Reactions of OH-radicals with procarbazine. A pulse radiolysis and computer simulation study.

Pulse radiolysis was applied to study the reactivity of *OH radicals with procarbazine (PC), a cytostatic agent widely used in radiation- and chemotherapy. An overall rate constant of k(*OH+PC) = 3.7 x 10(9) l.mol(-1).s(-1) was determined. The thereby formed transients had a strong absorption at 350 nm, epsilon350 = 4.46 x 10(3) l.mol(-1).cm(-1), and a weak absorption band around 530 nm. Computer simulation studies to elucidate the most probable sites of *OH attack on the PC molecule showed that *OH radical addition to the aromatic ring had the highest probability. These transients decayed by a first order reaction, k = 1.75 x 10(3) s(-1), whereby species having a maximum absorption at 300 nm and broad shoulder at 340-380 nm were formed. Similar absorptions were observed after gamma radiolysis of PC. A reaction mechanism is suggested. For the reaction of H-atoms with PC, a rate constant k(*H+PC) = 6.4 x 10(8) l.mol(-1).s(-1) was determined.

Radiation induced effect of the vitamins C, E and beta-carotene on sanazole efficiency. A study in vitro.

The effect of the vitamins C, E-acetate and beta-carotene on the sanazole (AK-2123) efficiency was investigated in vitro (E. coli bacteria AB 1157) under the influence of gamma-rays. The vitamins were applied with sanazole individually or in appropriate mixtures in airfree media as well as in media saturated with nitrous oxide (N2O) or air. The addition of vitamin C led in all cases to a very strong enhancement of the sanazole efficiency, namely: in airfree media to 2-fold, in the presence of N2O to 1.5-fold and in the presence of air to 2-fold. The influence of vitamin E-acetate and beta-carotene and mixtures of them depended on the respective gas used for saturation of the medium. An attempt was made to estimate the role of the reducing (e-aq, H-atoms) and oxidizing radicals (OH, O2.-) in respect to the influence of the sanazole activity. The results are of interest for the chemoradiation therapy of cancer.

Pulse radiolysis studies of beta-carotene in oxygenated DMSO solution. Formation of beta-carotene radical cation.

The spectroscopic and kinetic characteristics of beta-carotene radical cation (beta-carotene(.+)) were studied by pulse radiolysis in aerated DMSO solution. The buildup of beta-carotene(.+) with k(1) = (4.8 +/- 0.2) x 10(8) dm(3) mol(-1) s(-1) [lambda(max) = 942 nm, epsilon = (1.6 +/- 0.1) x 10(4) dm(3) mol(-1) cm(-1)] results from an electron transfer from beta-carotene to DMSO(.+). The beta-carotene(.+) species decays exclusively by first-order reaction, k = (2.1 +/- 0.1) x 10(3) s(-1), probably by two processes: (1) at low substrate concentration by hydrolysis and (2) at high concentrations also by formation of dimer radical cation (beta-carotene)(2)(.+). Under the experimental conditions, a small additional beta-carotene triplet-state absorption ((3)beta-carotene) in the range of 525 to 660 nm was observed. This triplet absorption is quenched by oxygen (k = 7 x 10(4) s(-1)), resulting in singlet oxygen ((1)O(2)), whose reactions can also lead to additional formation of beta-carotene(.+).

Enhancement of mitomycin C efficiency by vitamin C, E-acetate and beta-carotene under irradiation. A study in vitro.

Using E.coli bacteria (AB 1157) and leukemia cells (HL 60) as a model for in vitro studies it was established that the efficiency of mitomycin C (MMC) can be influenced in the presence of antioxidant vitamins. This synergistic effect of the vitamins C, E-acetate and beta-carotene on MMC activity is rather strong for E.coli bacteria under irradiation (15 and 50 Gy) in the presence of air. Vitamin C contributes more efficiently to the MMC-activity in leukemia cells than the other two vitamins. The effect is explained by a cascade electron transfer process from the vitamins to MMC, where vitamin C is acting as a major electron source. These results might be of importance in cancer therapy.

Reactivity of homocysteine-thiolactone and alpha-methylhomocysteine-thiolactone with e-(aq) and OH-radical: a pulse radiolysis study.

