Living photochemical damage

Upon absorption of UV radiation from sunlight the bases can proceed through photochemical reactions that can lead to photodamage in the nucleic acids. Photochemical reactions do occur in the bases, with thymidine dimerization being a primary result, but at low rates. The bases are quite stable to photochemical damage, having efficient ways to dissipate the harmful electronic energy, as indicated by their ultrashort excited state lifetimes. It had been known for years that the excited states were short lived, and that fluorescence quantum yields are very low for all bases [4, 81, 82], Femtosecond laser spectroscopy has, in recent years, enabled a much... 

Thus far in this book we have discussed one- or two-component photochemical systems which because of their relative simplicity lend themselves quite well to laboratory study. Consequently the mechanisms of many of the photoreactions we have discussed have been elucidated in exquisite detail. As we turn our attention in this chapter to some photochemical aspects of living systems, we shall find much more complex situations in which mechanistic details are just now beginning to be obtained. In some systems, such as those which exhibit phototaxis or phototropism, so little is known that our treatment must as a consequence be limited to only a brief discussion of these phenomena. The topics we will consider here are photosynthesis, vision, phototaxis and phototropism, and damage and subsequent repair of damage by light. Due to space limitations, a discussion of the very fascinating area of bioluminescence must be omitted. 

In a different, biological context the photochemical cydo-dimerization of carbonyl-conjugated alkenes is important as a major source of ultraviolet-induced damage to living cells. Thymine (2.77)... 

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