Photochemical transformations. XXX. Photolysis of thiobenzoic acid O-esters. I. Photolysis of O-cholesteryl thiobenzoate.
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Benzoates↗
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Cross-bridge kinetics in the presence of MgADP investigated by photolysis of caged ATP in rabbit psoas muscle fibres.
1. The interaction between MgADP and rigor cross-bridges in glycerol-extracted single fibres from rabbit psoas muscle has been investigated using laser pulse photolysis of caged ATP (P3-1(2-nitrophenyl)ethyladenosine 5'-triphosphate) in the presence of MgADP and following small length changes applied to the rigor fibre. 2. Addition of 465 microM-MgADP to a rigor fibre caused rigor tension to decrease by 15.3 +/- 0.7% (S.E.M., n = 24 trials in thirteen fibres). The half-saturation value for this tension reduction was 18 +/- 4 microM (n = 23, thirteen fibres). 3. Relaxation from rigor by photolysis of caged ATP in the absence of Ca2+ was markedly slowed by inclusion of 20 microM-2 mM-MgADP in the photolysis medium. 4. Four phases of tension relaxation occurred with MgADP in the medium: at, a quick partial relaxation (in pre-stretch fibres); bt, a slowing of relaxation or a rise in tension for 50-100 ms; ct, a sudden acceleration of relaxation; and dt, a final, nearly exponential relaxation. 5. Experiments at varied MgATP and MgADP concentrations suggested that phase at is due to MgATP binding to nucleotide-free cross-bridges. 6. Phase bt was abbreviated by including 1-20 mM-orthophosphate (Pi) in the photolysis medium, or by applying quick stretches before photolysis or during phase bt. These results suggest that phases bt and ct are complex processes involving ADP dissociation, cross-bridge reattachment and co-operative detachment involving filament sliding and the Ca(2+)-regulatory system. 7. Stretching relaxed muscle fibres to 3.2-3.4 microns striation spacing followed by ATP removal and release of the rigor fibre until tension fell below the relaxed level allowed investigation of the strain dependence of relaxation in the regions of negative cross-bridge strain. In the presence of 50 microM-2 mM-MgADP and either 10 mM-Pi or 20 mM-2,3-butanedione monoxime, relaxation following photolysis of caged ATP was 6- to 8-fold faster for negatively strained cross-bridges than for positively strained ones. This marked strain dependence of cross-bridge detachment is predicted from the model of A. F. Huxley (1957). 8. In the presence of Ca2+, activation of contraction following photolysis of caged ATP was slowed by inclusion of 20-500 microM-MgADP in the medium. An initial decrease in tension related to cross-bridge detachment by MgATP was markedly suppressed in the presence of MgADP. 9. Ten millimolar Pi partly suppressed active tension generation in the presence of MgADP.(ABSTRACT TRUNCATED AT 400 WORDS)
Adenosine Diphosphate↗
Flash photolysis studies of excitation-contraction coupling, regulation, and contraction in smooth muscle.
1. Flash photolysis of caged compounds of phenylephrine, inositol 1, 4, 5 trisphosphate (InsP3), GTP gamma S, ATP, and CTP has been successfully used to study excitation-contraction coupling, contractile regulation, and contraction in smooth muscle. Major processes explored with this method were (a) the delay between agonist-receptor interaction and contraction and between the rise in InsP3, Ca2+ release and contraction; (b) the effect of myosin light chain phosphorylation on the rate of force development and the respective contributions of phosphorylation and crossbridge kinetics to differences between phasic and tonic smooth muscles; (c) the kinetics of the crossbridge cycle. We have also reviewed recent results obtained by other methods and bearing on the mechanisms of pharmacomechanical Ca2+ release and modulation of the Ca2+ sensitivity of the regulatory/contractile apparatus. 2. The long delay (1.5 at 22 degrees C) following activation of alpha 1-adrenergic receptors through photolysis of caged phenylephrine and the high Q10 of this process are consistent with the hypothesis that activation of phospholipase C is the major mechanism of alpha-adrenergic pharmacomechanical Ca2+ release. 3. The delay between photolysis of caged InsP3 and Ca2+ release is short: 30 ms or less, while the latency of contraction is significant (0.3-0.5 s at 22 degrees C) and similar to the lag between the rise in [Ca2+]i and force development in intact smooth muscles. The latency of contraction following photolysis of caged ATP in permeabilized muscles in rigor, in the presence of Ca2+ and calmodulin, is similar, about 0.2-0.5 s at 22 degrees C. 4. In muscles in which the myosin light chains are maintained in a phosphorylated state during rigor, photolysis of caged ATP initiates contractions with a short delay (10 ms or less). This result and those summarized above (2 and 3) suggest that the major portion of the delay between agonist-receptor interaction and contraction is due to activation of phospholipase C and InsP3 production, and about 0.2-0.5 s of the delay (22 degrees C) can be ascribed to prephosphorylation reactions between Ca2+, calmodulin, and myosin light chain kinase, and/or to mechanical processes, or to the chemical kinetics of two-step reactions. 5. Force development from rigor, initiated by photolysis of caged ATP in the presence of Ca2(+)-calmodulin, is rate-limited by myosin light chain phosphorylation; it is significantly accelerated if the myosin light chains are already phosphorylated prior to photolysis.(ABSTRACT TRUNCATED AT 400 WORDS)
Animals↗
Elimination of residual tumor cells from autologous bone marrow grafts by dye-mediated photolysis: preclinical data.
