Changes in the Paramagnetic Centres in Irradiated and Heated Dental Enamel Studied Using Electron Paramagnetic Resonance

The EPR signals in dental enamel produced by radiation and by heat were studied. The inherent background signal at g = 2.005, and a radiation-produced signal at g = 2.002 have different saturation behaviour with microwave power, and this affords a method of signal optimization. Heating enamel at temperatures from 100 degrees C to 450 degrees C produces a range of radical species from g = 2.002 to g = 2.005, which have been characterized by their g-values, line widths and saturation behaviour. Standard dental drilling produces a range of radicals which appear to be similar to those produced by heat.

The solution conformation of triarylmethyl radicals

Hyperfine coupling tensors to 1H, 2H, and natural abundance 13C were measured using X-band pulsed electron nuclear double resonance (ENDOR) spectroscopy for two triarylmethyl (trityl) radicals used in electron paramagnetic resonance imaging and oximetry: methyl tris(8-carboxy-2,2,6,6-tetramethyl-benzo[1,2d:4,5-d']bis(1,3)dithiol-4-yl) and methyl tris(8-carboxy-2,2,6,6-tetramethyl(-d3)-benzo[1,2d:4,5-d']bis(1,3)dithiol-4-yl). Quantum chemical calculations using density functional theory predict a structure that reproduces the experimentally determined hyperfine tensors. The radicals are propeller-shaped with the three aryl rings nearly mutually orthogonal. The central carbon atom carrying most of the unpaired electron spin density is surrounded by the sulfur atoms in the radical and is completely shielded from solvent. This structure explains features of the electron spin relaxation of these radicals and suggests ways in which the radicals can be chemically modified to improve their characteristics for imaging and oximetry.

Electron spin relaxation of triarylmethyl radicals in fluid solution

Electron spin relaxation times of a Nycomed triarylmethyl radical (sym-trityl) in water, 1:1 water:glycerol, and 1:9 water:glycerol were measured at L-band, S-band, and X-band by pulsed EPR methods. In H(2)O solution, T(1) is 17+/-1 micros at X-band at ambient temperature, is nearly independent of microwave frequency, and exhibits little dependence on viscosity. The temperature dependence of T(1) in 1:1 water:glycerol is characteristic of domination by a Raman process between 20 and 80 K. The increased spin-lattice relaxation rates at higher temperatures, including room temperature, are attributed to a local vibrational mode that modulates spin-orbit coupling. In H(2)O solution, T(2) is 11+/-1 micros at X-band, increasing to 13+/-1 micros at L-band. For more viscous solvent mixtures, T(2) is much shorter than T(1) and weakly frequency dependent, which indicates that incomplete motional averaging of hyperfine anisotropy makes a significant contribution to T(2). In water and 1:1 water:glycerol solutions continuous wave EPR linewidths are not relaxation determined, but become relaxation determined in the higher viscosity 1:9 water:glycerol solutions. The Lorentzian component of the 250-MHz linewidths as a function of viscosity is in good agreement with T(2)-determined contributions to the linewidths at higher frequencies.

Linewidth analysis of spin labels in liquids. II. Experimental

This work demonstrates that homogeneous linewidths can be extracted from continuous wave electron paramagnetic resonance spectra and that they quantitatively agree with the predictions of existing relaxation theory. We suggest that relaxation theory can be used to predict experimental lineshapes provided that the simulations properly include sources of broadening. We have found that the rotational correlation times for spin labels in different percentages of glycerol/water mixtures are best modeled by a power law treatment for the viscosity, similar to that for translational diffusion. The translational diffusion coefficients themselves also have a power law dependence on the viscosity for glycerol/water mixtures. The linewidths were linearly dependent upon both the oxygen and the spin label concentration. The hyperfine splittings of all nuclei were observed to decrease linearly with increasing spin label concentration, completely at odds with existing theory which predicts a quadratic dependence upon concentration. The linear dependence was independent of hyperfine splitting until the magnitude of the hyperfine splitting was less than the homogeneous linewidth.

