PHOTORESPONSIVE POLYMERS - PHOTOMODULATION OF THE MACROMOLECULAR STRUCTURE IN POLY(L-LYSINE) CONTAINING SPIROPYRAN UNITS

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 --> alpha-helix transitions of the macromolecular chains. The back-reaction in the dark was accompanied by the progressive decrease of the a-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 coil --> helix transition of the macromolecules only in a very narrow range of HFP/NEt(3) or HFP/MeOH solvent composition; therefore, spiropyran-modified poly(L-lysine) can be considered a typical example of photoresponsive systems having a ''gated photoresponse''.