[Photochemical revelation of the stacking aggregation of thymine chromophores in water solutions of polythymidylic acid].

The structure heterogeneity of water solutions of polyribothymidylic acid at T(room) was studied from changes caused in their absorption spectra by the photodimerization reaction. Three fractions of thymine chromophores were revealed from the differential absorption spectra: (a) the main fraction consisting of weakly interacting (isolated chromophores) chromophores with the absorption spectrum maximum at approximately 270 nm; (b) pair chromophores of the first type with the absorption spectrum maxima at 260 and 290 nm (exciton splitting 4000 cm(-1)); and (c) pair chromophores of the second type with the absorption spectrum maxima at 250 and 280 nm (exciton splitting 4300 cm(-1)). The revealed aggregates have a relatively high photochemical activity in the photodimerization reaction in comparison with the isolated chromophores. They contribute little to the total absorption spectrum of solutions but make a great contribution to its changes at the initial stages of the UV irradiation of solutions.