An electron-electron double-resonance study of interactions between [14N]- and [15N]stearic acid spin-label pairs: lateral diffusion and vertical fluctuations in dimyristoylphosphatidylcholine

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Abstract
Vertical fluctuations of the terminal methyl groups of stearic acid acyl chains toward the surface of dimyristoylphosphatidylcholine (DMPC) bilayers were investigated by using spin-label electron-electron double-resonance (ELDOR) methodology. Spin-label pairs consisting of 2 populations of stearic acid spin-labels were employed, each at 0.25 mol % concentration, where the nitroxides of the 1st population were 15N substituted and the nitroxides of the 2nd contained 14N. Various combinations of labels with the nitroxide moieties located at carbons 5, 12 or 16 (C5, C12, C16) were used. ELDOR permits measurement of collision frequencies between the 2 constituents of the pair, for example, between 15N spin-labels at C5 and 14N labels at C16. Intramolecular contributions to the ELDOR effect including N nuclear relaxation are eliminated by the use of spin-label pairs. Above the main phase transiton temperature, bimolecular collisions between C5 and C16 occur with about half the frequency of C16:C16 collisions. Vertical fluctuations are very pronounced. A dependence of these fluctuations on temperature and pH has been observed. Lateral diffusion constants calculated from the bimolecular collision frequencies of C16:C16 pairs are 4.56 .times. 10-8, 5.77 .times. 10-8 and 8.09 .times. 10-8 cm2/s at 27.degree., 37.degree. and 47.degree. C. These values are in good agreement with previous measurements of lipid diffusion in DMPC.

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