DNA monomeric units in solution
In parallel with our studies of model helices we have measured, by femtosecond UV fluorescence spectroscopy , the fluorescence lifetimes and anisotropy decays of all the bases, the nucleosides and the nucleotides in aqueous solution, with the exception of guanine which is not sufficiently soluble. In general, the fluorescence decays of the monomeric DNA components are very fast (< 1 ps ). The observed deactivation rates do not depend on the wavelength and decrease slightly when going from the base to nucleotide. Due to the time-resolution of our setup, we have found that, contrary to what was reported in the literature, the fluorescence decays cannot be described correctly by mono-exponential functions. This shows that the relaxation processes occurring in the excited state(s) are complex. A significant part of total fluorescence (up to 75 % for dA and dAMP) decays at times faster than our temporal resolution (< 100 fs).