The efficient radiation protecting agents homocysteine-thiolactone x HCl (HCTL x HCl) and its alpha-alkylated derivative (alpha-methyl-homocysteine, alpha-MHCTL x HCl) have been investigated in respect to the identification of the primarily formed species after absorption of ionizing radiation using pulse radiolysis technique.The reaction of e-(aq) with the unprotonated form of HCTL (k = 2.1 x 10(9) dm3 mol(-1) s(-1)) is leading to the formation of a radical anion having two absorption bands: at 275 nm (epsilon = 2500 dm3 mol(-1) cm(-1)) and 510 nm (epsilon = 930 dm3 mol(-1) cm(-1)), which decay with 2k = 2.3 x 10(9) dm3 mol(-1) s(-1). The protonated form of HCTL reacts with e-(aq) with k = 4.0 x 10(10) dm3 mol(-1) s(-1). The OH-radicals react with HCTL with k = 1.95 x 10(9) dm 3 mol(-1) x s(-1) resulting in a transient spectrum with lambda(max) = 265 nm (epsilon =2000 dm3 mol(-1) cm(-1)). The transients disappear with 2k = 2.1 x 10(9) dm3 mol(-1) s(-1). The reactivity of e-(aq) with alpha-MHCTL was determined for both forms: for the protonated, k = 1.25 x 10(10) dm3 mol(-1) s(-1) and for the unprotonated, k = 2.6 x 10(9) dm3 mol(-1) s(-1). The transient absorption spectrum at pH = 8.4 shows two absorption bands: lambda = 275 nm (epsilon = 3500 dm3 x mol(-1) x cm(-1)) and 490 nm (epsilon = 1160 dm3 x mol(-1) x cm(-1)). The transients disappear with 2k = 2.2 x 10(9) dm3 x mol(-1) x s(-1). The reaction of OH with alpha-MHCTL x HCl,k = 8.4 x 10(9) dm3 x mol(-1) x s(-1) (pH = 8.6) is resulting in an absorption spectrum with lambda(max) < 260 nm and an absorption band at 350 nm (epsilon = 510 dm3 x mol(-1) x cm(-1)). Up to 50 micros after pulse the transients decay with 2k = 5.5 x 10(9) dm3 x mol(-1) x s(-1) and thereafter by a k = 8.4 x 10(9) dm3 x mol(-1) x s(-1) (pH = 8.6) is resulting in an absorption spectrum with lambda(max) < 260 nm and an absorption band at 350 nm (epsilon = 510 dm3 x mol(-1) x cm(-1)). Up to 50 micros after pulse the transients decay with 2k = 5.5 x 10(9) dm3 x mol(-1) x s(-1) and thereafter by a first order reaction. In addition, the formation of some products was also studied. The yield of ammonia resulting from alpha-MHCTL x HCl strongly depends on pH, e.g. at pH = 5.1 Gi (NH3) = 0.95, whereas at pH = 9.15 it increases to Gi = 3.1. Hydrogen sulphide is formed in airfree solutions, Gi (H2S) = 0.29, whereas in the presence of N2O it is reduced to Gi (H2S) = 0.10. Some probable reaction mechanisms are presented.

Reduction of extracellular dehydroascorbic acid by K562 cells.

K562 erythroleukaemic cells produced ascorbate when incubated with dehydroascorbic acid. The reduction depended on the number of cells and on the concentration of dehydroascorbic acid. The observed rate consists of a high affinity (apparent Km 7 mu M, Vmax 3 center dot 25 pmol min-1 (10(6) cells)-1 and a low affinity component, which was non-saturable up to 1 mM of DHA (rate increase of 0 center dot 1 pmol min-1 (10(6) cells)-1 (1 mu M of DHA-1). The rate was dependent on temperature and was stimulated by glucose and inhibited by phloretin, N-ethylmaleimide, parachloro-mercuribenzoate and the noyltrifluoroacetone. Although uptake of DHA proceeded at a higher rate than its extracellular reduction, the generation of extracellular ascorbate from DHA cannot be accounted for by intracellular reduction and the release of ascorbate, since the latter was not linear with time and had an initial rate of approximately 3 pmol min-1 (10(6) cells-1). At a concentration of DHA of 100 mu M this is 25 per cent of the observed reduction.

Radiation-induced formation of haematoporphyrin-transients in aqueous solution. A pulse radiolysis study.

The radiation-induced formation of some haematoporphyrin (HP) transients in aqueous solution was studied by pulse radiolysis. The OH radicals attack HP (rate constant k = (1.3 +/- 0.2) x 10(10) dm3 mol-1 s-1) and the resulting transients decay with 2k = (2.5 +/- 0.2) x 10(8) dm3 mol-1 s-1. The spectrum of the HP-OH species shows several maxima in the range of 450-850 nm (epsilon 590 = 3880, epsilon 640 = 3850, epsilon 800 = 3670 dm3 mol-1 cm-1). k for one electron reduction of HP (radical anion, HP.-) was determined by a direct eaq- attack on HP (at pH 9.0, k = (1.6 +/- 0.2) x 10(10) dm3 mol-1 s-1) as well as by electron transfer from hydroxyisopropyl radicals to the substrate (at pH 8.6, k = (3.2 +/- 0.1) x 10(8), at pH 3.8, k = (4.0 +/- 0.2) x 10(8), and at pH 2.0, k = (2.1 +/- 0.1) x 10(9) dm3 mol-1 s-1). In slightly alkaline media (pH 8.6 - 9.5) the HP.- transients have main absorption bands at lambda < 450 and 640 nm (epsilon 660 = 7500 dm3 mol-1 cm-1), whereas at pH 2.0 they are at lambda < 430 and 640 nm (epsilon 640 = 8170 dm3 mol-1 cm-1) and 710 nm (epsilon 710 = 8500 dm3 mol-1 cm-1). The one-electron oxidation of HP (formation of HP.+, radical cation) was studied by means of N3. or SO4.- species as electron acceptors. Using N3. at pH 9.5 k (HP + N3.) = (3.2 +/- 0.2) x 10(9) and for the decay 2k = (3.5 x 0.3) x 10(8) dm3 mol-1 s-1 were determined; at pH 3.6, k = (HP + N3.) = (7.1 +/- 0.3) x 10(8) and 2k = (2.6 +/- 0.3) x 10(7) dm3 mol-1 s-1 were found. In the case of SO4.- at pH9.8 a buildup (5.6 +/- 0.3) x 10.7 dm3 mol-1 s-1) was obtained. The spectrum of the HP.+ transients at pH 9.6 possesses several maxima with epsilon 345 = 38,800, epsilon 450 = 2400, epsilon 595 = 2880, and epsilon 660 = 3300 dm3 mol-1 cm-1. The corresponding values at pH 3.8 are epsilon 640 = 2450, epsilon 710 = 2600, and epsilon 780 = 1580 dm3 mol-1 cm-1.(ABSTRACT TRUNCATED AT 400 WORDS)

