John D. Zimbrick

PARAMAGNETIC RELAXATION OF TRAPPED ELECTRONS IN IRRADIATED ALKALINE ICES.

The paramagnetic relaxation characteristics of trapped electrons in γ‐irradiated alkaline ices were studied by power saturation methods. The relaxation times were measured as a function of radiation dose, deuteration, phase, and temperature. Linewidths were measured under various conditions. The trapped electron EPR line is inhomogeneously broadened by nuclear hyperfine interactions; the spin‐lattice relaxation mechanism is probably a cross relaxation process with O−. The change in relaxation time with radiation dose indicates that the electrons are trapped with a nonuniform spatial distribution in hydroxyl anion vacancies in radiation‐produced spurs of ∼30‐A radius.

Paramagnetic Relaxation of Trapped Hydrogen Atoms in Irradiated Frozen Aqueous Solutions

The paramagnetic relaxation characteristics of trapped hydrogen atoms in γ‐irradiated acidic and oxyanion ices were studied by power‐saturation techniques. Relaxation times were measured as a function of radiation dose, deuteration, phase, and solute. The EPR linewidth of the trapped hydrogen atom was measured under various conditions. The EPR lineshape and power‐saturation response indicate that the linewidth is due to contributions from both homogeneous and inhomogeneous interactions. The changes in relaxation time with radiation dose indicate that hydrogen atoms are trapped with a uniform distribution near oxyanions in the ice and are not trapped within radiation‐produced spurs. The presence of phase effects on relaxation times supports an interstitial‐defect model for the hydrogen‐atom trapping site and suggests that the trap potential energy well is shallower in the polycrystalline phase than in the glassy phase. In polycrystalline ices the coupling between trapped hydrogen atoms and lattice phonons ...

Track Structure in DNA Irradiated with Heavy Ions

Abstract Bowman, M. K., Becker, D., Sevilla, M. D. and Zimbrick, J. D. Track Structure in DNA Irradiated with Heavy Ions. Radiat. Res. 163, 447–454 (2005). The spatial properties of trapped radicals produced in heavy-ion-irradiated solid DNA at 77 K have been probed using pulsed electron paramagnetic double resonance (PELDOR or DEER) techniques. Salmon testes DNA hydrated to 12 water molecules per nucleotide was irradiated with 40Ar ions of energy 100 MeV/nucleon and LET ranging from 300 to 400 keV/μm. Irradiated samples were maintained at cryogenic temperature at all times. PELDOR measurements were made using a refocused echo detection sequence that allows dipolar interaction between trapped radicals to be observed. The EPR spectrum is attributed to electron loss/gain DNA base radicals and neutral carbon-centered radicals that likely arise from sugar damage. We find a radical concentration of 13.5 × 1018 cm−3 in the tracks and a track radius of 6.79 nm. The cross section of these tracks is 144 nm2, yielding a lineal radical density of 2.6 radicals/nm. Based on the yields determined previously for particles having calculated LET values of 300–400 keV/μm and our measured lineal density, we obtain an LET of 270 keV/μm, which is in good agreement with the calculated range of values. These measurements of radical density and spatial extent provide the first direct experimental determination of track characteristics in irradiated DNA.

Ethanol effects on synaptic glutamate receptor function and on membrane lipid organization

The enhancement of L-glutamic acid binding activity of brain synaptic membranes by low concentrations of ethanol (less than 50 mM) and the decrease in binding at high concentrations (greater than 100 mM) was not due to a direct action by ethanol on the glutamate binding protein. Biphasic effects of ethanol on membrane protein complexes such as the glutamate binding sites might be the result of biphasic changes in membrane lipid organization. Low ethanol concentrations (0.1-4.0 mM) were shown to decrease fatty acid chain motion detected by the EPR probe 5-doxyl stearic acid, whereas high concentrations (greater than 400 mM) increased lipid motion in egg phosphatidylcholine liposomes. The function of the L-glutamate receptor-ion channel complex in the presence of ethanol was also determined by measuring the changes in thiocyanate (SCN-) influx brought about by L-glutamate or ethanol. A low concentration of ethanol (9.4 mM) diminished the L-glutamate-induced depolarization of synaptic membranes, while a high concentration (93.7 mM) increased the passive SCN-influx and produced a transient overshoot in glutamate-stimulated SCN-flux.

