Photoresponsive peptide and polypeptide systems: part II. Photochromism of poly(l‐ornithine) containing various azo‐contents in side chains

Abstract

Abstract Poly(L‐ornithine)s having various azo‐contents in the side chains were synthesized by the water‐soluble carbodiimide procedure. The photochemical properties of the polypeptides poly[ N δ ‐ p ‐(phenylazo)benzoyl‐L‐ornithine] (PPABLO) containing 3–77 mol% azobenzene were investigated by absorption and circular dichroism spectroscopy in hexafluoro‐2‐propanol (HFIP) or water, and in HFIP‐water or methanol‐water solvent mixtures. The photochromism of the dichroic bands of the PPABLOs containing 20–77 mol% azobenzene in the visible and ultraviolet wavelength regions was found to be mostly reversible as a function of irradiation time at different wavelengths due to the photostationary state (above 80% trans‐cis photoisomerization) of the azo aromatic moieties. The PPABLO containing 3.2 mol% azobenzene in water exhibited conformational changes from random coil to helix by the addition of methanol or sodium dodecyl sulphate (SDS). The photo‐induced conformational change was observed in HFIP‐water‐SDS solvent mixtures, while no conformational change was seen in water and HFIP‐water solvent mixtures.