Radiation-induced luminescence in DNA: evidence for long-range electron migration.

The radiation-induced "in-pulse" luminescence emission from solid DNA containing either metronidazole or a highly electron-affinic 5-nitrofuran in the range 3-2000 (w:w) base pairs per additive molecule has been investigated in vacuo at 293 K using electron pulses of energy below 260 keV. The luminescence intensity at 450 nm from DNA decreases with increasing content of the additive in the sample and approaches a limiting level at high concentrations of the additives. At these higher concentrations the limiting value represents about 50% of that observed from DNA alone. It is shown that the efficiency of the additives in reducing the luminescence intensity is dependent upon their redox potential E1(7); this dependence is consistent with these additives acting as electron acceptors. It is concluded that the ability of the electron acceptors to reduce the luminescence is related to the electron affinity of E1(7) of the acceptors and electron migration distances of at least 300 base pairs are proposed.

Radiation-induced energy migration within solid DNA: the role of misonidazole as an electron trap.

The "in-pulse" luminescence emission from solid DNA produced upon irradiation with electron pulses of energy below 260 keV has been investigated in vacuo at 293 K to gain an insight into the existence of radiation-induced charge/energy migration within DNA. The DNA samples contained misonidazole in the range 3 to 330 base pairs per misonidazole molecule. Under these conditions greater than 90% of the total energy is deposited in the DNA. The in-pulse radiation-induced luminescence spectrum of DNA was found to be critically dependent upon the misonidazole content of DNA. The luminescence intensity from the mixtures decreases with increasing content of misonidazole, and at the highest concentration, the intensity at 550 nm is reduced to 50% of that from DNA only. In the presence of 1 atm of oxygen, the observed emission intensity from DNA in the wavelength region 350-575 was reduced by 35-40% compared to that from DNA in vacuo. It is concluded that electron migration can occur in solid mixtures of DNA over a distance of up to about 100 base pairs.

One-electron oxidation of C(2) and C(3) methyl substituted 5,6-dihydroxyindoles: model pathways of melanogenesis.

One-electron oxidation of a series of C(2)- and C(3)-methyl substituted analogues of 5,6-dihydroxyindole (DHI), in the pH range 5-9, was studied using the technique of pulse radiolysis with spectrophotometric detection. This investigation was undertaken to further our understanding of the involvement of free radical species in the polymerization processes leading to melanin formation. The optical absorption spectra of the protonated indole semiquinone radicals resulting from the one-electron oxidation of C(2)- and C(3)-methyl substituted indoles were similar to those of their corresponding hydroxylated indoles. From this similarity, it is inferred that methylation at C(2) and C(3) of DHI has little or no effect upon the initial radicals. The semiquinone radicals of these analogues subsequently decay to yield the corresponding quinone methide/imine. However, methyl substitution at C(2) and C(3) of the quinone methide derived from these analogues results in their stabilization. This stabilization contracts with the reactivity of the corresponding quinone methide from DHI. Further, these stabilized quinone methides do not interact with the azide ion (N3-), in contrast to the reaction of N3- with the quinone methide of DHI. It is concluded that methylation at C(2) and C(3) of DHI will modify the pathways so that the polymerization processes are less effective than those with DHI.

Diffuse reflectance pulse radiolysis of solid DNA: the effect of hydration.

Using diffuse reflectance pulse radiolysis, it has been demonstrated from spectral characteristics of the resulting transients that the chemical events following irradiation of DNA depend upon its state of hydration.