Photoresponsive peptide and polypeptide systems: 10. Synthesis and reversible photochromism of azo aromatic poly(L-alpha,beta-diaminopropionic acid).

Poly-L-alpha,beta-diaminopropionic acid) having azo aromatic side chain was synthesized by the water-soluble carbodiimide procedure. The photochemical properties of the azo polypeptide poly[N beta-p-(phenylazo)benzoyl-L-alpha,beta-diaminopropionic acid] (PPABLDPA) was investigated by absorption and circular dichroism (c.d.) spectroscopy in hexafluoro-2-propanol (HFIP) and dimethylformamide. The photochromism of the absorption band 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 (88% trans)-cis photoisomerization of the azo aromatic moieties. The c.d. spectra exhibited two and three-stage photochromism on irradiation by light. The reversible photo-induced solubility change was also studied. On irradiation PPABLDPA is soluble under ultraviolet light (cis) and precipitates under visible light (88% trans) in HFIP-water. A discussion was presented that includes our previous results on this azo aromatic polylysine homologue series.