Control by ring substitution of the conformation change dynamics in photochromic polypeptides

In order to develop new systems that couple photochromism with molecular co nformation change, a series of spiropyrans having different ring substituen ts were attached to poly(L-glutamic acid). The polypeptides were dissolved in hexafluoroisopropanol and dark adapted so that the dye was in the merocy anine form. Following adaptation by white light and dye photoconversion fro m the merocyanine to spiropyran forms, polypeptide dark-adaptation kinetics were monitored by circular dichroism (CD) and dye dark adaptation was moni tored by UV/visible, Light adaptation caused a light-induced coil-to-helix transition, with dark adaptation resulting: in relaxation back to the coil. The dark-adaptation rate constant measured by UV/visible was equal to that measured by CD, demonstrating close coupling between dye state and polypep tide conformation. By varying the substituents on the spiropyran ring, dark -adaptation half lives were varied from less than a minute to 2 h, represen ting nearly three orders of magnitude.