Restriction Of Molecular Motion Research Articles

Component analysis of proton NMR spectra of linear polyethylene in the α‐transition temperature range and phase distribution

AbstractBroad‐line 1H NMR spectra of linear polyethylene at temperatures in the α‐transition range can be analyzed in terms of contributions from the crystalline and noncrystalline components provided molecular motion in the crystalline region is adequately considered. The spectrum of solid n‐C32H66 or n‐C44H90 prior to melting is used to take account of the contribution of the crystalline region of the polymer to molecular motions. The temperature dependence of the component distribution in the polymer is briefly discussed for a wide range of temperatures, together with previously reported results at low temperatures. The noncrystalline component is in a rigid glassy state at very low temperatures but with rising temperature it transforms to a mobile glassy state with restricted molecular motion, and transforms partially to the rubbery state at high temperature. The crystalline component remains rigid at low temperature, but some molecular motion is associated with it at higher temperatures in the α‐transition range.

Synthetic Bilayer Membranes with Anionic Head Groups

Abstract Anionic amphiphiles which possess two normal alkyl groups as the hydrophobic residue and sulfonate, phosphate or carboxylate groups as the hydrophilic residue were prepared. Most of these compounds aggregate extensively in water to form vesicles and lamellae which are probably formed by the molecular bilayer. These aggregates are quite similar to the lecithin liposome with respect to the restricted molecular motion and cholesterol binding.

13C NMR studies of lipids in bilayers and membranes

Recent studies of lipid bilayers and membranes by 13C NMR 1,2) have indicated the inherent advantages of this nucleus of a wide range of chemical shifts and relatively sharp resonances in systems of restricted molecular motion. The experiments summarised here demonstrate that information about the molecular motion of lipids in bilayers can be obtained by 13C NMR relaxation measurements, and that the T 1 relaxation times are characteristic of both the structure of the lipid and its steric interactions with other lipid molecules. Spectra can also be obtained from some membranes at natural abundance, or by specific 13C enrichment of membrane phospholipids through biosynthetic incorporation of labelled fatty acids.

Anisotropic and restricted molecular motion in lecithin bilayers

A treatment has been developed which accounts for the prominent features of the PMR spectra and proton relaxation data of unsonicated lecithin bilayers. The spin-lattice relaxation time T 1 and the spin-spin relaxation time T 2 of the protons of a methyl top undergoing anisotropic and restricted reorientation have been calculated employing the theory of Woessner, but modified to include the effects of orientational anisotropies on long time scales. Analysis of the nmr data in terms of this theory permitted determination of the mobility of the lecithin hydrocarbon chains in the bilayer phase. These results indicate that the hydrophobic core of an unsonicated bilayer is more appropriately described as a soft solid rather than as a highly mobile fluid, contrary to the suggestions of recent esr spin label studies.

13 C NMR spectra of lecithin vesicles and erythrocyte membranes.

UNLIKE proton nuclear magnetic resonance (NMR) spectroscopy, 13C NMR has the potential advantages in studies of macromolecular systems of a wider range of chemical shifts (∼200 p.p.m.) and of reduced resonance broadening in structures with restricted molecular motion. Thus we have been able to observe 13C NMR spectra of unsonicated lecithin and erythrocytes, which do not yield high resolution proton spectra. We also report T1 relaxation times for 13C nuclei in the polar headgroup of the lecithin molecule and at both ends of the alkyl chains, in sonicated vesicles in D2O buffer and in deuterochloroform. The results are consistent with increasing chain motion towards the terminal methyl group of the alkyl chains for both systems, in agreement with relaxation data for stearic acid derivatives specifically labelled with 19F nuclei in lecithin vesicles1, and with the results of similar spin label experiments of Hubbell and McConnell2.

Nuclear relaxation in phospholipids and biological membranes.

ATTEMPTS have recently been made to apply high resolution proton magnetic resonance spectrometry to the problem of molecular association in biological membranes and in dispersions of phospholipids in water1–4. On the high resolution scale, line broadening gives the appearance of weak absorption by protons of fatty acid molecules. Because broad lines could result from restricted molecular motion and might indicate that fatty acid chains are immobilized in these systems, a central problem in the interpretation of spectra is the relationship between molecular mobility and linewidth. In steady-state absorption nuclear magnetic resonance (NMR) the apparent transverse relaxation time, T2, is given by the reciprocal of the linewidth. Often the T2 thus obtained is an inverse measure of correlation time and reflects molecular mobility. This interpretation is valid for isotropic solutions of small molecules, where rapid motion produces line narrowing and large values of T2.