MC540-mediated photolysis has several features that make it potentially attractive as a clinical purging procedure. (1) The experience with experimental tumors suggests that MC540-mediated photolysis is effective against a broad range of leukemias and solid tumors, including drug-resistant tumors (Sieber et al., 1984b). Drug-resistant tumor cells are likely to occur in heavily pretreated patients. (2) MC540-mediated photolysis is not cell-cycle dependent (Manna and Sieber, 1985). It kills both resting and cycling cells. In this regard, MC540-mediated photolysis is a valuable complement to cell-cycle specific cytotoxic drugs. (3) There is a large differential in sensitivity between normal pluripotent hematopoietic stem cells and leukemia and neuroblastoma cells. (4) The mechanism of action of MC540-mediated photolysis is different from that of lectins, antibodies and most cytotoxic drugs. MC540 binds to the lipid portion of the plasma membrane and membrane lipids are probably a primary target of the toxic photoproducts. Antibodies and lectins react with proteins and carbohydrates and most drugs have intracellular targets (e.g., nuclear DNA). We would therefore expect little cross-resistance if MC540-mediated photolysis were used in combination with other purging procedures.(5) The small amounts of dye that remain associated with the marrow graft and are infused into the patient are approximately 100,000-fold less than the LD(10) (in mice) and therefore unlikely to cause any harm. The outcome of the first clinical application of the technique supports this view (Sieber et al., 1986c). A better understanding of the underlying molecular mechanisms will undoubtedly lead to more effective applications of the technique and perhaps to the identification of more potent analogs of MC540.
Bone Marrow↗
Ultraviolet flash photolysis of gramicidin-doped lipid bilayers.
We have examined the rate of gramicidin channel conductance inactivation by ultraviolet photolysis using 0.1 millisecond light flashes. The lower limit on the channel photolysis reaction rate has been reduced by four orders of magnitude over previous observations. Monoolein/hexadecane bilayers formed in 1.0 M KCl were doped with (1-3) x 10(6) gramicidin A' channels and exposed to a broad-spectrum light flash. The flash reduced membrane conductance abruptly by approx. 16%. Following the flash, a further slow reduction of approx. 3% was observed followed by a slow recovery of approx. 4%. The post-flash decay and recovery may be due to slow chemical reactions, conformational relaxations, or changes in the equilibrium between aqueous, lipid-bound, and channel-forming dimerized gramicidin. Under our experimental conditions, gramicidin M was insensitive to light flashes compared to gramicidin A', demonstrating that for gramicidin A' the photolysis mechanism depends specifically on the tryptophan side-chain. Flash photolysis of a membrane containing a small population of channels (approx. 30) indicated that the decay is due to the sudden inactivation of several channels. The recovery appears to result from insertion of normal channels into the membrane. Flash photolysis of single-channel membranes showed that the flash causes abrupt, complete channel inactivation.
Electric Conductivity↗
OH radical formation by photolysis of aqueous porphyrin solutions. A spin trapping and e.s.r. study.