Pulse radiolysis of aromatic amino acids - State of the art.

The pulse radiolysis method as well as the primary processes of water radiolysis and the spectroscopic characteristics of H, OH, HO2/O2 (-) and e aq (-) are briefly presented. Subsequently, kinetic and spectroscopic data of the transients resulting from the resolved multi site attack on aromatic amino acids are discussed. The reactivity of H and e aq (-) with the same substrates, as well as the effect of oxygen on the major radiolytic processes are reviewed. Finally, the formation of tryptophan radical cation is mentioned shortly. The presented radiation mechanisms are the fundamentals for radiolytic processes occurring in proteins, enzymes and hormones in the living cells.

Hydrated electron formation in nanosecond and picosecond laser flash photolysis of hematoporphyrin in aqueous solution.

Nanosecond (lambda exc = 266, 355 and 532 nm) and picosecond (lambda exc = 355 nm) laser flash photolysis of hematoporphyrin (Hp) was performed in neutral (pH 7.4) and alkaline (pH 12) aqueous solution, as well as in the presence of 0.1% Triton X-100. The dependence of the yield of photoproduced hydrated electrons (e-aq) on laser pulse energy was studied over a wide range of energies (0.2 to greater than 1000 mJ cm-2). The results show that e-aq are predominantly formed in a two-photon process at lambda exc = 266 and 355 nm. One-photon quantum yields are higher at lambda exc = 266 nm than at lambda exc = 355 nm. Both one-photon and two-photon pathways are less efficient at higher Hp concentration, reflecting the influence of Hp self-aggregation. Two-photon e-aq formation is more efficient when 30 ps pulses are used for excitation, as compared to 10 ns pulses. No e-aq could be detected at lambda exc = 532 nm. Nanosecond pulse-induced transient spectra obtained at pH 7.4 are also discussed.

Advancements of radiation induced degradation of pollutants in drinking and waste water.

The radiation induced decomposition of some water pollutants such as chlorinated methanes, ethanes and olefines has been studied under various conditions. In addition to product analysis, spectroscopic and kinetic characteristics of CH3, C2H5 and their peroxides were also reinvestigated by pulse radiolysis techniques. Based on the experimental results, probable reaction mechanisms for the decomposition of some pollutant systems are presented. At the same time the present state of the research in this field is given.

Radiation induced decomposition of biological resistant pollutants in water.

Data and reaction mechanisms for oxidative decomposition of biological resistant chlorinated hydrocarbons in aerated water under the influence of ionizing radiation are reported. The radiolysis of the pollutants was studied as a function of the absorbed radiation dose, whereby the formation of C1- ions served as an indicator for the degradation process. In addition phenol was also investigated as a representative of aromatic compounds. Based on the experimental data a radiation pretreatment of water is proposed followed by conventional methods for water purification.

Photoejection of electrons from flavins in polar media.

Riboflavin and 12 of its derivatives have been shown to form solvated electrons under ultraviolet irradiation (253.7 nanometers) in various water-methanol solvent mixtures. The highest quantum yield of solvated electrons (about 0.03) was obtained for flavins containing tyrosine on a side chain in the isoalloxazine N-3 or N-10 position. The splitting of hydrogen atoms from excited flavin molecules was also observed. From the results presented here, it can be determined that the semiquinone transients are formed not only by way of the flavin triplet, as usually suggested, but also by the attack of the electrons and hydrogen atoms on flavin molecules in the ground state. This is important, because the flavin radicals remaining after the electron-ejection or hydrogen-splitting processes must also be considered in the subsequent reaction mechanisms. The electron-ejection process from electronically excited flavins has important implications in the photobiology of these compounds.