Linewidth analysis of spin labels in liquids. I. Theory and data analysis

We present a method of simulating the EPR spectra of spin labels in liquids using direct convolution of hyperfine splitting with Lorentzian linewidths. The aim is to simulate the experimental lineshape by considering all spectrometer characteristics as well as inhomogeneous and homogeneous linewidth effects. A major advance in this method is the correction for the broadening produced by Zeeman modulation commonly used to obtain EPR signals; this allows experimenters much more freedom to optimize their experimental conditions for the best signal-to-noise ratio. Microwave power broadening (saturation) effects on the EPR lines are significant even at very low observer levels. Successful simulation requires that all contributions from unresolved hyperfine splittings be explicitly included. Inhomogeneous broadening is dealt with by including all spins that interact with the electron (as a set of superhyperfine interactions); there is no "effective Gaussian" to substitute for the correct superhyperfine interactions. The effects of spin exchange on the linewidth and lineshape can be observed and must be taken into account in order to extract the fundamental linewidths.

Site-specific dynamics in DNA: experiments

This chapter reviews the dynamics information obtained from experimental magnetic resonance studies of site-specifically labeled duplex DNA. A previous review (43) discusses the dynamics of duplex DNA; it develops a theory that shows how magnetic resonance experiments are used to detect those dynamics. The methods for obtaining information about dynamics as well as a summary of what is now known about the site-specific dynamics of DNA are presented. This review contains two methods sections which present results using electron paramagnetic resonance and nuclear magnetic resonance active probes.

Molecular dynamics in liquids: spin-lattice relaxation of nitroxide spin labels

Time domain pulsed saturation recovery and electron-electron double resonance spectroscopies were used to measure the spin-lattice relaxation rates of the electron and the nitrogen nucleus in nitroxide spin labels in liquids. The rotational correlation time range covered is from picoseconds to milliseconds. These rates are quantitatively explained by isotropic rotational Brownian dynamics, which modulate the interactions between the electron spin and the molecular angular momentum; the nitrogen and electron spins; and the solvent protons with both the electron and the nitrogen spins. This solves a 20-year-old problem that has limited scientific applications of nitroxides.

Using nitroxide spin labels. How to obtain T1e from continuous wave electron paramagnetic resonance spectra at all rotational rates

Historically, the continuous wave electron paramagnetic resonance (CW-EPR) progressive saturation method has been used to obtain information on the spin-lattice relaxation time (T1e) and those processes, such as motion and spin exchange, that occur on a competitive timescale. For example, qualitative information on local dynamics and solvent accessibility of proteins and nucleic acids has been obtained by this method. However, making quantitative estimates of T1e from CW-EPR spectra have been frustrated by a lack of understanding of the role of T1e (and T2e) in the slow-motion regime. Theoretical simulation of the CW-EPR lineshapes in the slow-motion region under increasing power levels has been used in this work to test whether the saturation technique can produce quantitative estimates of the spin-lattice relaxation rates. A method is presented by which the correct T1e may be extracted from an analysis of the power-saturation rollover curve, regardless of the amount of inhomogeneous broadening or the rates of molecular reorientation. The range of motional correlation times from 10 to 200 ns should be optimal for extracting quantitative estimates of T1e values in spin-labeled biomolecules. The progressive-saturation rollover curve method should find wide application in those areas of biophysics where information on molecular interactions and solvent exposure as well as molecular reorientation rates are desired.

Stimulation of porphyrinogen oxidation by mercuric ion. I. Evidence of free radical formation in the presence of thiols and hydrogen peroxide

The etiology of mercury-induced porphyrinuria was investigated by testing the hypothesis that mercuric ions (Hg2+) promote free radical-mediated oxidation of reduced porphyrins (porphyrinogens) by compromising the antioxidant potential of endogenous thiols, particularly GSH. Studies in vitro demonstrated that porphyrinogens (uroporphyrinogen and coproporphyrinogen) readily undergo H2O2-dependent oxidization in the presence of Fe3(+)-EDTA and that this action is attenuated by GSH at biologically relevant concentrations (0.5-10 mM). At low concentrations, Hg2+ complexes with GSH in a 1:2 molar ratio to decrease the antioxidant effect of GSH. However, at Hg2+ concentrations approaching saturation-complexation with available GSH, stimulation of porphyrinogen oxidation to 2 to 3 times that mediated by the H2O2/Fe3(+)-dependent system alone is observed. Stimulation of porphyrinogen oxidation by Hg2+ plus GSH increases in a dose-related manner with the concentration of H2O2 in the reaction mixture but is independent of the presence of iron. No porphyrinogen oxidation is observed in reaction mixtures containing H2O2 and either Hg2+ or GSH alone or when Hg+ is substituted for Hg2+. Studies with reactive oxidant scavengers and ESR spectroscopy suggest the participation of free radical species in Hg:GSH-mediated porphyrinogen oxidation. A mechanism involving ligand exchange between Hg2+ and GSH, which leads to formation of GS radicals and subsequent propagation of reactive oxygen-based radical species, is proposed. These studies support the view that Hg2+ both compromises the antioxidant potential of GSH and promotes formation of reactive species via thiol complexation. These findings suggest a mechanistic basis underlying the porphyrinogenic as well as tissue-damaging properties of mercuric ions.