Radical production during the photolysis of deaerated aqueous alkaline solutions (pH 11) of some water-soluble porphyrins was investigated. Metal-free and metallo complexes of tetrakis (4-N-methylpyridyl)porphyrin (TMPyP) and tetra (4-sulphonatophenyl)porphyrin (TPPS4) were studied. Evidence for the formation of OH radicals during photolysis at 615, 545, 435, 408 and 335 nm of Fe(III) TPPS4 is presented. Fe(III) TMPyP, Mn(III) TPPS4 and Mn(III) TMPyP also gave OH radicals but only during photolysis at 335 nm. The method of spin trapping with 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) and 4-pyridyl-1-oxide-N-tert-butylnitrone (POBN) combined with e.s.r. was used for the detection of OH, H and hydrated electrons. With the spin trap DMPO, photolysis generated DMPO-OH adducts under certain conditions but no DMPO-H adducts could be observed. With POBN, no POBN-H adducts were found. The formation of OH was confirmed by studying competition reactions for OH between the spin traps and OH scavengers (formate, isopropanol) and the concomitant formation of the CO-2 adduct and the (CH3)2COH adduct with both DMPO and POBN. The photochemical generation of OH radicals was pH dependent; at pH 7.5 no OH radicals could be detected. Photolysis (615-335 nm) of dicyanocomplexes of the Fe(III) porphyrins did not produce OH radicals. When corresponding Cu(II), Ni(II), Zn(II) and metal-free porphyrins were photolysed at 615 and 335 nm, no OH radicals could be spin trapped. These results tend to associate the well-known phenomenon of photoreduction of Fe(III) and Mn(III) porphyrins with the formation of OH radicals. This process is described mainly as the photoreduction of the metal ion by the ligand-bound hydroxyl ion via an intramolecular process.
Electron Spin Resonance Spectroscopy↗
Calcium released by photolysis of DM-nitrophen stimulates transmitter release at squid giant synapse.
1. Transmitter release at the squid giant synapse was stimulated by photolytic release of Ca2+ from the 'caged' Ca2+ compound DM-nitrophen (Kaplan & Ellis-Davies, 1988) inserted into presynaptic terminals. 2. Competing binding reactions cause the amount of Ca2+ released by DM-nitrophen photolysis to depend on the concentrations of DM-nitrophen, total Ca2+, Mg+, ATP and native cytoplasmic Ca2+ buffer. Measurements of presynaptic [Ca2+] changes by co-injection of the fluorescent indicator dye Fura-2 show that DM-nitrophen photolysis causes a transient rise in Ca2+ followed by decay within about 150 ms to an increased steady-state level. 3. Rapid photolysis of Ca2(+)-loaded nitrophen within the presynaptic terminal was followed in less than a millisecond by depolarization of the postsynaptic membrane. As with action potential-evoked excitatory postsynaptic potentials (EPSPs), the light-evoked response was partially and reversibly blocked by 1-3 mM-kainic acid which desensitizes postsynaptic glutamate receptors. 4. Release was similar in magnitude and rate to normal action potential-mediated EPSPs. 5. The release of transmitter by photolysis of Ca2(+)-loaded DM-nitrophen was not affected by removal of Ca2+ from the saline or addition of tetrodotoxin. Photolysis of DM-nitrophen injected into presynaptic terminals without added Ca2+ did not stimulate release of transmitter nor did it interfere with normal action potential-mediated release. 6. Stimulation of presynaptic action potentials in Ca2(+)-free saline during the light-evoked response did not elicit increased release of transmitter if the ganglion was bathed in Ca2(+)-free saline, i.e. in the absence of Ca2+ influx. Increasing the intensity of the light or stimulating presynaptic action potentials in Ca2(+)-containing saline increased the release of transmitter. Therefore the failure of presynaptic voltage change to increase transmitter release resulting from release of caged Ca2+ was not due to saturation or inhibition of the release mechanism by light-released Ca2+. 7. Decreasing the temperature of the preparation increased the delay to onset of the light-evoked response and reduced its amplitude and rate of rise to an extent similar to that observed for action potential-evoked EPSPs.
Acetates↗
Photolysis primes biodegradation of benzo[a]pyrene.
14C-labeled benzo[a]pyrene (BaP) was used as a model-compound for polycyclic aromatic hydrocarbons (PAH) in order to assess the effect of photolytic pretreatment on the subsequent fate of BaP in sewage sludge and soil test systems. Photolysis was performed in methanolic solution with or without 0.1 M H2O2, under either UV light (300 nm) or natural sunlight. The presence of H2O2 greatly enhanced the rate of photolysis both with UV and with natural sunlight. Intact BaP resisted biodegradation in both test systems. Photolysis transformed BaP to polar materials that were subject to increased mineralization and binding in both biological test systems. As shown by the Ames assay, photolysis decreased the mutagenicity of BaP to test strains TA98 and TA104 only moderately. The photolysate had an increased acute toxicity and lost its need for activation by S-9 enzymes. However, during subsequent incubation in soil or sewage sludge, mutagenicity decreased rapidly by one to two orders of magnitude and acute toxicity disappeared due to the mineralization and binding of photoproducts to humic materials. Photolysis of BaP and similar PAH compounds represents a useful treatment option that could be applied to certain PAH-containing petroleum refinery sludge and to coal tar residues in order to facilitate their detoxification and environmentally safe disposal.
