Photoresponsive Polypeptides: Photochromism and Conformation of Poly(l-Glutamic Acid), Containing Spiropyran Units

Spirobenzopyran units were bound to the side chains of poly (L-glutamic acid) and partially methylated poly(L-glutamate)s. The modified polymers were found to exhibit ''reverse photochromism'' in hexafluoro-2-propanol (HFP), so the samples kept in the dark were characterized by an intense absorption band in the visible range of the spectrum, which was completely erased upon exposure to sunlight or irradiation at 500-550 nm. The CD spectra showed that the macromolecules adopted a random coil conformation in the dark, whereas the bleached solutions after exposure to light displayed the typical CD pattern of the alpha-helix. The back reaction in the dark was accompanied by the progressive decrease of the helix content and recovery of the original disordered conformation. The photoinduced conformational changes resulted in large and reversible viscosity variations. When spiropyran side chains were converted to ''spiropyran salts'' of trifluoroacetic acid, the system was still photochromic, but the macromolecules were disordered both in the dark and light conditions. However, when appropriate amounts of methanol were added as a cosolvent to the HFP solutions, the system responded to light, giving reversible variations of the alpha-helix content. Irradiation at appropriate solvent compositions allowed modulation of the extent of the photoresponse