The translational diffusion coefficient and the partition coefficient of a spin-labeled solute, di-tbutyl nitroxide, in an aqueous suspension of dipalmitoyl lecithin vesicles have been studied by electron spin resonance spectroscopy. When the lecithin is cooled through its phase transition temperature near 41'C, some solute is "frozen out" of the bilayer, and the standard partial molar enthalpy and entropy of partition go more positive by a factor of 8 and 6, respectively. However, the apparent diffusion constant in the lecithin phase is only slightly smaller than that in water, both above and below the transition temperature. The fraction of bilayer volume within which solute is distributed may increase with temperature, contributing to the positive enthalpy of partition. Comparison of time constants suggests that there is a permeability barrier to this solute in the periphery of the bilayer.A central problem of membrane biology is the understanding of solute permeation through biological membranes and lipid bilayers. The structure and dynamics of the membranes themselves play an important role in permeation, but in this study we concentrate on the behavior of the solute molecules, in particular, on the distribution and translational motions of a relatively small nonelectrolyte within the membrane. Although we have studied only one such system, and this in a preliminary manner, the general features observed may be reflected in other systems as well. Specifically, we have used electron spin resonance (ESR) techniques to determine the partition coefficient (K), self-diffusion coefficient (D), and a lower bound on the inter-phase transit time of the paramagnetic solute di-t-butyl nitroxide (DTBN) in an aqueous suspension of sonicated vesicles of the phospholipid, dipalmitoyl lecithin (DPL).Knowledge of K, D, and Membrane: Water Interfacial Resistances Is Essential for Understanding Permeability. Membrane permeability to solutes is usually described in terms of a permeability coefficient P = (A-1 dn/dt)/Ac, where A-1 dn/dt is the solute's molar flux across unit area of membrane, and Ac is the concentration difference of solute between the solutions on opposite sides of the membrane. P depends upon K (the equilibrium ratio of the solute's concentration in the membrane to its aqueous concentration), D (the solute's diffusion coefficient in the membrane), xo (membrane thickness), and r' and r" (resistances of the two membrane: water interfaces to solute flow). A relation (1) among these variables is: P = [r' + r' + fJo dx/K(x)D(x)]-, where the x-axis is taken perpendicular to the plane of the membrane. K and D are virtually certain to vary with position in a biological membrane or bilayer. Thus, a detailed analysis of permeability requires knowledge of K, D, and r. Few attempts (e.g., refs. 2-5) have been made to determine these quantities experimentally in biological membranes or bilayers.K and D Can Be Measured by Means of ESR Spectroscopy. Previous ESR studies have elucidated various motional characteristic...