Methods of physical labels--a combined approach to the study of microstructure and dynamics in biological systems

Anisotropic motion effects in CW non-linear EPR spectra: relaxation enhancement of lipid spin labels

Continuous-wave (CW) EPR measurements of enhancements in spin-lattice (T(1)-) relaxation rate find wide application for determining spin-label locations in biological systems. Often, especially in membranes, the spin-label rotational motion is anisotropic and subject to an orientational potential. We investigate here the effects of anisotropic diffusion and ordering on non-linear CW-EPR methods for determining T(1) of nitroxyl spin labels. Spectral simulations are performed for progressive saturation of the conventional in-phase, first-harmonic EPR signal, and for the first-harmonic absorption EPR signals detected 90 degrees -out-of-phase with respect to the Zeeman field modulation. Motional models used are either rapid rotational diffusion, or strong-jump diffusion of unrestricted frequency, within a cone of fixed maximum amplitude. Calculations of the T(1)-sensitive parameters are made for both classes of CW-experiment by using motional parameters (i.e., order parameters and correlation times), intrinsic homogeneous and inhomogeneous linewidth parameters, and spin-Hamiltonian hyperfine- and g-tensors, that are established from simulation of the linear CW-EPR spectra. Experimental examples are given for spin-labelled lipids in membranes.

Fluorescence studies on native and bound to trifluraline soy bean Lb"a" in the enhanced N2 fixation

The differences in the tryptophan (Trp) fluorescence of native (control) Lb"a" and experimental substance isolated from nodules of the Williams' soy beans variety treated with trifluraline at a concentration of 2.1 x 10(-10) M have been studied. A positively charged environment has been proved for the tryptophans of the native Lb"a" and a negative one for the tryptophans of the experimental Lb"a". The difference in the tryptophan emission spectra at lambdaex = 280 and 300 nm may be assigned to conformational alterations occurring in the experimental Lb"a". This is also confirmed by the greater energy transfer from tyrosine to tryptophan in the experimental Lb"a"--30% compared to the 10% in the native Lb"a". The value of the constant of acrylamide quenching (Ksv = 2.77 M(-1)) shows that the tryptophans are buried more deeply in the experimental Lb"a" than in the native Lb"a" (Ksv = 4 M(-1)). They are substantially lower than Ksv of the standard compound N-Ac-Trp-NH2 (16.30 M(-1)). The activation energy (Ea) of the thermal quenching of tryptophan fluorescence is higher for the experimental Lb"a" (37 kJ mol(-1)) as compared to the standard compound N-Ac-Trp-NH2 (24 kJ mol(-1)) and the native Lb "a" (32 kJ mol(-1)). The dissociation constant of the complex of trifluraline with Lb "a" (6.32 x 10(-11) M) has been determined as well as the stoichiometric ratio trifluraline/Lb"a" (1:1). The estimated nitrogenase activity (microM/gfrw h) and the total Lb (mg/gfrw) for trifluraline are higher as compared to those for the control.

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.

Membranotropic properties of the water soluble amino acid and peptide derivatives of fullerene C60

The modifying effects of the products of the equimolar addition Of DL-alanine and DL-alanyl-DL-alanine to fullerene C60 on the structure and permeability of the lipid bilayer of phosphatidylcholine liposomes has been studied using the luminescence probe technique. It is shown that these water soluble amino acid and dipeptide derivatives of fullerene (C60-AD) are quenchers of pyrene fluorescence and erythrosine phosphorescence of in both a water solution and liposomes. To study the permeability of the lipid bilayer a procedure based on the triplet probe technique has been developed. It has been found that the C60-AD derivatives under study are able to localize inside the artificial membrane, to penetrate into the liposomes through the lipid bilayer and to perform activated transmembrane transport of bivalent metal ions.

