Maria Grazia Saita

Copper(II) and nickel(II) binding modes in a histidine-containing model dodecapeptide

The formation of complexes of HGGGHGHGGGHG (HG12) with copper(II) and nickel(II) have been studied in aqueous solution under various experimental conditions, including different pH and metal to ligand ratios. The study has been carried out using visible absorption, circular dichroism and electron paramagnetic resonance spectroscopic methods. Moreover, electrospray ionisation mass spectrometry has been used to directly determine the stoichiometry of the copper(II) complexes. The results indicate that HG12 can easily accommodate two metal ions in as many binding sites. The solution structure of the main complex species formed in the reaction of copper(II) with HG12 has been inferred by comparison with the copper(II) complexes formed with the shorter peptide fragments HGGGHG–NH2 (HG6), Ac–HGGGHG–NH2 (AcHG6) and Ac–HGGG–NH2 (AcHG4). With an equimolar metal to ligand ratio, the copper(II) ion binds preferentially in the N-terminal region of HG12. Conversely, Ni(II) ions form identical complexes regardless of whether the metal to ligand ratio is 1∶1 or 2∶1. Finally, the circular dichroism spectra indicate a significant modification of the peptide conformation upon metal binding.

pH-Dependent stability of azithromycin in aqueous solution and structure identification of two new degradation products

ABSTRACT The degradation profile of azithromycin in buffered solutions was investigated using HPLC and found to be pH dependent in the range of 6.0–7.2. Desosaminylazitromycin, derived from hydrolytic loss of cladinose of the parent molecule, was the major degradation product at pH 6.0 but its amount progressively decreased moving toward pH 7.2. Two additional unreported degradation products were also observed and their structures were fully elucidated by MS‐ and NMR‐spectroscopy to be associated with opening of the macrocyclic lactone ring. HIGHLIGHTSDegradation of azithromycin in the pH interval 6.0–7.2 was investigated by HPLC.Impurity profiling of azythromycin was found dependent on the pH of the solution.Two novel degradation products were identified and spectroscopically characterized.Degradation of azythromycin can occur through opening of the lactone ring.