Electrospray mass spectrometric studies of L-carnosine (R-alanyl-L-histidine) complexes with copper(II) or zinc ions in aqueous solution

Electrospray ionization mass spectrometry (ESI-MS) was used for the speciation of supramolecular assemblies formed between equimolar amounts of carnosine and copper or zinc ions in dilute aqueous solutions. In the case of pure carnosine and carnosine/copper systems, the effect of pH changes, in the range 2-9, on the complexes surviving in solution was also explored. ESI data, besides supporting previous reported results on the formation of dimeric carnosine/copper and carnosine/zinc complexes, allowed a more complete speciation of the examined systems, bringing to light the existence of bis-complex species and, in the zinc case, the formation of oligomeric species. The data obtained for the systems investigated show that ESI-MS is not only a reliable and fast technique for the analysis of the metal/ligand systems, but also an interesting tool to obtain stoichiometric information on metal complexes formed in very low concentration solutions.