Cross-polarization/magic-angle-spinning nuclear magnetic resonance in selectively C-13-labeled synthetic eumelanins

We report a solid-state NMR study of synthetic eumelanins prepared by oxida tion of 5,6-dihydroxyindole (DHI) selectively C-13-labeled at positions 2 o r 3 of the indole ring. The C-13-H-1 couplings have been used to quantify t he carbons by selecting the non-protonated and protonated carbon resonances . By comparing the data of non-labeled melanin to that obtained using [2-C- 13]- and [3-C-13]-enriched DHI, it was possible to clearly demonstrate the high chemical reactivity of position 2 and, to a lesser extent, position 3 of the DHI unit. These two sites together are responsible for three-quarter s of the proton loss during polymerization. The cross-polarization/magic-an gle-spinning spectra likewise point to a partial oxidation of positions 2 a nd 3 to the carboxyl and carbonyl oxidation states during the formation of melanin. Furthermore, it is shown that C-13-C-13 dipolar. interactions in [ 2-C-13]-enriched DHI melanins can be observed using radiofrequency-driven d ipolar recoupling (RFDR) 2D experiments. An upper limit of about 4 Angstrom for the distance between the C-2 carbons is deduced from the RFDR experime nts. This result is in agreement with the basic arrangement of the differen t atoms expected in the DHI melanins. (C) 1999 Elsevier Science B.V. All ri ghts reserved.