Polarity and permeation profiles in lipid membranes

The isotropic N-14-hyperfine coupling constant, a(o)(N), of nitroxide spin labels is dependent on the local environmental polarity. The dependence of a(o)(N) in fluid phospholipid bilayer membranes on the C-atom position, n, of the nitroxide in the sn-2 chain of a spin-labeled diacyl glycerophosphol ipid therefore determines the transmembrane polarity profile. The polarity variation in phospholipid membranes, with and without equimolar cholesterol , is characterized by a sigmoidal, trough-like profile of the form {1 divid ed by exp [(n - n(o))/lambda]}(-1), where n = n(o) is the point of maximum gradient, or polarity midpoint, beyond which the free energy of permeation decreases linearly with n, on a characteristic length-scale, h, Integration over this profile yields a corresponding expression for the permeability b arrier to polar solutes, For fluid membranes without cholesterol, n(o) appr oximate to 8 and lambda approximate to 0.5-1 CH2 units, and the permeabilit y barrier introduces an additional diffusive resistance that is equivalent to increasing the effective membrane thickness by 35-80%, depending on the lipid. For membranes containing equimolar cholesterol, n(o) approximate to 9-10, and the total change in polarity is greater than for membranes withou t cholesterol, increasing the permeability barrier by a factor of 2, wherea s the decay length remains similar, The permeation of oxygen into fluid lip id membranes (determined by spin-label relaxation enhancements) displays a profile similar to that of the transmembrane polarity but of opposite sense . For fluid membranes without cholesterol n, - 8 and A - 1 CH2 units, also for oxygen. The permeation profile for polar paramagnetic ion complexes is closer to a single exponential decay, i.e., n(o) lies outside the acyl-chai n region of the membrane. These results are relevant not only to the permea tion of water and polar solutes into membranes and their permeabilities, bu t also to depth determinations of site-specifically spin-labeled protein re sidues by using paramagnetic relaxation agents.