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. 2001 Jul 3;98(14):7777-82.
doi: 10.1073/pnas.131023798.

Polarity and Permeation Profiles in Lipid Membranes

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Free PMC article

Polarity and Permeation Profiles in Lipid Membranes

D Marsh. Proc Natl Acad Sci U S A. .
Free PMC article

Abstract

The isotropic (14)N-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 glycerophospholipid 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 + 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, lambda. Integration over this profile yields a corresponding expression for the permeability barrier to polar solutes. For fluid membranes without cholesterol, n(o) approximately 8 and lambda approximately 0.5--1 CH(2) units, and the permeability 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) approximately 9--10, and the total change in polarity is greater than for membranes without cholesterol, increasing the permeability barrier by a factor of 2, whereas the decay length remains similar. The permeation of oxygen into fluid lipid 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(o) approximately 8 and lambda approximately 1 CH(2) 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-chain region of the membrane. These results are relevant not only to the permeation of water and polar solutes into membranes and their permeabilities, but also to depth determinations of site-specifically spin-labeled protein residues by using paramagnetic relaxation agents.

Figures

Figure 1
Figure 1
Polarity profiles of the isotropic 14N-hyperfine coupling constant, aformula image, of n-PCSL spin labels in fluid bilayer membranes of dipalmitoyl phosphatidylcholine (DPPC) (○) and of DPPC plus 50 mol % cholesterol (●). Data are from ref. and additional measurements. Errors in mean ao values are in the range ± 0.05–0.15 G. Lines represent nonlinear least-squares fits with Eq. 2. The fitting parameters and uncertainties are given in Table 1.
Figure 2
Figure 2
Permeation profiles, Π, for oxygen (O2, ●) and chromium oxalate (CrOx, ○) deduced from spin-lattice relaxation enhancements of n-PCSL spin labels in rod outer segment disk membranes (data from ref. 26). For CrOx, the values are scaled by a factor of 3. Solid line for O2 represents a nonlinear least squares fit with an expression equivalent to Eq. 2. Fitting parameters are: no = 7.8 ± 0.2, with the decay constant fixed at λ = 0.78. Solid line for CrOx is a simple exponential decay, starting at n = −9 with a decay constant λ = 4.35 ± 0.24. The spin labels positioned at n = −3.5 and n = −9 represent N-TEMPO-stearamide and N-TEMPO-phosphatidylcholine (TEMPO is 2,2,6,6-tetramethylpiperidine-N-oxyl) , respectively, with the nitroxide in the polar headgroup region of the lipid molecule.
Figure 3
Figure 3
Oxygen accessibilities, Π(O2), of site-directed spin labels attached to the first transmembrane segment of the SKC1 K+-channel. Experimental values (29) are given by ●, as a function of the labeled residue position in systematic cysteine-substitution mutants. Dashed lines give the envelope of the oxygen permeation profile at the position of the lipid-facing residues, according to Eq. 2. Solid lines are the complete oxygen concentration profiles, modulated by the helical residue periodicity of exposure, according to Eq. 6. The two halves of the membrane, residues 22–36 and 36–50, respectively, are fitted separately as described in the text.

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