Studies on radiosensitization of Escherichia coli cells by cis-platinum complexes

Abstract We recently reported that the antitumor drug cis -Pt(NH 3 ) 2 CI 2 (cis-DDP) produces significant radiosensitization of anoxic E Coli C cells 7 . We have extended these studies to three other platinum drugs, all of which have been shown to be more effective antitumor drugs than cis -DDP. The drugs are: cis -dichloro bis(ethylene imine) Pt(II) ( cis -DEP); cis-dichlorobicyclopentylamine Pt(II) ( cis -PAD); and Pt-thymine blue ( cis -PTB). Survival curve studies indicate that these drugs all produce greater anoxic radiosensitization of E coli C than cis -DDP at concentrations which are less toxic to the cells than similar concentrations of cis -DDP. If the cells are treated with any one of these drugs for two hours and then washed to remove the drug before irradiation, no detectable radiosensitization is found. We conclude that these drugs have the potential for being useful agents in combined modality therapy and that they warrant further study in mammalian systems.

Ethanol effects on synaptic glutamate receptors and on liposomal membrane structure

Exposure of synaptic plasma membranes to 50 mM ethanol in vitro brought about a 3.5 degrees C decrease in the transition temperature of the high affinity glutamate binding process in these membranes. Ethanol had no effect on the energy of activation of glutamate binding below the transition temperature but decreased the energy of activation above the transition temperature. Electron paramagnetic resonance (EPR) studies of lipid organization of egg lecithin and bovine brain phospholipid liposomes indicated that ethanol at low concentrations (0.04--2 mM) caused small increases in the rigidity of the membrane near the surface. At higher concentrations (0.04--2 M) ethanol brought about increasing fluidization of both the surface and inner areas of the bilayer. Even at 4 mM concentration ethanol enhanced the ordered to fluid state transition of liposome membranes as shown by a 5.2 degrees C and 1.9 degrees C decrease in the transition temperatures of the membrane determined with the cholestane EPR probe.

In vivo radiation-induced thymine residue release from E. coli DNA

Abstract Cells of E. coli C thy − 321 are examined for thymine residue release from DNA following gamma-irradiation from 5 to 15 krad. Experimental conditions are designed to inhibit enzyme activity that might promote base residue release. Enzyme action is restricted in order to assess the physicochemical action of radiation on cellular DNA, and to this end irradiation is done under O 2 , N 2 , and N 2 O saturating conditions. Both thymine and thymidine release from bacterial DNA are detected and quantitated, and three oxygen effects are noted in comparing yields of these products. No difference in effect is observed between N 2 and N 2 O gassing conditions, suggesting that the hydroxyl radical has little effect on thymine or thymidine release from irradiated DNA in vivo .

Spin trapping of reactive uracilyl radicals produced by ionizing radiation in aqueous solutions.

The DNA base analogue 5-bromouracil (BU) reacts with hydrated electrons to produce a highly reactive uracil-5-yl (uracilyl) radical. The uracilyl radical was spin trapped by 5,5-dimethylpyrroline-1-oxide (DMPO) and t-nitrosobutane (tNB). DMPO solutions (0.01 M at pH = 7, N2 or N2O saturated) without BU were irradiated with 60Co gamma-rays and the resulting spin adducts produced by e-aq, H., and OH. were identified. The irradiation of alkaline (pH 12, N2 saturated) solutions of DMPO alone simplified the e.p.r. spectrum of spin adducts in that only the H. adduct was observed. Irradiated DMPO solutions (pH 12, N2 saturated) containing BU yielded a composite of two six-line e.p.r. spectra, attributed to uracilyl spin adducts that have two distinct steric configurations, and a background spectrum of H.-DMPO adducts which was removed by computer-subtraction. Uracilyl-tNB spin adducts were detected by first trapping uracilyl radicals in irradiated photobleached 10 M NaOH glasses at 77 K and then dissolving these glasses in pH-adjusted tNB solutions.

Phytohaemagglutin-induced changes in spin label reduction in lymphocytes from tumor-bearing rats

Abstract Spin labeling techniques were utilized to investigate the rate of reduction of spin probes introduced into lymphocytes from normal and tumor-bearing animals. The response of the lymphocytes to phytohaemagglutin (PHA) stimulation was monitored by the spin labels Tempone (2,2,6,6-tetramethyl-4-oxo-piperidinooxy), PCA (2,2,5,5,-tetramethyl-1-pyrrolidinyl-oxy-3-carboxylic acid), and TMPN (2,2,6,6-tetramethyl-piperidinooxy). The EPR signal intensity of the nitroxide spin labels decreased according to first-order kinetics. For phytohaemagglutin challenged lymphocytes from tumor-bearing animals the Tempone signal loss was a factor of two less than the corresponding controls.

ESR studies of hydrogen trapped in alkali halides by proton irradiation

Abstract Trapped hydrogen atoms have been detected by electron paramagnetic resonance spectroscopy in proton irradiated KCl and NaCl single crystals at 77°K. The paramagnetic resonance characteristics indicate that the hydrogen atoms closely resemble U 3 centers.