Accessibility of spin-labeled phospholipids in anionic and zwitterionic bilayer membranes to paramagnetic relaxation agents. Continuous wave power saturation EPR studies.

The location of phospholipids, spin-labeled in the headgroup or at various positions of the sn-2 chain, incorporated in bilayer membranes of dimyristoylphosphatidylcholine, dimyristoylphosphatidylglycerol or dioleoylphosphatidylglycerol, has been calibrated in terms of their accessibility to paramagnetic relaxation agents. A power saturation approach has been used to determine the spin-label relaxation times, which in turn is influenced by spin-spin interactions with the different paramagnetic species. The effect of different paramagnetic relaxation agents on the power saturation behaviour of the spin-labeled lipids has been used to determine the relaxation enhancement which is quantified in terms of an accessibility parameter. Molecular oxygen, which dissolves preferentially in the lipid phase and the water-soluble, membrane-impermeant chromium oxalate anion are shown to report reliably on the accessibility of spin-labels located in one of the two phases. On the other hand, an uncharged, polar nickel-iminodiacetic acid complex is shown to enhance relaxation of spin-labels in both phases. These calibrations are essential for the study of the interaction of basic proteins with anionic lipid membranes.