Photoregulation of polymer conformation by photochromic moieties. I. Anionic ligand to an anionic polymer

Abstract— Photoregulated biological processes appear to make use of membrane-bound photochromic macromolecules. In order to elucidate various physicochemical pathways by which these processes can be triggered, model studies have been undertaken employing photochromic moieties bound to synthetic macromolecules with a labile fold structure. Inspired by Lovrien's 1967 work, attention was first focused on the anionic dis-azo stilbene dye chrysophenine (CHP) and poly(methacrylic acid) (PMA) in aqueous solution. This ligand is found to bind to and unfold PMA only if the degree of ionization of the polymer is below 0·75. Viscosity as well as equilibrium dialysis data indicate that maximally one CHP per ten monomer units PMA is bound. The apparent degree of ionization conferred to PMA by the ionic CHP ligand leading to polymer unfolding is the same as the known real degree of ionization leading to polymer unfolding in the absence of CHP. Upon trans→cis photoisomerization, the ligand either desorbs or creates a higher local dielectric constant because of the large cis-azo dipole moments. As a result some refolding to a more compact hydrodynamic volume occurs, as deduced from the viscosity measurements at a low degree of ionization. Simultaneously a lowering of the pK_app by 0·1 pK unit is observed at degrees of ionization below 0·075. Photoregulation of conformation as well as of pK_app indicates two possible pathways for the regulation of ionic fluxes such as have been postulated for photobiological transducers.