Induced optical activity of chondroitin sulfate C--acridine orange complexes

The vibronic absorption spectra and electronic states of acridine orange in aqueous solution

The time-dependent density functional theory (TD-DFT) was used to obtain vibronic absorption spectra of acridine orange dye (AO) in an aqueous solution that were in good agreement with the experiment. The protonated and neutral forms of the dye have been investigated. The results of calculations using various functionals and basis sets have been analyzed. The best agreement with experiment was given by the level of theory X3LYP/6-31G(d,p). AO molecular orbitals involved in electronic transitions due light absorption in the visible region of the spectrum have been obtained. The dipole moments and atomic charges of the ground and excited states of the AO molecule have been calculated. Maps of the electrostatic potential have been drawn. An insignificant photoinduced electron transfer was found in the central ring of the chromophore of the dye molecule. According to our calculations, the vibronic coupling and the Boltzmann distribution play a significant role in the absorption spectra of the AO.

Fragmentation and modification of iota-carrageenan and characterisation of the polysaccharide order-disorder transition in solution

A procedure is described in detail by which the algal polysaccharide, iota-carrageenan, can be modified and fragmented to products of more regular structure and shorter chain length. Since most chains of this product can form uninterrupted helices they undergo the disorder-order transition without the gelation that is a feature of the behaviour of the natural polymer. The disorder-order transition becomes much sharper and occurs at higher temperatures with increasing chain length. The sign and magnitude of the accompanying shift in optical rotation can be estimated satisfactorily from the solid-state geometry of the helix using our semi-empirical method for correlating optical rotation with the conformations at glycosidic linkages. In solutions of the helical form, but not the coil form, cationic dyes such as methylene blue or acridine orange show induced optical activity and a changed visible spectrum which we attribute to binding and stacking on the helix. This also causes a shift in the temperature of the helix-coil transition. We discuss the possibility that the tendency of dyes to stack could drive polysaccharides into conformations that they would not otherwise adopt, with implications for their use as 'conformational probes'.