Crystal Effects on Mesobilirubin: A Combined NMR Spectroscopic and Density Functional Theory Study.

We report solid-state NMR investigations of crystal effects in powdered mesobilirubin-IXα, an open-chain tetrapyrrole that is structurally related to bilirubin-IXα but hydrogenated at the 3- and 18-vinyl groups. 13 C and 15 N cross-polarization magic-angle spinning (CP/MAS) NMR experiments were performed on the compound at natural abundance. To facilitate the spectral analysis, density functional calculations were carried out at the B3LYP/6-311G(d,p) level of theory, using an enneameric cluster to simulate the solid. The 1 H, 13 C and 15 N chemical shift data calculated for the enneamer are in a good agreement with those observed in the experimental spectra, and the relative order of the calculated resonances was thus used to confirm the tentative assignments obtained mainly from the heteronuclear correlation spectra. The observed signal splittings of a small subset of the 13 C resonances in the peripheral regions of the two terminal rings provide evidence for microcrystalline heterogeneity of the powdered compound.