Novel photoresponsive p-phenylazobenzene derivative of an elastin-like polymer with enhanced control of azobenzene content and without pH sensitiveness

In the past years, photochromic derivatives of elastin-like polymers have proved to be an attractive alternative to other polypeptide and non-polypeptide photoresponsive polymers because they show high efficiency and display their properties in water solutions. Up to date, the chromophores have been conjugated to elastin-like polymers exclusively by amidation of carboxylic groups present in the polymer chains. However, the control of the number of attached chromophores and their intrinsic high sensibility to pH caused by unreacted COOH groups remain as main drawbacks of these polymers. A novel approach has been used in this work to synthesize an azobenzene derivative of poly(VPGVG), the copolymer poly[f(V)(VPGVG), f(X)(VPGXG)] (X = L-p-(phenylazo)phenylalanine; f(V) and f(X) = mole fractions), by introducing the azobenzene moiety by direct synthesis and not by conjugation in one of the two comonomers prior to polymerization. In this way, the azobenzene content was reasonably controlled by the comonomer ratio in the polymerization reaction. In addition, the resulting copolymer has not pH-sensitive groups such as carboxylic or amino groups in the amino acid side chains. Different compositions were assayed. Those above f(X) = 0.20 were water-insoluble, and those below f(X) = 0.15 showed poor photoresponsiveness. The optimum was established at fx = 0.15 in our experimental setup, which corresponds to 3 azobenzene groups per 100 amino acid residues in the polymer chain.