Electron-nuclear double resonance in melanins

Electron-nuclear double resonance (ENDOR) signals from matrix protons interacting with the stable free radicals of "A"- and "B"-type melanins have been observed as a function of pH. In all samples the single line is similar in width and unusually narrow. The ENDOR reduction varies by more than a factor of 10, indicating a large sensitivity of relaxation properties of melanin to sample type. Signals were observed over a wide range of experimental conditions with good signal-to-noise ratio, establishing feasibility for further more detailed ENDOR studies. Incubation in D2O resulted in little change, indicating that the free radical is well buried or protected. No resolved hyperfine structure was seen, consistent with the generally accepted view that melanin is a heterogeneous polymer.

Experimental considerations in biological ESR studies. I. Identity and origin of the 'tissue lipid signal': a copper-dithiocarbamate complex

The complex ESR signal previously observed in fatty tissues has been identified. It is a copper low-molecular-weight sulphur compound complex similar to copper-dithiocarbamate. The usual source of this signal is contact of copper-containing tissue with surgeon's gloves. This result emphasizes the care that must be taken in obtaining and interpreting ESR spectra of tissues. The formation of this complex by intentionally adding dithiocarbamate to tissue can be used as a very simple and sensitive analytical method for tissue copper. Copper levels of less than 0-1 ppm can be detected by this method. This is an order of magnitude more sensitive than determinations by atomic absorption.

Electron paramagnetic resonance study of single crystals of horse heart ferricytochrome c at 4.2 degrees K

Electron paramagnetic resonance (E.P.R.) spectra from single crystals of horse heart ferricytochrome c at 4.2 °K were analyzed to obtain the orientation of the principal g values relative to the crystallographic axes. The axis of the largest principal g value (g3 = 3.06) was within 5° of the heme normal direction reported in the X-ray structure of the same crystals (Dickerson et al.: J. Biol. Chem. 246, 1511 (1971)). The other two g axes (g1 = 1.25, g2 = 2.25) lie within 5° of the N–Fe–N directions in the heme ring, in contrast to met-myoglobin azide (Helcké et al.: Proc. R. Soc. B169, 275 (1968)) and cyanide (Blumberg, W. E.: personal communication) where they lie ~ 45° from the N–Fe–N directions. A version of Eisenberger and Pershan's theory (J. Chem. Phys. 47, 327 (1967)) was used to explain the 400–2000 G variation in linewidth on crystal rotation. The results were explained by combining the broadening produced by a distribution of rhombic crystal field potential (r.m.s. deviation 11%) over the molecular population, with that from a variation in the directions of the principal g values caused by misorientation (r.m.s. deviation 1.5°) of the molecules in the crystal.

The temperature dependence of the electron paramagnetic resonance spectrum of ferricytochrome c solutions between 4.2 degrees K and 77 degrees K

Electron paramagnetic resonance (E.P.R.) signals from tuna ferricytochrome c solutions were obtained in the temperature range 4.2–77 °K. The microwave power and temperature dependence of the spectra obtained fit the theory for a Kramer's doublet split by a magnetic field. The g-values obtained are 1.25, 2.24, and 3.05 in agreement with other workers (Salmeen, I., and Palmer, G.: J. Chem. Phys. 48, 2049 (1968)). The g = 3.05 line changes from 380 G wide at 20–40 °K with Gaussian shape to 700 G wide at 77 °K with Lorentzian shape. Analysis shows that the Gaussian shape can be explained by variation in the rhombic symmetry of the heme iron environment, a ± 6% spread in rhombic potential being required to give a 380 G line width. At higher temperatures the line appears to be determined by the electron spin-lattice relaxation time. Below 20 °K the E.P.R. absorption signal is saturated, and a large E.P.R. dispersion signal appears, due to fast passage effects, which can be used to give further information on electron spin relaxation times.