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- Electron spin resonance study of melanin treated with reduci...Electron spin resonance study of melanin treated with reducing agents
Biophysical Journal, Volume 15, Issue 6, 1 June 1975, Pages 565-572
L. Chauffe, J.J. Windle and M. Friedman
AbstractThe electron spin resonances (ESR) of several native and modified melanins have been determined. Melanins isolated from black wool and synthesized from 3,4-dihydroxyl-L-phenylalanine (L-DOPA) and tyrosine all show similar ESR signals. Modification of the isolated melanins by treatment with reducing agents causes some lightening in color and slight changes in the ESR spectra. Lithium and liquid ammonia (Birch) reduction applied to melanins from wool and L-DOPA gave very different results, as reflected by ESR spectra, but in both cases the changes were much greater than those produced by other treatments. In general, reductive treatments in nonaqueous media in the presence of metals increase the free radical content and line width, whereas treatment in aqueous media resulted in decreased free radical content. These observations are consistent with a melanin pigment which is an irregular polymer and has unpaired electrons localized on different but similar monomer units.
Abstract | PDF (443 kb) - Electron Spin Resonance Studies on MelaninElectron Spin Resonance Studies on Melanin
Biophysical Journal, Volume 4, Issue 6, 1 November 1964, Pages 471-490
M.S. Blois, A.B. Zahlan and J.E. Maling
AbstractElectron spin resonance (e.s.r.) observations of squid melanin have been conducted over the temperature range 500°K to 4.2°K, and the effect of various chemical treatments of the melanin upon the e.s.r. spectrum has been studied. The findings have shown that the paramagnetism of this melanin follows the Curie Law from 500°K to 4.2°K, that the spin signal can be eliminated by the addition of Cu to the melanin, and that the optical and e.s.r. absorptions of melanin are independent since either can be reduced or eliminated without affecting the other. Similar studies on synthetic melanins produced by autoxidation or by enzymatic oxidation of a number of biphenols were carried out. It was found that the e.s.r. signals of these synthetic melanins were strikingly similar (with respect to line width, line shape, and -value) with those of squid melanin. It is concluded that the unpaired electrons observed are associated with trapped free radicals in the melanin polymer, that the biosynthesis of melanin may involve a free radical mechanism, and that these physical data are in accord with the concept of Nicolaus that melanin is a highly irregular, three-dimensional, polymer.
Abstract | PDF (1144 kb) - Chemical and Structural Disorder in Eumelanins: A Possible E...Chemical and Structural Disorder in Eumelanins: A Possible Explanation for Broadband Absorbance
Biophysical Journal, Volume 90, Issue 3, 1 February 2006, Pages 743-752
M. Linh Tran, Ben J. Powell and Paul Meredith
AbstractWe report the results of an experimental and theoretical study of the electronic and structural properties of a key eumelanin precursor—5,6,-dihydroxyindole-2-carboxylic acid (DHICA)—and its dimeric forms. We have used optical spectroscopy to follow the oxidative polymerization of DHICA to eumelanin and observe red shifting and broadening of the absorption spectrum as the reaction proceeds. First principles density functional theory calculations indicate that DHICA oligomers (possible reaction products of oxidative polymerization) have the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital red-shifted gaps with respect to the monomer. Furthermore, different bonding configurations (leading to oligomers with different structures) produce a range of gaps. These experimental and theoretical results lend support to the chemical disorder model where the broadband monotonic absorption characteristic of all melanins is a consequence of the superposition of a large number of nonhomogeneously broadened Gaussian transitions associated with each of the components of a melanin ensemble. These results suggest that the traditional model of eumelanin as an amorphous organic semiconductor is not required to explain its optical properties and should be thoroughly reexamined. These results have significant implications for our understanding of the physics, chemistry, and biological function of these important biological macromolecules. Indeed, one may speculate that the robust functionality of melanins in vitro is a direct consequence of its heterogeneity, i.e., chemical disorder is a “low cost” natural resource in these systems.
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Copyright © 1976 The Biophysical Society. All rights reserved.
Biophysical Journal, Volume 16, Issue 10, 1165-1170, 1 October 1976
doi:10.1016/S0006-3495(76)85765-7
Research Article
Electron-nuclear double resonance in melanins
T. Sarna, C. Mailer, J.S. Hyde, H.M. Swartz and B.M. Hoffman
Abstract
Electron-nuclear double resonance (ENDOR) signals from matrix protons interacting with the stable free radicals of "A"- and "B"-type melanins have been observed as a function of pH. In all samples the single line is similar in width and unusually narrow. The ENDOR reduction varies by more than a factor of 10, indicating a large sensitivity of relaxation properties of melanin to sample type. Signals were observed over a wide range of experimental conditions with good signal-to-noise ratio, establishing feasibility for further more detailed ENDOR studies. Incubation in D2O resulted in little change, indicating that the free radical is well buried or protected. No resolved hyperfine structure was seen, consistent with the generally accepted view that melanin is a heterogeneous polymer.
