Narrow transmembrane pores, of clinical and physiological importance, are interesting because they provide a unique opportunity to examine the motions of molecules as they pass through a small space. Gramicidin is a pentadecapeptide which dimerizes to form a simple helical pore about 4 angstroms in diameter (1) having a smooth internal surface lined only with atoms from the polypeptide bakcbone. It usually produces membrane channels having a characteristic conductance but also has been shown to produce a broad spectrum of channels having a lower conductance (2). These are natural variants and are probably due to minor conformational changes in the peptide (appendix A). The simplicity of the gramicidin channel combines with its ability to form a variety of natural conformers to make it an ideal tool for the study of molecular interactions within small pores. We propose experiments using this tool which are particularly designed to examine, for the first time, the mechanism governing the interactions between ions in a pore. These primarily consist of measurements of the current-voltage relationships of single channels and the binding affinity of gramicidin containing vesicles in the presence of water substitutes which are expected to modify ion-ion interaction in specific ways. We also propose to further examine the functional and molecular origin of the natural-variant gramicidin channels. In particular, knowledge of the mechanism underlying previously observed transitions between variant and characteristic states is expected to provide helpful ideas to further the understanding of channel gating. This study is expected to provide experimental and theoretical tools for future analysis of permeation in biological channels.
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