Mechanism of Relaxation Enhancement of Spin Labels in Membranes by Paramagnetic Ion Salts: Dependence on 3d and 4f Ions and on the Anions

Abstract

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.

Phase Diagram of Ternary Cholesterol/Palmitoylsphingomyelin/Palmitoyloleoyl-Phosphatidylcholine Mixtures: Spin-Label EPR Study of Lipid-Raft Formation

“…Coexisting phases are expected to be characterized by populations of spin-labeled lipids with different intrinsic spin-lattice relaxation times (T 1,e ) (35), or with different relaxation enhancements induced by paramagnetic relaxants such as oxygen (36) or Ni perchlorate (37,30). Therefore, in addition to normal linear V 1 -EPR spectra, nonlinear out-of-phase V 0 1 -EPR spectra (29) were also studied in the putative coexistence regions.…”

The potential of manganese in construction of electrodes modified with thin liquid films of lipids

“…This could be ascribed, on the one hand, to specific interactions with the lipid headgroups, and on the other hand, to the small Mn 2þ ionic radius. Although the phosphate group is shown to be the main site of ion binding [30][31][32][33][34], some new EPR studies [35] show that, due to transient water-filled defects within the lipid layer, Mn 2þ diffuses even deeper into the acyl chain region. Thus it is not strange that, unlike species which are incapable of penetrating the film (e.g., ascorbate, Fe(CN) 3À=4À 6 , iodide I À [16][17][18][19][20]), manganese oxidation by the GCE is not hampered and can be used for mediation of redox reactions in the surrounding electrolyte.…”

Application of the out-of-phase absorption mode to separating overlapping EPR signals with different T1 values

“…Our studies show that membranes containing paramagnetic ions with long spin-lattice relaxation times (viz., Cu 2+ , Mn 2+ , and Gd 3+ ) give strong in-phase EPR signals in the same region as nitroxide spin labels [21]. In these cases, effective suppression of the signals from the paramagnetic ions can be achieved in the V 0 1 -spectral mode.…”

“…4B and A, respectively, for different microwave field strengths H 1 and different modulation frequencies x m . Ni 2+ ions have long been used as paramagnetic quenchers of aqueous spin labels (see, e.g., [20]), and Ni(ClO 4 ) 2 is a particularly effective relaxant for membrane-embedded spin labels [21]. As seen from spectrum (a) in Figs.…”