Microscopic acid–base equilibria of a synthetic hydroxamate siderophore analog, piperazine-1,4-bis(N-methylacetohydroxamic acid)

Abstract

The protonation behavior of the cyclic diaminodihydroxamate ligand, piperazine-1,4-bis(N-methylacetohydroxamic acid) (H₂L¹), has been studied at both the macroscopic and the microscopic level. Potentiometric and ¹H NMR techniques have been used for the study of this ligand as well as several model compounds: N-methylchloroacetohydroxamic acid, glycinehydroxamic acid and piperidino(N-methylacetohydroxamic acid). Molecular modeling calculations have also been performed to predict the most stable conformations and to estimate relevant contributions to the overall protonation process.

The results of the protonation microconstants show that the N-donors in H₂L¹ are much less basic than the O-donors. The protonated amine moieties release most of their protons in the acid region while the deprotonation of the hydroxamate moieties starts only above pH 5. The theoretical modeling calculations show the effect of electrostatic interactions and internal hydrogen bonds on the interactivity of the basic sites throughout the protonation process.