Identifying Guanosine Self Assembly at Natural Isotopic Abundance by High-Resolution 1H and 13C Solid-State NMR Spectroscopy

By means of the 1H chemical shifts and the proton–proton proximities as identified in 1H double-quantum (DQ) combined rotation and multiple-pulse spectroscopy (CRAMPS) solid-state NMR correlation spectra, ribbon-like and quartet-like self-assembly can be identified for guanosine derivatives without isotopic labeling for which it was not possible to obtain single crystals suitable for diffraction. Specifically, characteristic spectral fingerprints are observed for dG(C10)2 and dG(C3)2 derivatives, for which quartet-like and ribbon-like self-assembly has been unambiguously identified by 15N refocused INADEQUATE spectra in a previous study of 15N-labeled derivatives (Pham, T. N.; et al. J. Am. Chem. Soc. 2005, 127, 16018). The NH 1H chemical shift is observed to be higher (13–15 ppm) for ribbon-like self-assembly as compared to 10–11 ppm for a quartet-like arrangement, corresponding to a change from NH···N to NH···O intermolecular hydrogen bonding. The order of the two NH2 1H chemical shifts is also inverted, with the NH2 proton closest in space to the NH proton having a higher or lower 1H chemical shift than that of the other NH2 proton for ribbon-like as opposed to quartet-like self-assembly. For the dG(C3)2 derivative for which a single-crystal diffraction structure is available, the distinct resonances and DQ peaks are assigned by means of gauge-including projector-augmented wave (GIPAW) chemical shift calculations. In addition, 14N–1H correlation spectra obtained at 850 MHz under fast (60 kHz) magic-angle spinning (MAS) confirm the assignment of the NH and NH2 chemical shifts for the dG(C3)2 derivative and allow longer range through-space N···H proximities to be identified, notably to the N7 nitrogens on the opposite hydrogen-bonding face.