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The ion permeabilities of solid membranes of poly(L-glutamic acid) with azobenzene groups in the polymer side chains can be reversibly controlled by photo-irradiation.

ABSTRACT:Photoresponsive polypeptides, poly(L-glutamic acid) containing 1.9, 14.1, 46.3 mo!% azobenzenesulfonate moieties in the side chains (azo·S-PGA), have been prepared by the condensation reaction of poly(L-glutamic acid) (PGA, M,. = I.I 9 x 10 5 ) with 4-amino-l, I' -azobenzene-4' -sulfonic acid sodium salt in dimethylformamide (DMF) solution and the membranes were obtained by casting their DMF solution with 4,4'-diaminodiphenylmethane as a cross-linking agent. trans to cis Isomerization of the azobenzenesulfonate moieties induced by ultraviolet (UV) light resulted in oc-helix to coil transition in the membrane composed of azo · S-PGA containing 14.1 mol% azobenzenesulfonate moieties at adequate pHs, indicating the electrostatic repulsion between the sulfonate anions in the azo photochromic side chains and the neighboring glutamate anions to be enhanced by photoinduced changes in the geometry of the azo chromophores. As a result, the membrane potentials of azo·S-PGA membrane containing 14.1 mo!% azobenzenesulfonate moieties were strongly dependent on UV light irradiation. It was also found that photoinduced changes of the membrane structure and membrane potentials were irreversible. On the other hand, the structure of the azo · S-PGA membrane containing 1.9 and 46.3 mo!% azobenzenesulfonate moieties was shown to be independent of UV light irradiation at any pH. The former ineffectiveness of the light irradiation was attributed to small amounts of azo photochromic moieties and the latter to the fact that the azo · S-PGA membranes with large amounts of azobenzenesulfonate moieties are in random coil structures at any pH even in the drk.KEY WORDS Photoresponsive Polypeptide / Azobenzenesulfonate Side Chain/ Membrane/ Photoinduced Conformational Change/ Photoinduced Membrane Potential Change / It has been shown, as an example of the photoregulated function of artificial membranes, that the photochemical reaction of photochromic molecules, such as azobenzene, spiropyran and other dyes, entrapped in an inert membrane matrix can induce significant changes in membrane potentials across cellulose1 -4 and poly(vinyl chloride) 5 -s membranes. It has been reported, furthermore, that the membrane potential can be also photoregulated by using photoresponsive polymers having such photochromic molecules incorporated as pendant groups in the polymer chains. For example, Irie et al.,9 showed that visible light irradiation induced reversible changes in the potential acrnss a membrane composed of poly(methyacry\ic acid) containing spirobenzopyran moieties based on the photoinduced conformational change of the polymer chain in a high ionic concentration range and photoinduced decrease in the negative fixed charge density of the membrane in the low ionic