Complete 1H resonance assignment of β-maltose from 1H–1H DQ-SQ CRAMPS and 1H (DQ-DUMBO)–13CSQ refocused INEPT 2D solid-state NMR spectra and first principles GIPAW calculations

Abstract

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