New biophysical techniques and their application to the study of membranes.

Our present-day views of the structure and dynamics of biological membranes have evolved through the application of biophysical methods. Recent advances in the correlation of function with biomembrane structure have followed closely upon new developments in methodology and instrumentation, particularly in the field of spectroscopy. While much progress has been made in the study of lipid dynamics, little information is available on the structural dynamics of membrane proteins. The limits of the advances made in this field can be illustrated with the proteins involved in ion transport. The intramembraneous structure of the ionophore gramicidin A, a polypeptide only 15 amino acids in length, has been the subject of considerable debate for many years (Urry et al., 1971; Veatch et al., 1974; Lotz et al., 1976; Nabedryk et al., 1982; Wallace, 1984). Elucidation of the molecular dynamics of ion transport by gramicidin, which is perhaps the simplest model for intramembraneous proteins, is a logical first step in determining the mechanisms by which ions negotiate the dielectric barrier of biological membranes. As yet, the mechanistic proposals for gramicidin-mediated transport are largely speculative (Mackay et al., 1984). The complexities exhibited by gramicidin-mediated transport are minimal, however, when contrasted with the structurally complex proteins involved in active transport of ions and in the formation of gated ion channels. To these considerations we need to add that membrane protein structures may undergo conformational cfianges with variations in pH, temperature, the concentration of metal ions and 'trigger' molecules and, in some cases, under the influence of light. Recently, new techniques have been introduced into membrane biochemistry which may reveal the