Solid state nuclear magnetic resonance of membranes
Abstract
Lineshape analysis of solid state 2H NMR spectra is a very useful tool in the investigation of membranes. Resolution enhancement conferred by numerical deconvolution of powder patterns, which are a superposition of orientation dependent spectra, to spectra characteristic of single alignment greatly simplifies analysis. This is especially true for signals from multiple labelling sites. An innovative method to rapidly deconvolute (depake) spectra governed by axially symmetric second rank tensor interactions possessing a P2(cos &thetas;) dependence upon the angle &thetas; between the symmetry axis and the external magnetic field is developed. By utilizing time and frequency domain weighting functions to facilitate a fast Fourier transformation (FFT) based solution, the new approach differs substantially from previous procedures which are iterative or require matrix inversion and, hence, are slow. Its efficacy is demonstrated with data for model membranes. The properties of polyunsaturated membranes are not well understood. Aqueous multilamellar dispersions of isomeric [2H31] 16:0-α18:3 PC (1-[2H31] palmitoyl-2- cis, cis, cis-octa-9, 12, 15-trienoylphosphatidylcholine) and [2H31] 16:0-γ18:3 PC (1-[2H 31] palmitoyl-2-cis, cis, cis-octa-6, 9, 12-trienoylphosphatidylcholine) are compared by broadline 2H NMR spectroscopy to investigate the importance of double bond location. The α18:3 chain has double bonds at Δ9, 12 and 15 positions while in the γ18:3 chain they are at positions Δ6, 9 and 12. Moment analysis of spectra recorded as a function of temperature reveals that the gel to liquid crystalline transition for [ 2H31] 16:0-α18:3 PC membranes exhibits broad hysteresis characterized by a midpoint temperature of −9°C on heating and −20°C on cooling, while the phase transition of [2H31) 16:0-γ18:3 PC membranes exhibits no hysteresis and is centered on −27°C. In the liquid crystalline state, molecular ordering is 10% and 20% lower within [2H31] 16:0-α18:3 PC than within [ 2H31] 16:0-γ18:3 PC at, respectively, the same temperature and equal reduced temperature. Smoothed order parameter profiles derived from depaked spectra show that a depression of the plateau region of approximately constant order in the upper portion (C2–C7) of the 16:0 sn-1 chain in [2H31] 16:0-α18:3 PC is responsible. In the lower portion (C8–C16) the profiles gradually converge as order decreases towards the terminal methyl. The results emphasize that the position of double bonds must be considered in attempts to explain the biological role of polyunsaturation.
Degree
Ph.D.
Advisors
Wassall, Purdue University.
Subject Area
Molecules|Biophysics
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