Flavin-dependent alcohol oxidase from the yeast Pichia pinus. Spatial localization of the coenzyme FAD in the protein structure: hot-tritium bombardment and ESR experiments

The spatial localization of the coenzyme FAD in the quaternary structure of the alcohol oxidase from the yeast Pichia pinus was studied by tritium planigraphy and ESR methods. In the present paper we measured the specific radioactivity of FAD labelled as a part of the alcohol oxidase complex. The specific-radioactivity ratio for two FAD portions (FMN and AMP) was calculated. ESR experiments show 4 A (0.4 nm) to be the depth of immersion of paramagnetic isoalloxazines into alcohol oxidase octamer molecules. It is suggested that FAD molecules are bound to the surface of the octamer, rather than to the subunit interfaces. The orientation of the prosthetic group FAD in the alcohol oxidase protein is discussed.

A collision gradient method to determine the immersion depth of nitroxides in lipid bilayers: application to spin-labeled mutants of bacteriorhodopsin

Ten mutants of bacteriorhodopsin, each containing a single cysteine residue regularly spaced along helix D and facing the lipid bilayer, were derivatized with a nitroxide spin label. Collision rates of the nitroxide with apolar oxygen increased with distance from the membrane/solution interface. Collision rates with polar metal ion complexes decreased over the same distance. Although the collision rates depend on steric constraints imposed by the local protein structure and on the depth in the membrane, the ratio of the collision rate of oxygen to those of a polar metal ion complex is independent of structural features of the protein. The logarithm of the ratio is a linear function of depth within the membrane. Calibration of this ratio parameter with spin-labeled phospholipids allows localization of the individual nitroxides, and hence the bacteriorhodopsin molecule, relative to the plane of the phosphate groups of the bilayer. The spacing between residues is consistent with the pitch of an alpha-helix. These results provide a general strategy for determining the immersion depth of nitroxides in bilayers.

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.

Characterization of spin-labelled fatty acids and hematoporphyrin binding sites interactions in serum albumin

Electron paramagnetic resonance (EPR) was used to investigate the spin-labelled fatty acid (SLFA) binding equilibrium to human (HSA) and bovine (BSA) serum albumin. The number of 5-doxyl stearate (5-DS) and 16-doxyl stearate (16-DS) binding sites on HSA and BSA were found to be equal, while the association constants, KA values (especially those of the primary binding site) were different. The applied EPR spectra analysis permitting a quantitative distinguishing between slow macromolecular rotation (pi c) and fast anisotropic motion (steric restriction, S) of bound SLFA, allowed SLFA oxazolidinyl ring mobility to be estimated. The 5-DS nitroxide radical is completely immobilized within the HSA protein matrix (S approximately 1.0, pi c approximately 56 +/- 1 ns). The 5-DS when bound to BSA exhibited the presence of more extensive fluctuations (lower S and pi c values) and its immersion depth with respect to BSA surface was calculated to be 4 +/- 2 A. The 16-DS oxazolidinyl radical bound to HSA was found to undergo moderated fluctuations (both S and pi c are smaller with respect to 5-DS) and it is buried deeper within the protein core (rimm = 10 +/- 2 A with respect to BSA surface). The tetrapyrrole ligands hematoporphyrin (Hp) and hematoporphyrin derivative (HpD) were found to induce well detectable changes in the SLFA binding patterns to serum albumin. The action mode was determined to be different for 16-DS (primary) and 5-DS (secondary) serum albumin binding sites: (i) 5-DS is extruded from several binding sites accompanied by an increase in KA in the remaining ones; (ii) simultaneous binding of 16-DS and Hp consists of cooperative and non-cooperative phases (both the number of the independent sites and the parameter of cooperativity, alpha, being dependent on Hp/HSA ratio); (iii) in principal the mobilities of 5-DS and 16-DS bound to HSA are changed, depending on the porphyrin/HSA ratio; and (iv) the effective immersion depth of the paramagnetic centres with respect to the protein surface is increased when Hp is present as a second ligand (rimm = 7 +/- 2 and 16 +/- 2 A for 5-DS and 16-DS, respectively).