Mechanism of relaxation enhancement of spin labels in membranes by paramagnetic ion salts: dependence on 3d and 4f ions and on the anions.

Progressive saturation EPR measurements and EPR linewidth determinations have been performed on spin-labeled lipids in fluid phospholipid bilayer membranes to elucidate the mechanisms of relaxation enhancement by different paramagnetic ion salts. Such paramagnetic relaxation agents are widely used for structural EPR studies in biological systems, particularly with membranes. Metal ions of the 3d and 4f series were used as their chloride, sulfate, and perchlorate salts. For a given anion, the efficiency of relaxation enhancement is in the order Mn(2+) > or = Cu(2+) > Ni(2+) > Co(2+) approximately Dy(3+). A pronounced dependence of the paramagnetic relaxation enhancement on the anion is found in the order ClO(-)(4) > Cl(-) > SO(2-)(4). This is in the order of the octanol partition coefficients multiplied by spin exchange rate constants that were determined for the different paramagnetic salts in methanol. Detailed studies coupled with theoretical estimates reveal that, for the chlorides and perchlorates of Ni(2+) (and Co(2+)), the relaxation enhancements are dominated by Heisenberg spin exchange interactions with paramagnetic ions dissolved in fluid membranes. The dependence on membrane composition of the relaxation enhancement by intramembrane Heisenberg exchange indicates that the diffusion of the ions within the membrane takes place via water-filled defects. For the corresponding Cu(2+) salts, additional relaxation enhancements arise from dipolar interactions with ions within the membrane. For the case of Mn(2+) salts, static dipolar interactions with paramagnetic ions in the aqueous phase also make a further appreciable contribution to the spin-label relaxation enhancement. On this basis, different paramagnetic agents may be chosen to optimize sensitivity to different structurally correlated interactions. These results therefore will aid further spin-label EPR studies in structural biology.