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 water 1–4 . 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, T 2 , is given by the reciprocal of the linewidth. Often the T 2 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 T 2 .