Spectroscopic studies of poly(vinylamine) containing optically active thymine derivatives as grafted pendants

The optical properties of poly(vinylamine) containing optically active (+)‐ or (−)‐2‐(thymin‐1‐yl) propionyl groups as grafted pendants (PT) and the related monomer (MT) and dimer models (DT) were investigated by UV, circular dichroism (CD), and NMR spectroscopy. Highly syndiotactic PT has a smaller hypochromicity versus MT and a larger optical rotation than the less syndiotactic PT in various solutions. These results are attributed to an interaction between the configurational arrangement of thymines, the conformation of the polymers, and base stacking between thymines. The interactions of these polymers with poly(adenylic acid) (polyA) were also studied and the results compared with other vinyl‐type nucleic acid model polymers. The isomers of the optically active dimer models [prepared from meso and (dl)‐2,4‐diaminopentane] were separated. The CD spectra of (+)‐D(−)T in CHCl3 and trifluoroethanol (TFE) displayed extremely solvent‐dependent exciton coupling of the π–π* (B2u) transition of the base chromophore, which was not observed in the other models or polymers, except the meso‐type dimer model (meso)‐D‐(−)T. This coupling decreased with increasing solvent dielectric constant, while UV hypochromicity increased. This behavior as well as the 360‐MHz NMR spectra suggest that (+)‐D(−)T exists in an extended form in solvents of low dielectric constant and gradually assumes a stacked conformation as the dielectric constant increases.