This work is aimed at careful and accurate studies of the dynamic structure of model membranes using electron-spin relaxation techniques. The spin-relaxation analyses are based on the rigorous theories of Freed and co-workers for motional and slow-tumbling effects, which have previously been extensively applied to (thermotropic) liquid crystals. Our present studies are on well-aligned and thick multilayers of such systems as partially hydrated phospholipid bilayers, which we can prepare virtually defect-free; future work will focus on various (fully-hydrated) lipid phases. We shall determine molecular ordering, fluidity, chain tilt, headgroup cooperativity, and how these properties change at the various phase transitions. Based upon our methods for preparing well-aligned lipid multilayers containing polypeptides, we plan further in-depth studies of the polypeptid-lipid interaction, including the """"""""hardening-effect"""""""" vs. the """"""""disordering-effect"""""""" of the polypeptide on the bilayer structure. We shall extend these studies to protein-lipis interactions in biological membranes, in particular the toad plasma and rod outer segment (ROS) disc membranes containing rhodopsin. Also, we are developing new techniques for electron-spin relaxation experiments on membranes,which are designed to signeificantly improve sprectral resolution. These include a two-dimensional electron-spin-echo technique for the study of oriented lipids. It provides much enhanced sensitivity to molecular dynamics and ordering, and we plan to extensively exploit this new technique. Also, echo modulation patterns will be used to delineate structural features of model membranes. We shall be extending our ESR studies into the high-frequency and high-yield regime with the completion of our new 250 GHz (90kG) Far-Infrared ESR spectrometer. The increased spectral resolution form the high fields will enable us to make accurate measurements of magnetic tensors, to accurately determine orienting potentials and ordering tensors and to more accurately study the lipid-protein interaction. Comparative studies of spin-relaxation over a wide range of frequencies (1GHz, 9GHz, 35 GHz, 250 GHz) should lead to more detiailed information on the dynamic structure of model membranes. As a complement, we shall be using the Cornell synchrotron source to perform X-ray studies on the same oriented samples used in the ESR experiments. Also, our new ESR-Imaging technique will be adapted to enable convenient measurements of lateral diffusion un membranes.
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