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

•Cu(II)-insulin and Mn(II)-insulin complexes were examined using different methods.•Cu(II) binding to insulin can be monitored by UV–vis absorption spectroscopy.•Mn(II) binding to insulin cannot be monitored by UV–vis absorption spectroscopy.•Cu(II) ions showed higher affinity to insulin molecule than Mn(II) ions.