Absorption of UV radiation by DNA: spatial and temporal features

The present review focuses on studies carried out by our group on the interaction of UV radiation with DNA. In particular, we examine the way that the energy acquired by DNA helices following direct absorption of UVC radiation is extended spatially and how its effects evolve during the time. These effects depend on the base sequence and can be revealed by the study of model helices. The experimental results were obtained by optical spectroscopy, used in a refined way which allows detection of very weak absorbance changes (10(-3)) as well as of intrinsic emission from DNA components whose fluorescence quantum yields are as low as 10(-4). Measurements were performed both under continuous irradiation and using pulsed excitation which permitted us to follow early events, occurring from 10(-14) to 10(-1)s. The experiments were guided by theoretical calculations. The spatial features concern the extent of the excited states formed immediately upon UV absorption; these were shown to be delocalized over several bases under the effect of electronic coupling. Moreover, thanks to the spectral fingerprints governed by the electronic coupling; we probed local denaturation induced on a double helix following formation of cyclobutane dimers. Regarding the temporal features, three different topics are presented: (i) ultrafast excitation energy transfer occurring among the bases in less than 100 fs, (ii) electron ejection from DNA upon absorption of one photon at 266 nm and (iii) formation of (6-4) photo-adducts involving a reaction intermediate. The most important message emerging from these studies is that DNA bases may adopt a collective behaviour versus UV radiation. Furthermore, time-resolved studies unravel processes which are undetectable by investigations using continuous irradiation. All these pieces of information change our understanding of how DNA damage occurs upon absorption of UV radiation.