Dynamical and equilibrium properties of membrane constituents and nucleic acids from deuterium NMR spectroscopy

Abstract

The chemically non-perturbing deuterium (²H) spin probe provides a unique tool for investigating equilibrium and dynamical properties of lipids, integral membrane proteins, and nucleic acids in relation to their biological activity. Analysis of ²H nuclear magnetic resonance (NMR) relaxation rates gives information about reorientation rates and mean-squared amplitudes; whereas simulation of the spectral lineshapes obtained within the biological temperature range yields bond angles and degree of ordering. In the first part of the dissertation, the frequency dependent ²H NMR spin-lattice relaxation rates (R₁(Z)) for 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles has been analyzed in combination with the corresponding ¹³C R₁(Z) relaxation rates, which enabled one for the first time to unify the ²H and ¹³C relaxation data for lipids in terms of dynamical models for slow order fluctuations. However, none of the existing models is able to account simultaneously for the ²H R₁(Z) frequency dispersion and the orientation dependent ²H R₁(Z) and quadrupolar order (R1Q relaxation rates of DMPC. A new composite membrane deformation model is proposed which simultaneously describes both the frequency and orientation dependent data. Influence of cholesterol on lipid dynamics is also investigated by analyzing the orientational anisotropy of the ²H R₁(Z) and R₁(Q) relaxation rates for DMPC-d54:cholesterol (1/1). The composite membrane deformation model including variable residual coupling tensor predicts a significant variation in the degree of chain entanglement along the acyl chains as a result of lipid-cholesterol interaction, and a smaller contribution from collective motions indicating an increase in the bilayer rigidity versus pure lipid systems. The second part of the dissertation describes a general method for calculating ²H NMR lineshapes of uniaxially-oriented samples. Several intermediate transformations of the coupling tensor are introduced, describing the position of the bond with respect to the symmetry axis, the distribution of the symmetry axes, and the orientation of the specimen as a whole. Lineshapes are calculated using a Monte Carlo method which allows one to effectively treat complicated geometries. The closed-form expression is also derived. The method has been successfully applied to the ²H NMR spectra of bacteriorhodopsin, and helped to resolve previous controversies between the different interpretations of X-ray diffraction studies versus ²H NMR regarding the conformations of Na-DNA at low humidity.