Spin relaxation measurements using first-harmonic out-of-phase absorption EPR signals

Nondestructive Analysis of Mummification Balms in Ancient Egypt Based on EPR of Vanadyl and Organic Radical Markers of Bitumen

The black matter employed in the funeral context by ancient Egyptians is a complex mixture of plant-based compounds with variable amounts of bitumen. Asphaltene, the most resistant component of bitumen, contains vanadyl porphyrins and carbonaceous radicals, which can be used as paramagnetic probes to investigate embalming materials without sample preparation. Electron paramagnetic resonance (EPR) at the X-band, combining in-phase and out-of-phase detection schemes, provides new information in a nondestructive way about the presence, the origin, and the evolution of bitumen in these complex materials. It is found that the relative EPR intensity of radicals and vanadyl porphyrins is sensitive to the origin of the bitumen. The presence of nonporphyrinic vanadyl complexes in historical samples is likely due to the complexation of VO²⁺ ions by carboxylic functions at the interface between bitumen and other biological components of the embalming matter. The absence of such oxygenated vanadyl complex in natural bitumen and in one case of historical human mummy acquired by a museum in the 19th century reveals a possible, nondocumented, ancient restoration of this mummy by pure bitumen. The linear correlation between in-phase and out-of-phase EPR intensities of radicals and vanadyl porphyrins in balms and in natural bitumen reveals a nanostructuration of radicals and vanadyl porphyrin complexes, which was not affected by the preparation of the balm. This points to the remarkable chemical stability of paramagnetic probes in historical bitumen in ancient Egypt.

Phase diagram of ternary cholesterol/palmitoylsphingomyelin/palmitoyloleoyl-phosphatidylcholine mixtures: spin-label EPR study of lipid-raft formation

For canonical lipid raft mixtures of cholesterol (chol), N-palmitoylsphingomyelin (PSM), and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), electron paramagnetic resonance (EPR) of spin-labeled phospholipids--which is insensitive to domain size--is used to determine the ternary phase diagram at 23°C. No phase boundaries are found for binary POPC/chol mixtures, nor for ternary mixtures with PSM content <24 mol %. EPR lineshapes indicate that conversion from the liquid-disordered (L(α)) to liquid-ordered (L(o)) phase occurs continuously in this region. Two-component EPR spectra and several tie lines attributable to coexistence of gel (L(β)) and fluid phases are found for ternary mixtures with low cholesterol or low POPC content. For PSM/POPC alone, coexistence of L(α) and L(β) phases occurs over the range 50-95.5 mol % PSM. A further tie line is found at 3 mol % chol with endpoints at 50 and ≥77 mol % PSM. For PSM/chol, L(β)-L(o) coexistence occurs over the range 10-38 mol % chol and further tie lines are found at 4.5 and 7 mol % POPC. Two-component EPR spectra indicative of fluid-fluid (L(α)-L(o)) phase separation are found for lipid compositions: 25%<PSM<65%, 5%<chol<30-35%, 65%>POPC>10%, and confirmed by nonlinear EPR. Tie lines are identified in the L(α)-L(o) coexistence region, indicating that the fluid domains are of sufficient size to obey the phase rule. The three-phase triangle is bounded approximately by the compositions 40 and 75 mol % PSM with 10 mol % chol, and 60 mol % PSM with 25 mol % chol. These studies define the compositions of raft-like L(o) phases for a minimal realistic biological lipid mixture.

Spin-Label EPR for Determining Polarity and Proticity in Biomolecular Assemblies: Transmembrane Profiles

Hyperfine couplings and g-values of nitroxyl spin labels are sensitive to polarity and hydrogen bonding in the environment probed. The dependences of these electronic paramagnetic resonance (EPR) properties on environmental dielectric permittivity and proticity are reviewed. Calibrations are given, in terms of the Block-Walker reaction field and local proton donor concentration, for the nitroxides that are commonly used in spin labeling of lipids and proteins. Applications to studies of the transverse polarity profiles in lipid bilayers, which constitute the permeability barrier of biological membranes, are reviewed. Emphasis is given to parallels with the permeation profiles of oxygen and nitric oxide that are determined from spin-label relaxation enhancements by using nonlinear continuous-wave EPR and saturation recovery EPR, and with permeation profiles of D(2)O that are determined by using (2)H electron spin echo envelope modulation spectroscopy.

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

The use of 90 degrees-out-of-phase first-harmonic absorption (V1'-) EPR to resolve the spectra from nitroxide spin labels with differing T1-relaxation times is described. Non-linear V1'-EPR spectra recorded under moderate saturation have sharper lines compared with the in-phase V1-EPR spectra, and amplitudes that preferentially enhance components with longer T1-relaxation. Discrimination between V1'-spectral components can be increased further by means of selective paramagnetic relaxation enhancement agents. Examples are given of biophysical applications to double labelling in single-component membranes and phase separation in two-component membranes, to lipid-peptide complexes, and to binding of spin-labelled reagents. It is concluded that optimal resolution in V1'-EPR spectroscopy is obtained at relatively low Zeeman modulation frequencies (20-30 kHz) and moderate saturation (H1 approximately 0.2-0.3 G).

