Two-dimensional ELDOR in the study of model and biological membranes

Recent studies on model and biological membranes by two-dimensional (2-D) electronelectron double resonance (ELDOR) are reviewed and discussed. The studies include (1) the phase behavior of dispersions of phospholipid-cholesterol membrane vesicles; (2) the effect of the ion-channel-forming peptide gramicidin A on the lipid membrane; and (3) the effects of stimulation by antigen of the immunoglobulin E receptors in plasma membrane vesicles upon the lipid structure. In the first studies it is shown that the 2-D ELDOR spectra enable clear distinctions amongst the different phases, and this leads to a reliable temperature and composition-dependent phase diagram. In the second studies it is possible to distinguish bulk and boundary lipids and to describe their different dynamic structures. In the third studies we could distinguish both liquid-ordered and liquid-disordered spectral components, and one finds that the fraction of the latter increases as a result of stimulation. Emphasis is placed on the new "full Sc- method" of processing the 2-D ELDOR data to significantly enhance spectral resolution, which is particularly important in studying the spectra from coexisting phases or components. In the full Sc- method, one utilizes both the real and imaginary parts of the signal, instead of their magnitude; the needed phase corrections are obtained as part of the nonlinear least-squares fitting of the 2-D ELDOR data.