MECHANISM OF THE PHOTOHYDRATION OF PYRIMIDINES: A FLASH PHOTOLYSIS STUDY

Disclosing the Role of C4-Oxo Substitution in the Photochemistry of DNA and RNA Pyrimidine Monomers: Formation of Photoproducts from the Vibrationally Excited Ground State

Oxo and amino substituted purines and pyrimidines have been suggested as protonucleobases participating in ancient pre-RNA forms. Considering electromagnetic radiation as a key environmental selection pressure on early Earth, the investigation of the photophysics of modified nucleobases is crucial to determine their viability as nucleobases' ancestors and to understand the factors that rule the photostability of natural nucleobases. In this Letter, we combine femtosecond transient absorption spectroscopy and quantum mechanical simulations to reveal the photochemistry of 4-pyrimidinone, a close relative of uracil. Irradiation of 4-pyrimidinone with ultraviolet radiation populates the S₁(ππ*) state, which decays to the vibrationally excited ground state in a few hundred femtoseconds. Analysis of the postirradiated sample in water reveals the formation of a 6-hydroxy-5H-photohydrate and 3-(N-(iminomethyl)imino)propanoic acid as the primary photoproducts. 3-(N-(Iminomethyl)imino)propanoic acid originates from the hydrolysis of an unstable ketene species generated from the C4-N3 photofragmentation of the pyrimidine core.

Characterization of intensity-dependent optical rotation phenomena in chiral molecules in solution

The rotation of the plane of polarization of linearly polarized light by chiral molecules in solution is due to a forward scattering event. Ordinary optical rotation, a single-photon effect, is independent of intensity. As the light intensity is increased, other effects can appear, such as two-photon scattering or alignment of the molecule by one photon and scattering with a change of polarization by another. Both of these effects result in intensity-dependent (or nonlinear) optical rotation. A polarimeter was used to measure the nonlinear optical rotation of solutions in a heterodyne experiment. No nonlinear optical rotation was found in molecules lacking an absorption band near the laser frequency. In the three pyrimidine nucleosides studied, which do have such an absorption band, a nonlinear optical rotation was identified that was cumulative with each laser pulse. The effect persisted with a time constant that was on the order of seconds and characteristic of the molecule.

Radiobiology and photobiology on Earth and in space: points of encounter and protection considerations

All radiations originate in space, and the spectrum of radiations reaching the troposphere is limited only because of their range and absorption by the ozone layer above the atmosphere. Ultraviolet-C and the very heavy ions are therefore produced on earth only artificially, by special lamps and in accelerators. The range of biological effects of the different UV radiations and low and high LET radiations have been studied extensively, yet only recently new facts such as the production of DNA strand breaks by long wave UV light were established, adding to the various points of encounter existing between ionizing and nonionizing radiations. There are some similarities in radiation products, and the resulting effects of insult by radiation on biological systems very often are similar, if not the same. A common phenomenon that exists in all healthy biological cells is the ability to repair damage to DNA and thus either survive or mutate, and although the specific mechanisms of repair are somewhat different, the end result is the same. Recently a mechanism of improved radioprotection was found to involve an effect of certain radioprotective compounds on DNA repair. It is suggested that improved, and nontoxic, modes of protection may be offered by employing such compounds as biological response modifiers and natural substances. Further research is needed and is under way.

A 5-4 pyrimidine-pyrimidone photoproduct produced from mixtures of thymine and 4-thiouridine irradiated with 334 nm light

The nucleoside 4-thiouridine, present in some bacterial tRNA species, is known to be a chromophore and a target for near-UV light-induced growth delay and also mediates both photoprotection and near-UV cell killing in various bacterial strains. To investigate the photoreaction of 4-thiouridine with DNA or its precursors, we irradiated aqueous mixtures of thymine and 4-thiouridine with 334 nm light and then separated photoproducts using two or more stages of reversed-phase high performance liquid chromatography. The two equally abundant major photoproducts were analyzed by UV absorbance spectrophotometry, fast-atom bombardment and electron-impact mass spectrometry, and 1H- and 13C-NMR spectroscopy, and have been identified as two diastereomers of 6-hydroxy-5-[1-(beta-D-erythro-pentofuranosyl)-4'-pyrimidin-2'- one]dihydrothymine (O6hThy[5-4]Pdo), of molecular weight = 370.32. These two diastereomers, although stable at room temperature or below, are interconvertible by heating (90 degrees C for 5 min) in aqueous solution. The possible biological significance of this photoproduct is discussed, and an application as a crosslinker for oligonucleotides to selectively block replication is suggested.

Transients of uracil and thymine derivatives and the quantum yields of electron ejection and intersystem crossing upon 20 ns photolysis at 248 nm

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.