Collisions between nitrogen-14 and nitrogen-15 spin-labels. 1. Lipid-lipid interactions in model membranes.

A new method is described to evaluate collision frequencies between spin-labeled molecules in membranes. This method utilizes a mixture of 14N and 15N nitroxides. The extent of spin-spin interaction between the two different isotopes can be determined by comparing experimental ESR spectra with simulated spectra. The latter are calculated by using modified Bloch equations and by assuming that the spin exchange process is the dominant broadening mechanism. A more approximate analysis is also described; it is based on the evaluation of the line broadening from the measurement of peak heights. Both analyses show that relatively low concentrations of label can be utilized: 1-5% labeled lipids in a membrane give a broadening sufficient to allow quantitation. Collision frequencies equal to or larger than 0.1 MHz can be evaluated by this method. Because of the limited overlap between 15N and 14N spectra, this method allows the investigation of the collisions between unlike molecules of very different concentration. From the collision frequencies, the topological domain explored by a probe can be inferred as well as its lateral diffusion constant. In this paper, it is shown that in spite of the low order of the chain residues near their methyl terminal, a probe linked at the omega - 2 position of an ionized stearic acid does not interact with a probe positioned near the polar region. Nevertheless the former probe, deeply embedded in the membrane, explores a larger volume. It is also shown that if the nitroxide is attached to a fatty ester, the probe "sees" evenly the whole hydrophobic region of the membrane.