Complete H-1 resonance assignment of beta-maltose from H-1-H-1 DQ-SQ CRAMPS and H-1 (DQ-DUMBO)-C-13 SQ refocused INEPT 2D solid-state NMR spectra and first principles GIPAW calculations
A disaccharide is a challenging case for high-resolution H-1 solid-state NMR because of the 24 distinct protons (14 aliphatic and 10 OH) having H-1 chemical shifts that all fall within a narrow range of approximately 3 to 7 ppm. High-resolution H-1 (500 MHz) double-quantum (DQ) combined rotation and multiple pulse sequence (CRAMPS) solid-state NMR spectra of beta-maltose monohydrate are presented. H-1-H-1 DQ-SQ CRAMPS spectra are presented together with H-1 (DQ)-C-13 correlation spectra obtained with a new pulse sequence that correlates a high-resolution H-1 DQ dimension with a C-13 single quantum (SQ) dimension using the refocused INEPT pulse-sequence element to transfer magnetization via one-bond C-13-H-1 J couplings. Compared to the observation of only a single broad peak in a H-1 DQ spectrum recorded at 30 kHz magic-angle spinning (MAS), the use of DUMBO H-1 homonuclear decoupling in the H-1 DQ CRAMPS experiment allows the resolution of distinct DQ correlation peaks which, in combination with first-principles chemical shift calculations based on the GIPAW (Gauge Including Projector Augmented Waves) plane-wave pseudopotential approach, enables the assignment of the H-1 resonances to the 24 distinct protons. We believe this to be the first experimental solid-state NMR determination of the hydroxyl OH H-1 chemical shifts for a simple sugar. Variable-temperature H-1-H-1 DQ CRAMPS spectra reveal small increases in the H-1 chemical shifts of the OH resonances upon decreasing the temperature from 348 K to 248 K.