Photoresponsive peptide and polypeptide systems IV: Light-induced conformational changes in ϵ-poly(l-lysine) wit photochromic side chains

Abstract

ϵ(Poly(l-lysine) containing a high mole percentage of azo aromatic side chains was synthesized by the active ester method. The photochemical properties of the azo polypeptide, ϵ-poly(N^α-phenylazobenzoyl-l-lysine) (ϵ-PPABLL), were investigated by absorption and circular dichroism (CD) spectroscopy in hexafluoro-2-propanol (HFIP). The photochromism of the absorption band in the near UV and visible wavelength region was found to be mostly reversible as a function of irradiation time at different wavelengths (360 nm and 460 nm) owing to the trans—cis photoisomerization of the azo aromatic moieties. The CD spectrum exhibited multistage photochromism on irradiation by light. The whole CD spectrum, especially at the dichroic bands at 300 nm (positive), 340 nm (negative) and 365 nm (trans, positive), showed a peculiar reversible photochromism on irradiation by 360 nm and 460 nm light, except the first irradiation time course. The position and magnitude of the negative dichroic band at 225 nm suggested that the backbone conformational change of ϵ-PPABLL was a light-induced partial reversible transition from a β-form-rich structure to a random-coil-rich structure.