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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Photochromic vinyl polymers, such as polyacrylates containing spiropyran groups, were found to undergo photoinduced variations of their viscosity.1"4 Since the viscosity of a polymer system is

Poly(L-lysine) was reacted with spiropyran reagents to give polypeptides containing 46 and 25 mol % spiropyran units in the side chains. The resulting modified polymers were soluble in hexafluoro-2-propanol (HFP), where they exhibited reverse photochromism. In pure HFP, the photochromic polymers were random coils and the structure was not affected by the photoisomerization of the side chains. When appropriate amounts of cosolvents such as triethylamine or methanol were added to the HFP solutions, exposure to visible light (500 nm) or to sunlight for a few seconds produced the coil --helix transitions of the macromolecular chains. The back-reaction in the dark was accompanied by the progressive decrease of the -helix content and recovery of the original disordered conformation. CD measurements showed that the photoisomerization of the photochromic side chains was able to trigger the coilhelix transition of the macromolecules only in a very narrow range of HFP/NEts or HFP/MeOH solvent composition; therefore, spiropyran-modified poly(L-lysine) can be considered a typical example of photoresponsive systems having a "gated photoresponse".

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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.

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