Dipolar Recoupling in MAS NMR:  A Probe for Segmental Order in Lipid Bilayers

Novel two-dimensional magic-angle spinning (2D MAS) NMR experiments designed to measure the magnitudes and signs of ¹³C−¹H dipolar interactions in fluid phase lipid bilayers are presented. MAS is employed throughout the experiments while dipolar recoupling is achieved (by radio frequency irradiation) during the evolution period. Multiple ¹³C−¹H dipolar couplings are measured for a natural abundance sample of L_α phase dimyristoylphosphatidylcholine (DMPC). The magnitudes of ¹³C−¹H dipolar interactions are determined by fitting numerical simulations of recoupled powder line shapes with experimental data while the signs of these interactions are obtained by monitoring the buildup of antiphase magnetization by ¹³C detection. A comparison of the order parameters obtained by ¹³C−¹H dipolar recoupling with those previously obtained for DMPC by ²H NMR indicates that dipolar recoupling is a viable method for determining segmental order in fluid phase lipid bilayers without recourse to isotopic enrichment. Measurement of the signs of ¹³C−¹H dipolar couplings provides additional structural information that is unavailable through ²H NMR. The results obtained for DMPC suggest that the accuracy of the dipolar recoupling experiments presented in this work is competitive with that of previous techniques which require switched-angle spinning for the measurement of the magnitudes and signs of ¹³C−¹H dipolar interactions in lipid bilayers.