Stacking aggregation and photodimerization of thymine chromophores in aqueous solutions of polythymidylic acid

Heterogeneity of aqueous solutions of polythymidylic acid (poly-T) at room temperature is established by luminescence and absorption spectroscopy. It is found that there exist two fractions of thymine chromophores: (1) a photoactive fraction consisting of two types of stacking aggregates with a structure optimal for photodimerization, the first type having absorption bands at 250 and 280 nm (exciton splitting 4300 cm$^{-1}$), and the second type, at 260 and 290 nm (exciton splitting 4000 cm$^{-1}$), and (2) a photochemically intact fraction with an absorption maximum at 270 nm without evidence of exciton splitting, which probably consists of relatively isolated chromophores.