High molecular weight poly(L-glutamic acid) was chemically modified with a spiropyran reagent to give polypeptides containing 35 and 85 mol % spiropyran units in the side chains. To investigate the photochromic and conformational behavior in acid conditions, the polypeptides were dissolved in hexafluoro-2-propanol (HFP) and a small amount of trifluoroacetic acid was added (TFA, c = 5 x 10(-4) g/mL). In the absence of acid, the same polypeptides were found to respond to light, giving reversible coil/alpha-helix transitions. In the presence of TFA, by contrast, photoisomerization of the chromophores in the side chains did not; result in any conformational variation of the macromolecular main chains, and CD spectra showed that the macromolecules adopted a random coil structure, both in the dark and after light exposure. However, when appropriate amounts of cosolvents such as methanol (MeOH) or trifluoroethanol (TFE) were added to the HFP/TFA solutions, the system again responded to light giving reversible coil/alpha-helix transitions of the macromolecular structure. The extent of the photoinduced conformational changes depended on solvent composition and the photoresponse could be modulated by the combined action of light; and chemical environment. For HFP/MeOH solvent mixture, it is possible to trigger the photostimulated coil/alpha-helix transition in a narrow window of solvent composition, so that the system is characterized by a gated photoresponse. The molecular processes responsible for the observed photoinduced conformational changes are discussed on the basis of absorption and fluorescence results. These results also allow us to set up a detailed picture of the molecular mechanisms responsible for the described chemo-modulated photosensory system.