QUANTITATIVE-ANALYSIS BY C-13 SOLID-STATE NMR OF EUMELANINS

The technique of cross-polarization (CP/MAS) offers many possibilities in the study of insoluble molecules such as the eumelanins. A dipolar dephasing of 43-mus permitted a determination of the resonances due t o quaternary carbons. Moreover, after an extremely short polarization transfer time (35-mus), it is possible to selectively differentiate th e protonated carbons. The spectra obtained by these two methods associ ated with those resulting from a cross-polarization experiment for a d uration of 2-ms permitted a quantitative measurement of the ratio of p rotonated to non-protonated carbons in natural and synthetic melanins. The quantitative aspect of CP/MAS was verified by a comparison of MAS and CP/MAS spectra.