Signal intensities in 1H-13C CP and INEPT MAS NMR of liquid crystals

Spectral editing with CP and INEPT in C-13 MAS NMR enables identification of rigid and mobile molecular segments in concentrated assemblies of surfactants, lipids, and/or proteins. In order to get stricter definitions of the terms rigid and mobile, as well as resolving some ambiguities in the interpretation of CP and INEPT data, we have developed a theoretical model for calculating the CP and INEPT intensities as a function of rotational correlation time tau(c) and C-H bond order parameter S-CH, taking the effects of MAS into account. According to the model, the range of tau(c) can at typical experimental settings (5 kHz MAS, 1 ms ramped CP at 80-100 kHz B-1 fields) be divided into four regimes: fast (tau(c) < 1 ns), fast-intermediate (tau(c) approximate to 0.1 mu s), intermediate (tau(c) approximate to 1 mu s), and slow (tau(c) > 0.1 ms). In the fast regime, the CP and INEPT intensities are independent of tau(c), but strongly dependent on vertical bar S-CH vertical bar, with a cross-over from dominating INEPT to dominating CP at vertical bar S-CH vertical bar > 0.1. In the intermediate regime, neither CP nor INEPT yield signal on account of fast T-1 rho and T-2 relaxation. In both the fast-intermediate and slow regimes, there is exclusively CP signal. The theoretical predictions are tested by experiments on the glass-forming surfactant n-octyl-beta-D-maltoside, for which tau(c) can be varied continuously in the nano- to millisecond range by changing the temperature and the hydration level. The atomistic details of the surfactant dynamics are investigated with MD simulations. Based on the theoretical model, we propose a procedure for calculating CP and INEPT intensities directly from MD simulation trajectories. While MD shows that there is a continuous gradient of tau(c) from the surfactant polar headgroup towards the methyl group at the end of the hydrocarbon chain, analysis of the experimental CP and INEPT data indicates that this gradient gets steeper with decreasing temperature and hydration level, eventually spanning four orders of magnitude at completely dry conditions. (C) 2013 Elsevier Inc. All rights reserved.