CHEMICAL CHARACTERIZATION OF EUMELANINS WITH SPECIAL EMPHASIS ON 5,6-DIHYDROXYINDOLE-2-CARBOXYLIC ACID CONTENT AND MOLECULAR-SIZE

Mammalian melanins exist in two chemically distinct forms: the brown t o black eumelanins and the yellow to reddish pheomelanins. Eumelanins are derived from copolymerization of 5,6-dihydroxyindole (DHI) and 5,6 -dihydroxyindole-2-carboxylic acid (DHICA). Eumelanins can be quantifi ed by HPLC analysis of the oxidation product pyrrole-2,3,5-tricarboxyl ic acid (PTCA) and by our spectrophotometric method (Sp.EM) We also de veloped a spectrophotometric method for assaying the total amount of e u- and pheomelanins by dissolving them in Soluene-350 (TM). In additio n, we previously showed that Sp.EM/TM and PCTA/TM ratios are significa nt parameters in characterizing eumelanins produced in follicular mela nocytes. The objectives of this study were (1) to clarify the signific ance of Sp.EM/TM and PTCA/TM ratios in synthetic eumelanins and (2) to apply these methods to characterizing natural eumelanins with various DHI/DHICA ratios and molecular sizes. The results obtained show that (1) the Sp.EM/TM ratio of synthetic eumelanins increases as polymeriza tion proceeds, (2) the Sp.EM/TM and PTCA/TM ratios in copolymers of DH I and DHICA correlate to the percentage content of DHICA-derived units , and (3) combination of the Sp.EM/TM and PTCA/TM ratios serves to est imate the DHICA content and the degree of polymerization in natural eu melanins. (C) 1997 Academic Press.