Monitoring Cu(II)-insulin and Mn(II)-insulin complexes using potentiometric, chromatographic, UV–vis absorption and fluorescence emission spectroscopic techniques

Abstract

In the present work, Cu(II)-insulin and Mn(II)-insulin complexes were investigated using potentiometric acid-base titration, UV–vis absorption and fluorescence (FL) emission spectroscopy, and liquid chromatography (UFLC). Cu(II) and Mn(II) ions are essential nutritional trace elements. They have important activities with enzymes and proteins in the human body. Insulin is a peptide hormone that regulates glucose level in the body. Cu(II) and Mn(II) metal ions are bound to insulin molecules, and their protein complexes form. The binding behaviors of the essential nutritional metal ions to insulin molecules have physiological importance in a human body or drug formulations. The binding properties of Cu(II) and Mn(II) ions to insulin molecule were examined using different methods and compared with each other. It was shown that the Cu(II)-insulin complex formation could be monitored by potentiometric acid-base titration, UV–vis absorption, UFLC and FL methods. However, UV–vis absorption, UFLC and FL methods are not suitable for the monitoring Mn(II)-insulin complexes. The binding mechanism of Cu(II) ions is different than the binding mechanism of Mn(II) ions. The Cu(II) binding is governed by the chelating mechanism, while the Mn(II) binding is by ionic interaction. The acid-base potentiometric titration method is a cheap practical method to check whether the binding of metal ions to protein is or not as well as Cu(II) and Mn(II) metal ions. UV–vis absorption spectroscopy can be used for protein-metal ion interactions via a chelating mechanism.