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 ha ve been performed on spin-labeled lipids in fluid phospholipid bilayer memb ranes to elucidate the mechanisms of relaxation enhancement by different pa ramagnetic ion salts. Such paramagnetic relaxation agents are widely used f or structural EPR studies in biological systems, particularly with membrane s. 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 enh ancement is in the order Mn2+ greater than or equal to Cu2+ > Ni2+ > Co2+ a pproximate to Dy3+. A pronounced dependence of the paramagnetic relaxation enhancement on the anion is found in the order ClO4- > Cl- > SO42-. This is in the order of the octanol partition coefficients multiplied by spin exch ange rate constants that were determined for the different paramagnetic sal ts in methanol, Detailed studies coupled with theoretical estimates reveal that, for the chlorides and perchlorates of Ni2+ land Co2+), the relaxation enhancements are dominated by Heisenberg spin exchange interactions with p aramagnetic ions dissolved in fluid membranes. The dependence on membrane c omposition of the relaxation enhancement by intramembrane Heisenberg exchan ge indicates that the diffusion of the ions within the membrane takes place via water-filled defects. For the corresponding Cu2+ salts, additional rel axation enhancements arise from dipolar interactions with ions within the m embrane. For the case of Mn2+ salts, static dipolar interactions with param agnetic ions in the aqueous phase also make a further appreciable contribut ion to the spin-label relaxation enhancement. On this basis, different para magnetic agents may be chosen to optimize sensitivity to different structur ally correlated interactions. These results therefore will aid further spin -label EPR studies in structural biology. (C) 2001 Academic Press.