Signal intensities in 1H-13C CP and INEPT MAS NMR of liquid crystals
Available online 14 March 2013
Publication year: 2013
Source:Journal of Magnetic Resonance
Spectral editing with CP and INEPT in 13C 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 ?c and C-H bond order parameter SCH, taking the effects of MAS into account. According to the model, the range of ?c can at typical experimental settings (5 kHz MAS, 1 ms ramped CP at 80-100 kHz B1 fields) be divided into four regimes: fast (?c < 1 ns), fast-intermediate (?c ? 0.1 ?s), intermediate (?c ? 1 ?s), and slow (?c > 0.1 ms). In the fast regime, the CP and INEPT intensities are independent of ?c, but strongly dependent on |SCH|, with a cross-over from dominating INEPT to dominating CP at |SCH|>0.1. In the intermediate regime, neither CP nor INEPT yield signal on account of fast T1? and T2 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-?-d-maltoside, for which ?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 ?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.
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