Spectrophotometric, voltammetric and electrospray mass spectrometric study of the association of pyocyanine with Mn(II) in aqueous media

Spectrophotometry, voltammetry and electrospray mass spectrometry have been used to examine dilute aqueous solutions containing pyocyanine and manganous ions. Spectrophotometry shows no evidence of complex formation between the two constituents in either non-buffered (pH similar to 5) or buffered (pH 6.4) media at concentrations up to 100 mu mol l(-1) pyocyanine and 150 mu mol l(-1) Mn2+. Direct voltammetry with buffered solutions confirms the spectrophotometric results, however the results obtained with non-buffered solutions, although significantly different from those for the buffered solutions, do not provide unambiguous evidence of complex formation. Complexes of Mn2+ with one, two and three pyocyanine molecules are present in high intensity in the electrospray mass spectra obtained with unbuffered aqueous solutions but are almost entirely absent with the buffered solutions. It is concluded that the concentrations of complexes in the solutions are extremely low and that their mass spectrometric observation is due to their formation in the electrospray process and not to their pre-formation in the solutions. (C) 1998 Elsevier Science B.V. All rights reserved.