localized electronic states in condensed-phase eumelanin

“…Actually, polymerization is recognized as a central issue in the study of the electronic structure of melanins to such an extent that recent experimental studies have been carried out with the aim to grow single crystals containing eumelanin monomers by inhibiting the polymerization through the addition of suitable side groups. 6 First-principles density-functional theory ͑DFT͒ calculations of the electronic structure of the melanin monomers 7,8 and model oligomers 9 have been published lately. Even if synthetic methods of preparing isolated monomers of DHI are available, it is not simple to test these calculations because these molecules are highly unstable and spontaneously polymerize in an oxidizing environment.…”

Effect of Dimerization on Vibrational Spectra of Eumelanin Precursors†

“…Using DHICA in such a study is problematic because DHICA is able to bind in the 4, 7 and possibly even the 3 positions, yielding several structurally and possibly optically distinct compounds (9). Therefore, we have synthesized a new compound, N ‐methyl‐5‐hydroxy‐6‐methoxy‐indole (MHMI), which is ideally suited to such studies (22). Functional groups in MHMI have been strategically placed on the indole framework, sterically hindering binding to the 2 and 7 positions and allowing dimerization via 4‐4′ coupling alone.…”

The effect of hydration on the UV absorption coefficient of intact melanosomes

“…In particular, Nighswander-Rempel et al reported absorption data for of N-methyl-5-hydroxy-6methoxyindole (MHMI), a model for DHI. 20 They find that the oscillator strength of MHMI in the UV decreases with a decrease in solvent polarity; decreasing about 35% comparing acetonitrile to benzene. This observation is consistent with our conclusion that dehydration in vacuum would result in a decrease in the absorption coefficient of the melanosome.…”