pH-tunable equilibria in azocrown ethers with histidine moieties.

The crown ethers with electro- and photoactive azo moieties containing substituents with mobile protons such as in the -COOH groups of histidine, show unique effect of pH switched on/off presence of the azo form. The differences observed for the electrochemical behavior of azocrown ethers with N-acetylhistidine and imidazole moieties reveal the interference of a chemical reduction pathway in strongly acidified solutions. This chemical reduction process leads to the formation of a hydrazine derivative which can be detected by its further electroreduction on the electrode surface. The involvement of chemical reduction is seen clearly in the presence of mobile protons of the -COOH group and mercury as the electrode substrate. The behaviour of the N-acetylhistidine azomacrocyle is similar to that of compounds known to exist in quinone-hydrazone tautomeric equilibria.