Through-bonds and through-space solid-state NMR correlations at natural isotopic abundance: Signal assignment and structural study of Simvastatin

J. Brus^1*, A. Jegorov²,

¹⁾ Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic,
²⁾ IVAX Pharmaceutical sro, Research Unit, Branisovska 31, 370 05 Ceske Budejovice, Czech Republic.

J. Phys. Chem. A, 108(18); 3955, (2004).

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Abstract

Through-bond and through-space solid-state NMR correlation experiments at natural isotopic abundance (refocused CP-INADEQUATE, 1H-13C HETCOR, 1H-13C MAS-J-HMQC) are presented on moderately sized molecule of simavastatin (C25H38O5). Refocused 13C-13C CP-INADEQUATE with an optimized performance of high-power decoupling provides full correlation spectrum within a day of acquisition. Complete unambiguous 1H and 13C signal assignment was achieved by 1H-13C MAS-J-HMQC and dipolar heteronuclear correlation spectroscopy. Dipolar correlation 1H-13C HETCOR with Lee-Goldburg cross-polarization (LG-CP) was used to promote long-range heteronuclear polarization transfer. More than 80 heteronuclear contacts were detected in 2D spectra measured with gradually increasing LG-CP. Majority of these interactions reflects polarization transfer between neighboring structural units. Approximately 30 cross-peaks correspond to desired long-range correlation and 20 of them can be unambiguously used to derive interatomic distances. Formation of additional coherences indicating 1H-1H polarization transfer was observed applying standard Hartmann-Hahn on-resonance CP as a mixing period in 2D HETCOR. No sharp border in distance between inter- and intramolecular correlations was found by careful analysis of x-ray diffraction data. Intermolecular 1H-13C contacts correspond to polarization transfer range of ca. 3.0-5.1 ?. Minority of the detected intermolecular interactions was identified by comparison with corresponding intramolecular contacts, 1H-13C distances (> 5 ?) being beyond the detection limit. Short-range intermolecular interactions (3.0 -3.3 ?) are indicated by NMR aggregation shifts. In many cases correlation signals must be considered as a contribution of both inter- and intramolecular polarization transfer events. High selectivity and good resolution of 2D LG-CP HETCOR restricts the size of the mutually interacting spin system reflected by a single cross-peak. As the number of protons interacting with one carbon does not usually exceed 3-4, the observed dipolar oscillations of correlation signals can be analyzed with respect to limited set of heteronuclear dipolar couplings. The obtained heteronuclear 1H-13C and homonuclear 1H-1H contacts used in conjunction with 13C and 1H NMR aggregation shifts thus appear to be a practical and efficient tool to determine conformation as well as mutual orientation of well-organized molecules within crystal at natural isotopic abundance.