The interaction of water molecules with purple membrane suspension using 2H double-quantum filter, 1H and 2H diffusion nuclear magnetic resonance

Bacteriorhodopsin is a membrane protein of the purple membrane (PM) of Halobacterium salinarum, which is isolated as sheets of highly organized two-dimensional hexagonal microcrystals and for which water molecules play a crucial role that affects its function as a proton pump. In this paper we used single- and double-quantum 2H NMR as well as 1H and 2H diffusion NMR to characterize the interaction of water molecules with the PM in D2O suspensions. We found that, under the influence of a strong magnetic field on a concentrated PM sample (0.61 mM), the PM sheets affect the entire water population and a residual quadrupolar splitting ((upsi)q ~5.5 Hz, 298 K, at 11.7 T) is observed for the D2O molecules. We found that the residual quadrupolar coupling, the creation time in which a maximal DQF signal was obtained ((tau)max), and the relative intensity of the 2H DQF spectrum of the water molecules in the PM samples (referred to herein as NMR order parameters) are very sensitive to temperature, dilution, and chemical modifications of the PM. In concentrated PM samples in D2O, these NMR parameters seem to reflect the relative organization of the PM. Interestingly, we have observed that some of these parameters are sensitive to the efficiency of the trimer packing, as concluded from the apo-membrane behavior. The data for dionized blue membrane, partially delipidated sample, and detergent-treated PM show that these D2O NMR order parameters, which are magnetic field dependent, are sensitive to the structural integrity of the PM. In addition, we revealed that heating the PM sample inside or outside the NMR magnet has, after cooling, a different effect on the NMR characteristics of the water molecules in the concentrated PM suspensions. The difference in the D2O NMR order parameters for the PM samples, which were heated and cooled in the presence and in the absence of a strong magnetic field, corroborates the conclusions that the above D2O order parameters are indirect reflections of both microscopic and macroscopic order of the PM samples. In addition, 1H NMR diffusion measurements showed that at least three distinct water populations could be identified, based on their diffusion coefficients. These water populations seem to correlate with different water populations previously reported for the PM system.