Molecular motion of small nonelectrolyte molecules in lecithin bilayers

Summary We have used magnetic resonance spectroscopy, both ESR and¹³C spin relaxation, to measure translational and rotational mobilities and partition coefficients of small nitroxide solutes in dipalmitoyl lecithin liposomes. Above the bilayer transition temperature,T _c, the bilayer interior is quite fluid, as determined from the solutes' rapid rotational and moderately rapid translational motion; the rotational and translational viscosities within the bilayer are _R <1cP and =6–10cP, respectively. and _R are independent of molecular size for all solutes studied, but all were small compared to the size of the phospholipids. , and probably _R , are relatively independent of temperature aboveT _c, but both increase very sharply as temperature is lowered belowT _c; at 32°C, _R increases to 6cP and is greater than 1000 cP. Anisotropy of rotational motion increases gradually as temperature is lowered toT _c, and changes little belowT _c; anisotropy of translational motion was not investigated.¹³C nuclear spin relaxation measurements indicate that translational motion of nitroxide solutes is more rapid in the center of the bilayer than near the polar interface. It takes at least 100 nsec for a solute molecule to cross the bilayer/water interface. We estimate a lower limit of 2 sec/cm for the interfacial resistance to solute diffusion; this result indicates that interfacial resistance dominates permeation across the membrane. The relative solubility, or partition coefficient, is a strong function of solute structure, and decreases abruptly on cooling through the transition temperature. From the partition coefficient and its temperature dependence we calculate the free energy, enthalpy, and entropy of partition. Effects of cholesterol on partition and diffusion coefficients are compatible with the interpretation that bilayers containing cholesterol consist of two phases.