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.