Photoresponsive Polypeptides. Photomodulation of the Macromolecular Structure in Poly(Nε((phenylazophenyl)sulfonyl)-l-lysine)

Abstract

Poly(l-lysine) was reacted with p-phenylazobenzenesulfonyl chloride to give polypeptides containing azobenzene units linked to the Lys chains by means of sulfonamide functions. The azo-modified polymers were soluble in hexafluoro-2-propanol (HFP), in which they exhibited photochromism due to the trans/cis photoisomerization of the azobenzene units. The sulfonate azobenzene moieties were found to be thermally stable at room temperature, so that interconversions between the two isomers were achieved only photochemically, by irradiating at 340 and 417 nm, respectively. Previously reported poly(Nε-(phenylazobenzoyl)-l-lysine) was found to adopt the α-helix structure in HFP and did not give photoinduced conformational changes. For poly(Nε-((phenylazophenyl)sulfonyl)-l-lysine), by contrast, CD spectra showed that the macromolecules adopted a disordered structure in HFP, when azo units were either in trans or in cis configuration. However, when appropriate amounts of methanol or 1,2-dichloroethane were added to the HFP solutions, irradiation at 340 and 417 nm, alternately, produced reversible formation of a helical and random coil structure of the macromolecules. The photostimulated structural changes have been discussed on the basis of the interactions between HFP and azobenzenesulfonyl groups, which seem to be different depending on whether the azo units are in trans or in cis configuration.