Gramicidin A is thought to form a dimer channel through which alkali cations and hydrogen ions can passively permeate lipid bilayer membranes. The present work describes four conformational species which have been isolated from a single organic solvent system and individually characterized by circular dichroism, proton nuclear magacetate, and dioxane were Spectrograde (Matheson Coleman and Bell). Ethanol was "Rossville Gold Shield" (Commercial Solvents Corporation); dimethyl sulfoxide was "spectroanalyzed" (Fisher); and the chloroform was analytical grade (Baker).Thin-Layer Chromatography. All silica thin-layer chromatography (tic) plates were from Quantum Industries. The plates were developed in dioxane-water (100:1) (v/v) and visualized with the tryptophan reagent, A(A-dimethylaminobenzaldehyde (Greenstein and Winitz, 1961). Absolute spot mobilities relative to the solvent front were approximately 0.4 for species 4, 0.3 for 3, 0.15 for 2, and 0.04

We have used a combination of a shift reagent and mathematical filtering or presaturation of the extracellular sodium resonance for the quantitative investigation of the intracellular sodium and lithium relaxation times in the perfused frog heart. While the T1 of the intracellular sodium was found to consist of a single-exponential time constant (approximately 23 ms), the T2 was better fit as a double-exponential decay with time constants of approximately 2 and 17 ms. However, the relative amplitudes of the two time constants in the T2 decay were found to be inconsistent with those which would be expected from a homogeneous pool of nuclei undergoing quadrupolar interactions. The relaxation times were not changed by a fivefold increase in the intracellular sodium level (due to perfusion with a ouabain-containing buffer). The T1 and T2 of the intracellular lithium (after perfusion with lithium-containing buffer) were both well fit by single exponentials (700- and 31-ms time constants, respectively).

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