Photochemical genetics

Photochemical Genetics. I. The Ionic Nature of Uracil Photohydration... 

Photochemical Genetics. II. The Kinetic Role of Water in the Photohydration of Uracil and 1,3-Dimethyluracil... 

We present experimental results on photophysical deactivation pathways of uracil and thymine bases in the gas phase and in solvent/solute complexes. After photoexcitation to the S2 state, a bare molecule is tunneled into and trapped in a dark state with a lifetime of tens to hundreds of nanoseconds. The nature of this dark state is most likely a low lying nn state. Solvent molecules affect the decay pathways by increasing IC from the S2 to the dark state and then further to the ground state, or directly from S2 to S0. The lifetimes of the S2 state and the dark state are both decreased with the addition of only one or two water molecules. When more than four water molecules are attached, the photophysics of these hydrated clusters rapidly approaches that in the condensed phase. This model is now confirmed from other gas phase and liquid phase experiments, as well as from theoretical calculations. This result offers a new interpretation on the origin of the photostability of nucleic acid bases. Although we believe photochemical stability is a major natural selective force, the reason that the nucleic acid bases have been chosen is not because of their intrinsic stability. Rather, it is the stability of the overall system, with a significant contribution from the environment, that has allowed the carriers of the genetic code to survive, accumulate, and eventually evolve into life s complicated form. 

McHugh PJ, Knowland J. Characterization of DNA damage inflicted by free radicals from a mutagenic sunscreen ingredient and its location using an in vitro genetic reversion assay. Photochem Photobiol 1997 66(2) 276—81. 

Naim, R.S., D.C. Morizot, S. Kazianis, A.D. Woodhead and R.B. Setlow. Nonmammalian models for sunlight carcinogenesis genetic analysis of melanoma formation in Xiphophorus hybrid fish. Photochem. Photobiol. 64 440-448, 1996. 

Bilger W and Bjorkman O (1991) Temperature dependence of violaxanthin de-epoxidation and non-photochemical fluorescence quenching in intact leaves of Gossypium hirsutum L. and Malva parviflom L. Planta 184 226-234 Bilger W, Fisahn J, Brummet W, Kossmann J and Willmitzer L (1995) Violaxanthin cycle pigment contents in potato and tobacco plants with genetically reduced photosynthetic edacity. Plant Physiol 108 1479-1486... 

Fig. 9. DIS2 fluorescence image of a single petri dish. The plate is identical to the one in Fig. 8, but the instrument has been reconfigured for fluorescence imaging. Broadband blue-green light has been used for illumination, and the camera lens has been covered with an 865 nm long-pass filter. LHII is normally fluorescent in this bacterial strain because the reaction center (photochemical trap) has been genetically deleted. The same five colonies have been expanded below note that the colony on the right is highly fluorescent.

Light-minus-dark RC difference spectra under continuous actinic illumination show that only the Trp 2 °- Phe RCs are photochemically active (as indicated by partial bleaching of the 850 nm absorption maximum of the special pair (summarized in Table 1)). But addition of exogenous quinone (1 mM 2-methyl-1,4-naphthoquinone, MKq) restores complete bleaching in all the genetically modified RCs under these conditions. Hence, loss of quinone from the binding site appears to be responsible for the lack of photochemical activity in the untreated samples. 

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