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Effect of Dimerization on Vibrational Spectra of Eumelanin Precursors †
Author(s) -
NighswanderRempel Stephen P.,
Olsen Seth,
Mahadevan Indumathy B.,
Netchev George,
Wilson Brian C.,
Smith Sean C.,
RubinszteinDunlop Halina,
Meredith Paul
Publication year - 2008
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.2007.00273.x
Subject(s) - indole test , chemistry , dimer , pyrrole , density functional theory , raman spectroscopy , monomer , spectral line , benzene , ring (chemistry) , computational chemistry , planarity testing , molecular vibration , stereochemistry , molecule , photochemistry , crystallography , polymer , organic chemistry , physics , astronomy , optics
We have synthesized a compound ideally suited to the study of structure‐function relationships in eumelanin synthesis. N ‐methyl‐5‐hydroxy‐6‐methoxy‐indole (MHMI) has key functional groups strategically placed on the indole framework to hinder binding in the 2, 5, 6 and 7 positions. Thus, the dimer bound exclusively in the 4‐4′ positions was isolated and characterized. In order to study the difference in vibrational structure between the MHMI monomer and dimer, Raman spectra were acquired of both compounds, as well as indole, indole‐2‐carboxylic acid and 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA). Peaks were assigned to particular vibrational modes using B3LYP density functional theory calculations, and experimental and theoretical spectra displayed good agreement. Addition of functional groups to either benzene or pyrrole rings in the indole framework impacted vibrational spectra attributed to vibrations in either ring, and in some cases, peaks appearing unchanged between two compounds corresponded to different contributing vibrations. Dimerization resulted in an expected increase in the number of vibrational modes, but not a significant increase in the number of apparent peaks, as several modes frequently contributed to an individual observed peak. Comparison of spectral features of the monomer and dimer provides insight into eumelanin photochemistry, but final conclusions depend on the planarity of oligomeric structure in vivo .