Photoresponsive behavior of self-assembling systems by amphiphilic α-helix with azobenzene unit

A photoresponsive amphiphilic polypeptide composed of two amphiphilic alpha-helical polypeptides, poly[(gamma-methyl L-glutamate)-co-(L-glutamic acid)],joined with an azobenzene moiety, was prepared via the monolayer reaction method. We investigated the photoinduced changes in the aggregate structure of the polypeptide in aqueous solution. The polypeptides were soluble in aqueous solution and associated with each other to form a globular aggregate approximate to 10 nm in diameter in the dark. The globular aggregate grew into a twisted ribbon-like structure which could be observed with an optical microscope. The trans-to-cis isomerization of the azobenzene unit in the main chain induced a bending of the polypeptide rod, which resulted in a dissociation of the aggregate. In this process, the degree of disaggregation was not linear to the irradiated light intensity, i.e., the photoinduced effect appeared over a certain value of the light intensity. Such on-off deformations of the supramolecular order may be due to a native character of self-organizing systems supported by weak but cooperative intermolecular interactions. (C) 1998 Elsevier Science S.A.