2H-13C correlation solid-state NMR for investigating dynamics and water accessibilities of proteins and carbohydrates

Site-specific determination of molecular motion and water accessibility by indirect detection of H-2 NMR spectra has advantages over dipolar-coupling based techniques due to the large quadrupolar couplings and the ensuing high angular resolution. Recently, a Rotor Echo Short Pulse IRrAdiaTION mediated cross polarization ((CP)-C-RESPIRATION) technique was developed, which allowed efficient transfer of H-2 magnetization to C-13 at moderate H-2 radiofrequency field strengths available on most commercial MAS probes. In this work, we investigate the H-2-C-13 magnetization transfer characteristics of one-bond perdeuterated CD (n) spin systems and two-bond H/D exchanged C-(O)-D and C-(N)-D spin systems in carbohydrates and proteins. Our results show that multi-bond, broadband H-2-C-13 polarization transfer can be achieved using H-2 radiofrequency fields of similar to 50 kHz, relatively short contact times of 1.3-1.7 ms, and with sufficiently high sensitivity to enable 2D H-2-C-13 correlation experiments with undistorted H-2 spectra in the indirect dimension. To demonstrate the utility of this H-2-C-13 technique for studying molecular motion, we show H-2-C-13 correlation spectra of perdeuterated bacterial cellulose, whose surface glucan chains exhibit a motionally averaged C6 H-2 quadrupolar coupling that indicates fast trans-gauche isomerization about the C5-C6 bond. In comparison, the interior chains in the microfibril core are fully immobilized. Application of the H-2-C-13 correlation experiment to H/D exchanged Arabidopsis primary cell walls show that the O-D quadrupolar spectra of the highest polysaccharide peaks can be fit to a two-component model, in which 74% of the spectral intensity, assigned to cellulose, has a near-rigid-limit coupling, while 26% of the intensity, assigned to matrix polysaccharides, has a weakened coupling of 50 kHz. The latter O-D quadrupolar order parameter of 0.22 is significantly smaller than previously reported C-D dipolar order parameters of 0.46-0.55 for pectins, suggesting that additional motions exist at the C-O bonds in the wall polysaccharides. H-2-C-13 polarization transfer profiles are also compared between statistically deuterated and H/D exchanged GB1.