The generation of action potentials in nerve and muscle requires cell membranes with steeply voltage-dependent ionic permeabilities. The voltage-dependent, ion-selective pathways responsible for excitation have been characterized for numerous excitable tissues such as nerve axon, muscle, electric organ, algae and epithelia (Aidley, 1971; Hodgkin, 1964; Cole, 1968). The process of activating ionic pathways by some stimulus, such as a change in membrane potential, is called gating.

The ability of an organism to cope with foreign substances (antigens) depends in part on its capacity to synthesize antibodies (immunoglobulins) of the proper binding specificity to recognize and combine with these antigens. In view of the great variety of possible antigens, antibodies, or more specifically their combining sites, display considerable variation and possess structural properties such as to enable them to bind the antigenic determinants. A vast amount of immunoglobulin sequence data has become available and the three-dimensional structures of a number of immunoglobulin fragments have been elucidated. With these results we can now begin to understand the structural aspects of antibody–antigen reactions. The crystallographic results and the sequence data have been reviewed elsewhere (Capra & Kehoe, 1975; Davies, Padlan & Segal 1975a, b; Gally, 1973; Kabat, 1976; Nisonoff, Hopper & Spring, 1975; Poljak, 1975a, b). Here, an attempt is made to interpret these structural data in terms of the structural evolution of the antibody combining site and the structural basis for the specificity of the binding of antibody to antigen. First, the pertinent crystallographic and sequence data will be presented. Next, the structural studies which reveal the complementarity between antibody and ligand will be described. Then, structural mechanisms by which different combining site structures could be generated will be reviewed.

There is an inherent anisotropy in biological and model membrane systems and this anisotropy has a profound influence on the nuclear magnetic resonance spectra of such systems. For nuclei with a quadrupole moment the quadrupole coupling usually dominates the NMR spectrum, while for nuclei with spin quantum number I = ½ dipolar couplings provides the most important effects. The quadrupole coupling only affects isolated spins and usually gives rise to simple spectra. The dipole interactions on the other hand couple several spins. In a lipid bilayer with a multitude of spin-½ nuclei these couplings can give rise to complicated manybody effects. Thus the interpretation of the NMR spectra of membrane systems poses problems but at the same time there is a lot of information to be gained.