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The radical cations of naturally occurring furanochromones visnagin (VI) and khellin (KH) have been generated and identified for the first time by use of laser flash photolysis and pulse radiolysis techniques. The lifetimes of VI(.+) and KH(.+) are determined as approximately 6 and approximately 35 micros under these conditions, respectively. Direct 308-nm excitation of VI in aqueous buffer at physiological pH results in monophotonic photoionization to generate VI(.+), with a quantum yield of 0.075, which is much higher than that of 8-methoxypsoralen and KH under identical conditions. Though VI(.+) is a more powerful oxidant than KH(.+), both of them react with guanosine mononucleotide (k=1.2x10(9) and 3.8x10(7) dm(3) mol(-1) s(-1), respectively) via electron transfer to give the guanine radical cation. Furthermore, selective oxidation of guanine in single and double strand DNA by VI(.+) was also observed. These novel findings suggest that electron transfer reactions involving furanochromone radical cations may be of considerable importance in furanochromone photochemotherapy.

By use of pulse radiolysis techniques, the radical cations of purine nucleotides have been successfully produced by the SO4- ion oxidation. Time-resolved spectroscopic evidence is provided that the one-electron-oxidized radicals of dAMP and dGMP can be efficiently repaired by aromatic amino acids (including tyrosine and tryptophan) via electron transfer reaction. As a model peptide, Arg-Tyr-AcOH was also investigated with regard to its interaction with deprotonated purine radical cations. The rate constants of the electron transfer reactions were determined to be (1 approximately 5) x 10(8) dm(3) mol(-1) s(-1). These results suggest that the aromatic amino acids in DNA-associated proteins may play some role in electron transfer reactions through DNA.

The reactions of tea polyphenol derivatives, including epicatechin (EC) and epigallocatechin gallate (EGCG), with nitrogen dioxide radical (NO 2 ) and carbonate radical (CO 3 − ) have been studied in detail using time-resolved pulse radiolysis technique. In all the cases, the corresponding phenoxyl radical was formed through electron transfer reaction. From the build-up kinetics of the phenoxyl radicals and the decay kinetics of CO 3 − radical, the reaction rate constants of EC, EGCG with NO 2 and CO 3 − were determined to be 9.0×10 7 , 1.2×10 8 and 5.6×10 8 , 6.6×10 8 dm 3 mol −1 s −1 , respectively. Therefore, tea polyphenol derivatives proved to be efficient scavengers of NO 2 and CO 3 − radicals.

The reactions of etoposide (VP 16, 4′-demethyl-epipodophyllotoxin ethylidene- β - d -glucoside) and 4′-demethylepipodophyllotoxin (DMEP) with the primary radiolytic products of water, such as e − aq , H and OH/O − radicals, and the secondary radical (SO − 4 ) in aqueous solution were studied by use of the techniques of pulse radiolysis, respectively. The absorption spectra of reaction products with e − aq , H and OH/O − and SO − 4 radicals were observed, and the rate constants of them were determined by following the build-up kinetics of radicals produced or the decay of hydrated electron observed at 600 nm, respectively.

Using 248 nm laser flash photolysis, it has been demonstrated that etoposide (VP16, 4′-demethyl-epipodophyllotoxin ethylidene-β-D-glucoside) in aqueous solution is ionized via a monophotonic process giving a hydrated electron and neutral radical that results from rapid deprotonation of the radical cation of etoposide. The quantum yield of photoionization is determined to be 0.17 at room temperature with KI solution used as a reference. On the basis of comparison of the result with that obtained from pulse radiolysis, the reaction mechanism has been unambiguously identified.

The electron transfer reactions between the trichloromethylperoxyl radical (Cl3COO*) and hydroxycinnamic acid derivatives, including chlorogenic acid, sinapic acid, caffeic acid, ferulic acid and 3,4-(methylenedioxy)cinnamic acid, have been studied by pulse radiolysis. The hydroxycinnamic acid derivatives, especially sinapic acid, are identified as good antioxidants for reduction of Cl3COO* via electron transfer reactions. From buildup kinetic analysis of phenoxyl radical, the rate constant for reaction of Cl3COO* with sinapic acid has been determined to be 8.2x10(7) dm3 mol(-1) s(-1), while the rate constants of electron transfer from other hydroxycinnamic acid derivatives to Cl3COO* were obtained to be about 2x10(7) dm3 mol(-1) s(-1). The reaction of 3,4-(methylenedioxy) cinnamic acid with Cl3COO* was investigated as an evidence for the electron transfer mechanism.