Eur. J. Org. Chem.

The octapeptide [134-141] related to the active-site fragment of thioredoxin reductase, in which three residues outside the characteristic Cys-Xaa-Yaa-Cys motif of thiol/disulfide oxidoreductases were replaced by lysines, was head-to-tail- cyclized by using the suitably functionalized (4- aminomethyl)phenylazobenzoic acid (AMPB). The resulting monocyclic and disulfide-bridged bicyclic compounds underwent light-induced cis/trans isomerization in a fully reversible manner, with well-defined conformational transitions as a result of the strong differences in the molecular geometries of the trans and cis-azobenzene units. Correspondingly, the trans and cis forms of the cyclic bis(cysteinyl)-AMPB peptide were characterized by significantly differentiated redox potentials, which were exploited to catalyze the oxidative refolding of reduced RNase A with distinct efficiencies. The experimental results showed that the incorporation of the azobenzene moiety into conformationally restricted bis(cysteinyl) peptide systems provided folding adjuvants that photocontrolled the rates of oxidative protein folding