TRANSIENTS OF URACIL and THYMINE DERIVATIVES and THE QUANTUM YIELDS OF ELECTRON EJECTION and INTERSYSTEM CROSSING UPON 20 ns PHOTOLYSIS AT 248 nm

Abstract

Transients of uracil and a series of 17 correlated pyrimidines, e.g. methylated bases, nucleosides, nucleotides, and polyuridylic acid [poly(U)] were studied after 248 nm excitation by 20 ns laser pulses. The transient absorption spectra in aqueous solution at room temperature reveal the triplet state and the hydrated electron (e-aq), while the corresponding radical cation could not be observed at pH 6-7. Fast loss of the chromophore in the 260-290 nm range within 0.1 microsecond was observed in aqueous solution in some cases [e.g. poly(U), 5'-UMP, uridine, uracil] and in others (thymine, thymidine) virtually not. This photobleaching is assigned to formation of the photohydrate. The concentration of e-aq shows a quadratic dependence on the laser pulse intensity (IL) in the range (0.2-2) x 10(7) W cm-2 and the quantum yield of electron ejection (phi c-) thus depends linearly on IL. This behaviour, suggesting that the photoionization involves a two-step absorption process, was found for poly(U) and all pyrimidine monomers examined. At a constant IL value of 2 x 10(7) W cm-2, phi c- ranges from 3 x 10(-3) for 1,3-dimethylthymine to 4 x 10(-2) for poly(U). The triplet state shows a much larger transient absorbance (delta A, typically in a broad range, e.g. 290-500 nm) than that of the neutral radical resulting from the radical cation. The triplet state in organic solvents (acetonitrile and ethanol) shows generally a significantly larger delta A value than in aqueous solution. The estimated quantum yields of intersystem crossing at room temperature are compared with those of phosphorescence at -196 degrees C.

Photoionization of DNA and RNA Bases, Nucleosides and Nucleotides Through a Combination of One- and Two-photon Pathways upon 266 nm Nanosecond Laser Excitation¶

“…Understanding these pathways would assist in predicting how low‐intensity UV radiation damages DNA and RNA in living organisms. Monophotonic ionization of DNA bases has been confirmed experimentally for 193 nm laser pulses in aqueous solution (7–11).…”

“…We also investigated the effect of the substituent at pyrimidine N1 and purine N9 on PI. Substituents have been found to affect the photophysical and photochemical properties of the DNA bases (8,19–21). The lifetimes of electron and triplet states, the relative yields of triplet formation and the quantum yields of hydrated electrons of some uracil and thymine (Thy) derivatives in aqueous and organic solutions are dependent on the substituents and on their position of attachment (8).…”

“…Substituents have been found to affect the photophysical and photochemical properties of the DNA bases (8,19–21). The lifetimes of electron and triplet states, the relative yields of triplet formation and the quantum yields of hydrated electrons of some uracil and thymine (Thy) derivatives in aqueous and organic solutions are dependent on the substituents and on their position of attachment (8). UV photoelectron studies have shown that a sugar substituent at pyrimidine N1 and purine N9 causes a reduction of the vertical ionization potential (IP) of the free nucleotide, which sometimes exceeds 0.6 eV (20).…”

LASER‐INDUCED STRAND BREAK FORMATION IN DNA and POLYNUCLEOTIDES

“…From the initial linear increase of Qe-vs intensity (at low values) it has been concluded that the electrons and respective base radical cations are produced by consecutive biphotonic excitation Opitz and SchulteFrohlinde, 1987;Wala et al, 1990;Gorner, 1990a). Also for RNA and DNA Nikogosyan (1990) and his group have shown that Qe-increases with increasing ps laser pulse intensities and reaches a maximum.…”

“…Some values for GC-of nucleic acids are compiled in Table 1. The absorption of the resulting radical cations has not yet been observed with nucleic acids and their constituents since either the extinction coefficients are too low or the lifetimes of the radical cations under those conditions are too short (Wala et at., 1990;Bothe et al, 1990;Gorner, 1990a).…”