Benzo(a)pyrene↗
Flash photolysis of enzymes.
The photoionization of aromatic residues constitutes a major initial photochemical reaction in the flash photolysis of proteins at gamma greater than 250 nm. The ejected electrons have been observed as eaq- and the disulphide bridge electron adduct, and also must be trapped at unidentified sites. The number of tryptophyl (or tyrosyl) residues photo-ionized at 5 musec delay is approximately equal to the number of exposed residues. The flash photolysis data have been related to inactivation by considering how photolysis of these "photolabile" residues can affect enzymic activity, based on the microstructure and available information about permanent alterations and residue specificities. This analysis indicates that hen lysozyme and papain are inactivated by photolysis of an essential Trp residue, that bovine trypsin is inactivated by photolysis of a Trp residue adjacent to the key catalytic Ser and other pathways initiated by excitation of Tyr and Cys, that the efficient photoionization of Tyr and RNase A is not an important inactivating reaction, and that aromatic residues in subtilisn Carlsberg are photosensitive.
Amino Acids↗
Induced photolysis of DDT.
Photolysis of mixtures of certain alkyl halides and aromatic amines produces dehalogenation of the halide. These reactions involve a photoinduced charge transfer from the amine to the halide. Photolysis of tritolylamine and carbon tetrachloride produces tritolylaminium chloride. Photolysis of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) and diethylaniline at 3100 angstroms yields 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE), 1,1-dichloro-2,2-bis(p-chlorophenyl)ethane (DDD), p,p'-dichlorobenzophenone (DDCO), and hydrogen chloride. Photolysis of DDT does not occur unless an inducer which has a low ionization potential, such as diethylaniline, is present. The DDT-diethylaniline mixture is stable in the dark, and the induced photolysis is not affected by triplet quenchers.
Amines↗
Microtubule assembly and oscillations induced by flash photolysis of caged-GTP.
Microtubule assembly and oscillations have been induced using the rapid liberation of GTP by UV flash photolysis of caged-GTP and monitored by time-resolved X-ray scattering. The flash photolysis method of achieving assembly conditions is much faster than the temperature jump method used earlier (msec vs. s range). However, the structural transitions and their rates are similar to those described previously. This means that the rates of the transitions in microtubule assembly observed before are determined by the protein itself, and not by the rate at which assembly conditions are induced. The advantages and limitations of using the photolysis of caged-GTP in microtubule assembly studies are compared with temperature jump methods. Caged-GTP itself reduces the rate of microtubule assembly and oscillations at mM concentrations, consistent with a weak interaction between the nucleotide analogue and the protein. X-rays are capable of slowly liberating GTP and other breakdown products from caged-GTP, even in the absence of UV flash photolysis, thus causing an apparent "X-ray-induced" microtubule assembly. This effect depends on the X-ray dose but is independent of the caged-GTP concentrations used here (mM range), suggesting that the breakdown of caged-GTP is caused not by the direct absorption of X-rays by the compound but by another intermediate reaction such as the generation of radicals by the X-rays.
Animals↗
Synthesis of organic compounds from carbon monoxide and water by UV photolysis.
The photolysis of water vapor with carbon monoxide at 1849 A yields alcohols, aldehydes and organic acids, with an overall quantum yield of 3.3 x 10(-2). This rather high quantum yield could have led to a contribution of approximately 10(11) organic molecules cm-2 sec-1 to the pool of organic material on the primitive Earth. The reactions are initiated by the photolysis of water molecules and the resulting hydrogen atoms reduce the carbon monoxide to a variety of one and two carbon compounds. The organic molecules are dissolved in water and thus escape destruction by photolysis. Photolysis of water vapor with carbon dioxide did not yield organic compounds under these conditions.
Carbon Monoxide↗
Effects of ethylene glycol on the kinetics of contraction on flash photolysis of caged ATP in rat psoas muscle fibres.
