MICROSCOPIC ACID-BASE EQUILIBRIA OF A SYNTHETIC HYDROXAMATE SIDEROPHORE ANALOG, PIPERAZINE-1,4-BIS(N-METHYLACETOHYDROXAMIC ACID)

The protonation behavior of the cyclic diaminodihydroxamate ligand, pi perazine-1,4-bis(N-methylacetohydroxamic acid) (H2L1), has been studie d at both the macroscopic and the microscopic level. Potentiometric an d H-1 NMR techniques have been used for the study of this ligand as we ll as several model compounds: N-methylchloroacetohydroxamic acid, gly cinehydroxamic acid and piperidino(N-methylacetohydroxamic acid). Mole cular modeling calculations have also-been performed to predict the mo st stable conformations and to estimate relevant contributions to the overall protonation process. The results of the protonation microconst ants show that the N-donors in H2L1 are much less basic than the O-don ors. The protonated amine moieties release most of their protons in th e acid region while the deprotonation of the hydroxamate moieties star ts only above pH 5. The theoretical modeling calculations show the eff ect of electrostatic interactions and internal hydrogen bonds on the i nteractivity of the basic sites throughout the protonation process.