Kinetics of Melanin Polymerization during Enzymatic and Nonenzymatic Oxidation

Abstract

Melanin is an abundant biopigment in the animal kingdom, but its structure remains poorly understood. This is a substantial impediment to understanding the mechanistic origin of its observed functions. Proposed models of melanin structure include aggregates of both linear and macrocyclic units and noncovalently held monomers. Both models are broadly in agreement with current experimental data. To constrain the structural and kinetic models of melanin, experimental data of high resolution with chemical specificity accompanied by atomistic modeling are required. We have addressed this by obtaining electronic absorption, infrared, and ultraviolet resonance Raman (RR) spectra of melanin at several wavelengths of excitation that are sensitive to small changes in structure. From these experiments, we observed kinetics of the formation of different species en route to melanin polymerization. Exclusive chemical signatures of monomer 3,4-dihydroxyphenylalanine (dopa), intermediate dopachrome (DC), and early-time polymer are established through their vibrational bands at 1292, 1670, and 1616 cm respectively. Direct evidence of reduced heterogeneity of melanin oligomers in tyrosinase-induced formation is provided from experimental measurements of vibrational bandwidths. Models made with density functional theory show that the linear homopolymeric structures of 5,6-dihydroxyindole can account for experimentally observed wavenumbers and broad bandwidth in Raman spectra of dopa-melanin. We capture resonance Raman (RR) signature of DC, the intermediate stabilized by the enzyme tyrosinase, for the first time in an enzyme-assisted melanization reaction using 488 nm excitation wavelength and propose that this wavelength can be used to probe reaction intermediates of melanin formation in solution.

Effects of pH and Oxidants on the First Steps of Polydopamine Formation: A Thermodynamic Approach

“… 48 , 53 , 58 Detailed kinetic analysis of iron- and copper-induced oxidation of dopa and dopamine can be found in the literature. 29 , 42 , 48 , 49 …”

“… 20 Both polydopamine and eumelanin have complex and still debated structures, in which the major structural moieties are assumed to be catecholic or quinonoid 5,6-dihydroxyindole (DHI) units, their derivatives, and oligomers. 11 , 21 − 29 The structural components are bound together via covalent or noncovalent interactions; neither eumelanin nor polydopamine can be considered true polymers but represent a mixture of various oligomeric species with structural and redox disorder. 30 …”

Melanogenesis: A Search for Pheomelanin and Also, What Is Lurking Behind Those Dark Colors?

“…Thus, a vast quantity of literature does exist on the synthesis, characterization and applications of eumelanins derived from L-DOPA, dopamine or other catecholic precurosors. 4,[13][14][15] and references within. Much less literature exists on the synthesis, characterization and potential applications of the pheomelanins.…”

Melanogenesis Using Tyrosinate (Not Tyrosinase)

“…[33] b) Any study of the enzymes involved in the conversion of (1) into MN-like materials should include a careful evaluation of the non-enzymatic contribution of (1) to the rates of the various downstream reactions shown in Scheme 1. [13][14][15] A specific point that could be addressed is whether the non-enzymatic contribution of (1) has any effect on the ratio of DHI to DHICA (see Scheme 1) as this ratio may influence the physical properties of the MN material. [9] c) This report suggests a third, non-enzymatic, mechanism to generate darkly colored pigments from a multitude of catecholic precursors.…”