Investigation of reduction of Cu(II) complexes in positive‐ion mode electrospray mass spectrometry

Abstract

The electrospray ionization mass spectrometry (ESI-MS) behavior of seven Cu(II) complexes with tetradentate ligands has been studied. An unexpected reduction process, in positive ion mode, of the Cu oxidation state was observed, and shown to be due to charge transfer between the metal complex and the solvent molecules in the gas phase. Ion trap collision-induced dissociation experiments and deuterated solvents were used to support the proposed mechanism that is not a common electrochemical redox reaction at the ESI tip, but a gas-phase process. A series of solvents (acetonitrile, methanol, ethanol, propanol and iso-butanol) were tested, and a correlation between ionization energy (IE) and the amount of Cu(I) produced in ESI has been demonstrated for the alcohols, although some other solvent properties should also be taken into account. The electrochemical reduction potential of the complexes in solution is also an important parameter, since complexes more easily reduced in solution are also easier to reduce in the gas phase. Copyright Electrospray ionization mass spectrometry (ESI-MS) has emerged as an important technique for the analysis of a wide range of molecules. Although its major application is in the biochemistry field, the capacity of ESI to deal with inorganic and organometallic complexes was explored very soon after its introduction. 1Electrospray is used extensively nowadays in the field of metal ion complexes because of its capability to produce gasphase ions that are strongly correlated to the solution structure.2 Despite this strong correlation, sometimes structures and characteristics of metal complexes can be modified during the electrospray process, and rearrangements can occur when transferring ions from solution to the gas phase. Electrospray ionization of neutral or non-polar compounds is less efficient than that of polar compounds, and sometimes can produce molecular radical cations through an electrochemical oxidation process in the ESI needle at the liquid-metal interface. Redox reactions have been reported to occur in ESI-MS; 5 either electrochemical oxidation or reduction can be observed depending on the high voltage polarity applied to the ESI capillary (i.e., positive or negative mode). An electrospray source can be described as ªan electrolytic cell of a somewhat special kindº. 6 These redox reactions are essential to maintain the charge imbalance inside the formed droplets, although the importance of such electrochemical processes in understanding ESI and its analytical significance remains a much argued question, see, e.g., de la Mora et al. 7In transferring ions from solution to the gas phase, additional changes and rearrangements can also occur since ions are transferred into a very different environment. New conditions can induce ion structure transformation. For example, multiply charged ions, in which the charges are localized on one atom or a small group of atoms (e.g. multiply charged metal ions, M z ), because of the high columbic repulsion, are difficult to observe in the...

Electrochemical generation of Cu(I) complexes in aqueous solutions studied by on-line mass spectrometry

“…The 2BCP-Cu + absorbance was higher than the Cu + complex formed following the dissolution of a copper(II) salt (dot line) in the spray medium. This small quantity of the 2:1 complex in solution after 30 min could come from reduction of Cu 2+ complexes by methanol, where the reaction has a small kinetic constant, similar to those observed by Gianelli et al [26].…”

“…Schrö der et al have also observed this phenomenon by increasing the cone voltage tension [25]. Gianelli et al have reported the effect of the presence of solvents on Cu 2+ reduction and correlated the production of Cu + with the ionization energy of alcohols [26]. Cu + ions formed in the gas-phase can react with ligands offering a better affinity for Cu + .…”

The role of copper in cysteine oxidation: study of intra- and inter-molecular reactions in mass spectrometry

“…solvent or analyte. [41][42][43][44] So, the presence of Cu + when using a copper electrode can be explained by two processes. 37 First, electrogenerated copper(II) ions can be reduced in solution or in the gas phase.…”

On-line electrogeneration of copper-peptide complexes in microspray mass spectrometry

“…In comparison with the experiment, these data clearly show that the mono-copper complexes are constituted by 76% Ϯ 3% of Cu 2ϩ under these experimental conditions. This can be due either to the binding of copper ions on different sites, (RVYIH(Cu 2ϩ )PF or R(Cu ϩ )VYIHPF), or to a mixed population of Cu 2ϩ / Cu ϩ on a given site, (e.g., RVYIH(Cu 2ϩ/ϩ )PF), induced by an in situ reduction of Cu 2ϩ -histidine complexes in the gas phase [20,[22][23][24]35]. As the copper electrode is able to produce either Cu 2ϩ or Cu ϩ aqueous ions, Cu 2ϩ -histidine and/or Cu ϩ -arginine complexes can in principle be concomitantly formed.…”