ATP (1-1.2 mM) was photoreleased from caged ATP (5 mM) in skinned fibres from rat psoas muscle at 15-17 degrees C, to examine the effects of ethylene glycol (EG; 20% in solvent) on the kinetics of isometric contraction. Muscle fibres were stretched by 0.5-2% before photolysis, so that force just before photolysis was almost equal to the steady-state force after photolysis. At the phase of steady-state contraction, force and 500 Hz-stiffness in the presence of EG were 50% and 70% of the controls, respectively, resulting in a higher stiffness-to-force with EG, as reported previously. Following photolysis, force fell before rising to a steady-state plateau. The estimated rate constant of the force decay was approximately 90 s-1, and in the presence of EG was 80-85% of the control. This suggested a small effect of EG on the crossbridge detachment induced by ATP. The rate of force redevelopment was approximately 70 s-1, and EG decreased this rate to 50% of the control. This suggested that EG greatly slows the transition of the crossbridges from the detached state to the reattached force-producing state. The time course of the stiffness signals was consistent with this interpretation. The high stiffness-to-force ratio with EG indicated that EG not only reduces the rate constants which were directly examined in this study but also modifies other aspects of the crossbridge reaction.
Adenosine Triphosphate↗
Kinetics of sickle hemoglobin polymerization. I. Studies using temperature-jump and laser photolysis techniques.
Using a combination of laser photolysis and temperature-jump techniques, the kinetics of hemoglobin S polymerization have been studied over a wide range of delay times (10(-3) to 10(5)s), concentrations (0.2 to 0.4 g/cm3) and temperatures (5 to 50 degrees C). A slow temperature-jump technique was used to induce polymerization in samples with delay times between 10(2) seconds and 10(5) seconds by heating a solution of completely deoxygenated hemoglobin S. For samples with shorter delay times, polymerization was induced by photodissociating the carbon monoxide complex in small volumes (10(-9) cm3) using a microspectrophotometer equipped with a cw argon ion laser. The photolysis technique is described in some detail because of its importance in studying hemoglobin S polymerization at physiological concentrations and temperatures. In order, to establish conditions for complete photodissociation with minimal laser heating, a series of control experiments on normal human hemoglobin was performed and theoretically modeled. The concentration dependence of the tenth time is found to decrease with increasing hemoglobin S concentration. In the range 0.2 to 0.3 g/cm3, the tenth time varies as the 36th power of the hemoglobin S concentration, while in the range 0.3 to 0.4 g/cm3 it decreases to 16th power. As the tenth times become shorter, the progress curves broaden, with the onset of polymerization becoming less abrupt. For tenth times greater than about 30 seconds, measurements with the laser photolysis technique on small volumes yield highly irreproducible tenth times, but superimposable progress curves, indicating stochastic behavior. The initial part of the progress curves from both temperature-jump and laser photolysis experiments is well fit with an equation for the concentration of polymerized monomer, delta (t) = A[cosh (Bt) -1], which results from integration of the linearized rate equations for the double nucleation mechanism described in the accompanying paper (Ferrone et al., 1985). The dependence of the parameters A and B on temperature and concentration is obtained from fitting over 300 progress curves. The rate B has a large concentration dependence, varying at 25 degrees C from about 10(-4) S-1 at 0.2 g/cm3 to about 100 s-1 at 0.4 g/cm3.
Hemoglobin, Sickle↗
A long-lived transient resulting from flash photolysis of hematoporphyrin in aqueous solution.
A long-lived transient with a lifetime of several hundred microseconds was observed following the flash photolysis of aqueous solutions of hematoporphyrin buffered at pH 7.5. The transient-ground state difference absorption spectrum was determined 500 microseconds after flash photolysis. The yield of this species was found to increase with increasing hematoporphyrin concentration and it was also found to depend on the excitation wavelength. The lifetime of the species is not significantly affected by the presence of oxygen. Because the triplet state of hematoporphyrin is not the only long-lived species produced by flash photolysis of aqueous hematoporphyrin solutions, the observed triplet state extinction coefficients will be lower than the true value and hence the triplet state yields of hematoporphyrin determined by the flash photolysis, "complete conversion" technique, are only upper limits. The formation of the long-lived species is discussed in terms of electron transfer between the monomer partners in hematoporphyrin dimer and aggregates which are present in aqueous solutions of hematoporphyrin, particularly in concentrated solutions.
Hematoporphyrins↗
Photolysis intermediates of the artificial visual pigment cis-5,6-dihydro-isorhodopsin.
