The effect of mercuric ions on the excited states of DNA-intercalated ethidium bromide.

The first excited triplet state of DNA-intercalated ethidium bromide is produced with a quantum yield of 0.01 +/- 0.002 on irradiation at 532 nm. A difference extinction coefficient of 1.5 +/- 0.2 x 10(3) m2 mol-1 is measured for the triplet state at 380 nm. Mercuric ions quench the first excited singlet state of DNA-intercalated ethidium bromide via induced spin orbit coupling to give an increased yield of ethidium triplet states. The same mercuric ion that quenches the singlet state then quenches the triplet state, via the same mechanism, with a rate constant of ca 3.5 x 10(3) s-1. An upper limit for the rate of detachment of Hg2+ from its binding site in DNA may be fixed at ca 10(3) s-1.

The reactivity of various free radicals with hyaluronic acid: steady-state and pulse radiolysis studies.

The reactions of the primary water radicals with the biopolymer hyaluronic acid have been studied by pulse radiolysis. Bimolecular rate constants, expressed in terms of the disaccharide repeating sub-unit of hyaluronic acid, for OH., H. and eaq- were found to be 7 X 10(8) M-1 X s-1, 5 X 10(7) M-1 X s-1 and less than 5 X 10(6) M-1 X s-1, respectively. By comparing the viscosities of samples, gamma-irradiated in the steady state under a variety of conditions, with unirradiated controls, the efficiencies with which selected radicals cause chain breakage have been determined. Efficiencies of 30%, 15%, 0%, 0.2% and 5% were estimated for OH., H., eaq-, methanol radicals and tert-butanol radicals, respectively. The presence of oxygen during irradiation increased the extent of chain breakage by a factor of 1.75.

Radiation sensitivity of DNA-metal complexes: a pulse radiolysis study.

The effect of silver(I) and mercury(II) complexing upon the rate of reaction of both the hydrated electron and the OH radical with DNA has been investigated using the technique of pulse radiolysis. The results presented do not support the hypothesis that the mechanism of radiation sensitivity by these metals is mediated through increased rate of free radical attack upon DNA. Also, there is evidence presented that neither metal, when complexed with DNA, reacts at a significant rate with the free radical species e-aq and OH. The technique of pulse radiolysis was also used to study the complexation of both silver(I) and mercury(II) with DNA (calf-thymus, 40 per cent GC). Scatchard plots, derived from the experimental data, gave association constants of 1.85 x 10(5) dm3 mol-1 and 9.9 x 10(4) dm3 mol-1 and n values (number of complexing sites per nucleotide) of 0.71 and 0.31 for silver(I) and mercury(II) respectively.

The reactivities of furocoumarin excited states with DNA in solution. A laser flash photolysis and fluorescence study.

The effect of DNA on both the fluorescence emission spectra and yields and lifetimes of the triplet stae of psoralen and 8-methoxypsoralen in aqueous solution has been determined. The changes in the fluorescence spectra are similar in nature for both of these furocoumarins and are attributed to binding of the drug to DNA. The yield of the 8-methoxypsoralen triplet state when bound to DNA was found to be similar, if not identical, to that measured in the absence of DNA. This contrasts sharply with data obtained for psoralen from which it is concluded that either the yield of bound psoralen triplet states is very low, if not zero, or that the lifetime of such species is less than 50 ns. The relevance of this data to the molecular basis of skin photosensitisation by furocoumarins is discussed.

A pulse radiolysis study of the complexing of furocoumarins with DNA and proteins.

The complexing of furocoumarins with DNA in aqueous solution has been investigated using the fast-reaction technique of pulse radiolysis. Association constants are given for 8-methoxypsoralen, psoralen, khellin, angelicin, 5,7-dimethoxycoumarin, 5-methoxypsoralen and 4'-aminomethyl-4,5',8-trimethylpsoralen. In addition, the complexing of some furocoumarins to the protein, bovine serum albumin, has also been studied.

A laser flash photolysis study of the reactivities of the triplet states of 8-methoxypsoralen and 4,5',8-trimethylpsoralen with nucleic acid bases in solution.

The extinction coefficients, quantum yields and reactivities of the triplet states of 8-methoxypsoralen and 4,5',8-trimethylpsoralen in methanolic solution have been determined using laser flash photolysis techniques. The second-order rate constants for the quenching of these triplet states by pyrimidine and purine bases were found to be several orders of magnitude lower than those found for other furocoumarin derivatives. This may suggest, therefore, that the skin photosensitising ability of such compounds does not necessarily correlate with in vitro triplet state reactivity. Preliminary experiments on the reactivity of the psoralen triplet state with DNA itself indicate that no transient absorptions due to psoralen excited states can be observed when a photon is absorbed by the psoralen-DNA complex.