Through-Bonds and Through-Space Solid-State NMR Correlations at Natural Isotopic Abundance: Signal Assignment and Structural Study of Simvastatin

Abstract

Through-bond and through-space solid-state NMR correlation experiments at natural isotopic abundance (refocused cross-polarization incredible natural abundance double quantum transfer experiment (CP-INADEQUATE), 1H−13C HETCOR, 1H−13C magic-angle spinning J heteronuclear multiple-quantum coherence (MAS-J-HMQC)) are presented on a 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 1H−13C correlation with Lee−Goldburg CP (LG-CP) was used to promote long-range polarization transfer. More than 80 heteronuclear contacts were detected in 2D spectra measured with gradually increasing LG-CP. The 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 distance border between inter- and intramolecular correlations was found by careful analysis of X-ray diffraction data. Intermolecular 1H−13C contacts correspond to a polarization transfer range of 3−5 Å. Short-range intermolecular interactions (3.0−3.3 Å) 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 heteronuclear dipolar couplings. The obtained 1H−13C and 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 and mutual orientation of well-organized molecules within crystal at natural isotopic abundance.