Model-free estimation of the effective correlation time for C-H bond reorientation in amphiphilic bilayers: 1H-13C solid-state NMR and MD simulations

Molecular dynamics (MD) simulations give atomically detailed information on structure and dynamics in amphiphilic bilayer systems on timescales up to about 1 mu s. The reorientational dynamics of the C-H bonds is conventionally verified by measurements of C-13 or H-2 nuclear magnetic resonance (NMR) longitudinal relaxation rates R-1, which are more sensitive to motional processes with correlation times close to the inverse Larmor frequency, typically around 1-10 ns on standard NMR instrumentation, and are thus less sensitive to the 10-1000 ns timescale motion that can be observed in the MD simulations. We propose an experimental procedure for atomically resolved model-free estimation of the C-H bond effective reorientational correlation time tau(e), which includes contributions from the entire range of all-atom MD timescales and that can be calculated directly from the MD trajectories. The approach is based on measurements of C-13 R-1 and R-1 rho relaxation rates, as well as H-1-C-13 dipolar couplings, and is applicable to anisotropic liquid crystalline lipid or surfactant systems using a conventional solid-state NMR spectrometer and samples with natural isotopic composition. The procedure is demonstrated on a fully hydrated lamellar phase of 1-palmitoyl-2-oleoyl-phosphatidylcholine, yielding values of tau(e) from 0.1 ns for the methyl groups in the choline moiety and at the end of the acyl chains to 3 ns for the g(1) methylene group of the glycerol backbone. MD simulations performed with a widely used united-atom force-field reproduce the tau(e)-profile of the major part of the acyl chains but underestimate the dynamics of the glycerol backbone and adjacent molecular segments. The measurement of experimental tau(e)-profiles can be used to study subtle effects on C-H bond reorientational motions in anisotropic liquid crystals, as well as to validate the C-H bond reorientation dynamics predicted in MD simulations of amphiphilic bilayers such as lipid membranes. (C) 2015 AIP Publishing LLC.