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

Through-bond and through-space solid-state NMR correlation experiments at natural isotopic abundance (refocused cross-polarization incredible natural abundance double quantum transfer experiment (CPINADEQUATE),H-1-C-13 HETCOR, H-1-C-13 magic-angle spinning J heteronuclear multiple-quantum coherence (MAS-J-HMQC)) are presented on a moderately sized molecule of simavastatin (C25H38O5). Refocused C-13-C-13 CP-INADEQUATE with an optimized performance of high-power decoupling provides full correlation spectrum within a day of acquisition. Complete unambiguous H-1 and C-13 signal assignment was achieved by H-1-C-13 MAS-J-HMQC and dipolar heteronuclear correlation spectroscopy. Dipolar H-1-C-13 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 H-1-H-1 polarization transfer was observed applying standard Hartrnann-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 H-1-C-13 contacts correspond to a polarization transfer range of 3-5 Angstrom. Short-range intermolecular interactions (3.0-3.3 Angstrom) 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 H-1-C-13 and H-1-H-1 contacts used in conjunction with C-13 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.