Determination of T1-spin-lattice relaxation time in a two-level system by continuous wave multiquantum electron paramagnetic resonance spectroscopy in a presence of tetrachromatic microwave irradiation

Applicability of continuous wave multiquantum EPR methods to study relaxation times at X-band is examined. Multiquantum transitions excited in a two-level system by tetrachromatic irradiation are used for these studies. The Bloch equation model is applied to simulate lineshapes of the three quantum transitions as a function of frequency difference between exciting fields. The dependence of multiquantum transition signals on relaxation times and microwave amplitude is shown. On this basis a method of deducing relaxation times from these signals is formulated. The case of a homogeneously and inhomogeneously broadened resonance line is considered. Two experimental methods are used to verify the proposed hypothesis: the X-band continuous wave multiquantum EPR with four frequencies microwave field and saturation recovery EPR. The values of T(1) obtained from CW MQ EPR and SR EPR are compared.

Oxygen permeation profile in lipid membranes: comparison with transmembrane polarity profile

Permeation of oxygen into membranes is relevant not only to physiological function, but also to depth determinations in membranes by site-directed spin labeling. Spin-lattice (T(1)) relaxation enhancements by air or molecular oxygen were determined for phosphatidylcholines spin labeled at positions (n = 4-14, 16) of the sn-2 chain in fluid membranes of dimyristoyl phosphatidylcholine, by using nonlinear continuous-wave electron paramagnetic resonance (EPR). Both progressive saturation and out-of-phase continuous-wave EPR measurements yield similar oxygen permeation profiles. With pure oxygen, the T(2)-relaxation enhancements determined from homogeneous linewidths of the linear EPR spectra are equal to the T(1)-relaxation enhancements determined by nonlinear EPR. This confirms that both relaxation enhancements occur by Heisenberg exchange, which requires direct contact between oxygen and spin label. Oxygen concentrates in the hydrophobic interior of phospholipid bilayer membranes with a sigmoidal permeation profile that is the inverse of the polarity profile established earlier for these spin-labeled lipids. The shape of the oxygen permeation profile in fluid lipid membranes is controlled partly by the penetration of water, via the transmembrane polarity profile. At the protein interface of the KcsA ion channel, the oxygen profile is more diffuse than that in fluid lipid bilayers.

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.

Spin relaxation measurements using first-harmonic out-of-phase absorption EPR signals: rotational motion effects

A recent survey of nonlinear continuous-wave (CW) EPR methods revealed that the first-harmonic absorption EPR signal, detected 90 degrees out of phase with respect to the Zeeman modulation (V(1)(')-EPR), is the most appropriate for determining spin-lattice relaxation enhancements of spin labels (V. A. Livshits, T. Páli, and D. Marsh, 1998, J. Magn. Reson. 134, 113-123). The sensitivity of such V(1)(')-EPR spectra to molecular rotational motion is investigated here by spectral simulations for nitroxyl spin labels, over the entire range of rotational correlation times. Determination of the effective spin-lattice relaxation times is less dependent on rotational mobility than for other nonlinear CW EPR methods, especially at a Zeeman modulation frequency of 25 kHz which is particularly appropriate for spin labels. This relative insensitivity to molecular motion further enhances the usefulness of the V(1)(')-EPR method. Calibrations of the out-of-phase to in-phase spectral intensity (and amplitude) ratios are given as a function of spin-lattice relaxation time, for the full range of spin-label rotational correlation times. Experimental measurements on spin labels in the slow, intermediate, and fast motional regimes of molecular rotation are used to test and validate the method.

Fatty acid binding sites of serum albumin probed by non-linear spin-label EPR

A novel form of non-linear EPR spectroscopy, viz. the first harmonic absorption spectrum recorded in phase quadrature with respect to the Zeeman field modulation, is used here to investigate spin-lattice relaxation enhancements of nitroxide spin labels bound to serum albumin that are induced by spin-spin interactions with aqueous paramagnetic ions. The advantage of this EPR method is that it is directly sensitive to spin-lattice relaxation and affected relatively little by other spectral parameters (Livshits et al., J. Magn. Reson. 133 (1998) 79-91). Relaxation enhancements by ferricyanide of bound fatty acids (n-SASL) spin-labelled at different positions, n, in the chain are compared with those of different maleimide spin label derivatives attached at the single free -SH group, as well as with those of the spin labels free in solution. It was found that: (1) the encounter frequency of ferricyanide with 5-SASL and 12-SASL bound to serum albumin is more than two times less than that with 16-SASL; (2) the accessibility of ferricyanide to 16-SASL is comparable to that of the more immobilised covalently bound spin labels; and (3) the absolute values of the encounter frequencies for the bound spin-labelled fatty acids are approximately a factor of ten smaller than for the corresponding free spin labels, but the latter show a dependence on position of labelling that is similar to the bound labels. A kinetic scheme that is consistent with these relative differences involves rapid reversible transitions between an 'open' and 'closed' state, in which interaction with aqueous paramagnetic agents is possible only in the 'open' state. The equilibrium strongly favours the 'closed' state, which is further enhanced at low temperatures.