title: Cross-polarization/magic-angle-spinning nuclear magnetic resonance in selectively 13C-labeled synthetic eumelanins.
abstract: We report a solid-state NMR study of synthetic eumelanins prepared by oxidation of 5,6-dihydroxyindole (DHI) selectively 13C-labeled at positions 2 or 3 of the indole ring. The 13C-1H couplings have been used to quantify the carbons by selecting the non-protonated and protonated carbon resonances. By comparing the data of non-labeled melanin to that obtained using [2-(13)C]- and [3-(13)C]-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-quarters of the proton loss during polymerization. The cross-polarization/magic-angle-spinning spectra likewise point to a partial oxidation of positions 2 and 3 to the carboxyl and carbonyl oxidation states during the formation of melanin. Furthermore, it is shown that 13C-13C dipolar interactions in [2-(13)C]-enriched DHI melanins can be observed using radiofrequency-driven dipolar recoupling (RFDR) 2D experiments. An upper limit of about 4 A for the distance between the C-2 carbons is deduced from the RFDR experiments. This result is in agreement with the basic arrangement of the different atoms expected in the DHI melanins.
authors.affiliation: Institut de Chimie, UMR 7510 CNRS, Bruker S.A., Université Louis Pasteur, Strasbourg, France.
authors.forename: P
authors.initials: P
authors.lastname: Reinheimer
authors.forename: J
authors.initials: J
authors.lastname: Hirschinger
authors.forename: P
authors.initials: P
authors.lastname: Granger
authors.forename: P
authors.initials: P
authors.lastname: Breton
authors.forename: A
authors.initials: A
authors.lastname: Lagrange
authors.forename: P
authors.initials: P
authors.lastname: Gilard
authors.forename: M A
authors.initials: MA
authors.lastname: Lefebvre
authors.forename: N
authors.initials: N
authors.lastname: Goetz
mesh_terms: D002247:Carbon Isotopes; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008543:Melanins / Q000737:chemistry*
chemical_list: D002247:Carbon Isotopes; D008543:Melanins; C041877:eumelanin