SUMMARY A conformational transition of a macromolecule is often postulated to occur in biological energy transductions. When +he energy of a photon is transduced, a photochromic ligand mediates in the transfer of energy from the sun to the macromolecule. In this thesis the molecular mechanism of those processes in which this transduction leads to information transmission was investigated by means of model systems. The models consisted of poly(methacrylic acid) (PMA), in aqueous solution, and of water-swollen gels of poly(hydroxy¬ethylmethacrylate) (PHEMA), with several aromatic azo dyes. The influence of light was studied on the potentiometric titration curve of PMA in the presence of a cationic dye, an anionic dye and with covalently linked dyes, and on the length of a complex of the PHEMA gel with an anionic dye. Three different molecular mechanisms were found to be of importance in energy transduction in the models. The creation with light of a dipole on the azo group of the positively charged dye, via electrostatic shielding, influenced the acidity of the poly acid. In a buffered system this pK switch led to a charge in the viscosity and in the swelling of a gel (Chapter 6). Secondly, a change in binding properties of the negatively charged dye to PMA at small degrees of ionization also led to a decrease in the pK (Chapter 5). Finally, a photo-induced change in the water compatibility of the negatively charged Lazo ligand, in the PHEMA gel, was measured as a reversible change in length (Chapter 9). In that case most likely the state of aggregation of the macromolecular matrix is regulated with light. In all three cases the observed effect was amplified by the ability of the macromolecules to undergo a cooperative transition. It is discussed in Chapter 10 that the three mechanisms underlying the reversible photoregulation of the effects demonstrated in this thesis are not limited to those specific -effects. In principle the photoregulation of for example the permeability of polymer membranes is now possible. The relevance of the results of the investigations is evaluated for such biological processes as visual excitation, phyto¬chrome action and enzyme regulation.