The photolysis intermediates of an artificial bovine rhodopsin pigment, cis-5,6-dihydro-isorhodopsin (cis-5,6,-diH-ISORHO, lambda max 461 nm), which contains a cis-5,6-dihydro-9-cis-retinal chromophore, are investigated by room temperature, nanosecond laser photolysis, and low temperature irradiation studies. The observations are discussed both in terms of low temperature experiments of Yoshizawa and co-workers on trans-5,6-diH-ISORHO (Yoshizawa, T., Y. Shichida, and S. Matuoka. 1984. Vision Res. 24: 1455-1463), and in relation to the photolysis intermediates of native bovine rhodopsin (RHO). It is suggested that in 5,6-diH-ISORHO, a primary bathorhodopsin intermediate analogous to the bathorhodopsin intermediate (BATHO) of the native pigment, rapidly converts to a blue-shifted intermediate (BSI, lambda max 430 nm) which is not observed after photolysis of native rhodopsin. The analogs from lumirhodopsin (LUMI) to meta-II rhodopsin (META-II) are generated subsequent to BSI, similar to their generation from BATHO in the native pigment. It is proposed that the retinal chromophore in the bathorhodopsin stage of 5,6-diH-ISORHO is relieved of strain induced by the primary cis to trans isomerization by undergoing a geometrical rearrangement of the retinal. Such a rearrangement, which leads to BSI, would not take place so rapidly in the native pigment due to ring-protein interactions. In the native pigment, the strain in BATHO would be relieved only on a longer time scale, via a process with a rate determined by protein relaxation.
Isomerism↗
Rate of release of Ca2+ following laser photolysis of the DM-nitrophen-Ca2+ complex.
The determination of the rate of release of Ca2+ by pulsed photolysis of the photolabile chelator DM-nitrophen is important for its use in time-resolved physiological studies: the rate of substrate or effector release should be faster than the processes they initiate. Flash photolysis of DM-nitrophen using a 50-ns pulse from a frequency-doubled ruby laser (with emission at 347 nm having energy of ca. 10-20 mJ) yields short-lived photochromic or aci-nitro intermediates. At pH 6.9, double-exponential decay of a photochromic intermediate was observed for DM-nitrophen itself and its Ca2+ complex (tau 1/2 values of 24 and 570 microseconds, and 32 and 220 microseconds respectively), while only monoexponential decay of the DM-nitrophen-Mg2+ complex was detected (tau 1/2 = 31 microseconds). Only the photochemistry of DM-nitrophen-Ca2+ was found to be pH sensitive (monoexponential decay, tau 1/2 approximately 115 microseconds at pH 7.9 and 8.9). Use of the Ca(2+)-sensitive metallochromic dye antipyrylazo III in conjunction with pulsed photolysis of DM-nitrophen-Ca2+ enabled an upper limit of the half-time of release of Ca2+ to be established of ca. 180 microseconds (the rate of association of Ca2+ with the dye was probably rate determining). The rate of Ca2+ photorelease may, however, be faster than this. Thus, the DM-nitrophen-Ca2+ complex releases Ca2+ on photolysis sufficiently rapidly for the study of many Ca(2+)-dependent physiological processes with improved kinetic resolution over conventional mixing methods.
Acetates↗
E.S.R. of spin-trapped radicals in aqueous solutions of 5-halo derivatives of nucleic acid constituents: reactions of hydrated electrons, hydroxyl radicals and U.V. photolysis.
In order to obtain information concerning the mechanism of radio- and photosensitization due to 5-halogen substituted nucleic acid constituents, the free radicals produced in iodo-, bromo-, chloro- and fluoro-derivatives of uracil, uridine and deoxyuridine by reaction with hydrated electrons and with hydroxyl radicals and by direct U.V. photolysis have been studied by e.s.r. and spin-trapping. t-Nitrosobutane was used as the spin-trap. From 5-halogenated bases (except 5-fluorouracil) U.V. photolysis and reactions with hydrated electrons produced the uracilyl radical which was subsequently spin-trapped. When hydroxyl radical reactions were studied, the free radical at the N(1) position of the base was identified. From 5-fluorouracil U.V. photolysis generated the alpha-halo radical at the C(5) position of the base. For 5-halogenated ribonucleosides and deoxyribonucleosides, free radicals located on the sugar moiety were observed for reactions with hydrated electrons, hydroxyl radicals and for U.V. photolysis. The implications of these results for understanding the mechanism of radio- and photosensitization by 5-halogenated nucleic acids are discussed.
Electron Spin Resonance Spectroscopy↗