Polypeptides containing azobenzene or spiropyran units attached to the macromolecules respond to light or dark conditions giving reversible variations of their structure. In this Account we provide a short overview of current research in the field and describe the most significant experimental examples of photoresponse effects. They include photoinduced random coil/alpha-helix transitions, helix-sense reversal, photostimulated aggregation/disaggregation processes, and photomechanical effects. These fascinating properties suggest that photoresponsive polypeptides may become suitable materials for designing sensors and devices that can be photomodulated. Findings also demonstrate that it is possible to synthesize model systems which respond to light similarly to naturally occurring photoreceptors.

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Phototaxis action spectra have been determined in three different types of Euglena gracilis; green and dark-bleached cells containing the stigma and streptomycin-treated white mutant with an absorptionless stigma. In all three types of microorganisms the flavin containing paraflagellar body is present. The shape of the action spectrum is the same for the three types of cells and proportional to the absorption spectrum of flavoproteins. It has been shown that the structure of the action spectrum does not depend on the presence of screening organelles, on which, instead, depends the direction of the response to the light stimulus. It is concluded that the flavin chromophores present in the photoreceptor are the pigments responsible for phototaxis in Euglena gracilis.

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 × 10-4 g/mL). In the absence of acid, the same polypeptides were found to respond to light, giving reversible coil/α-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/α-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/α-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.