Michel Prudent
Metal ions are essential elements in all living systems. Their detections and identifications in biological processes require different methodologies. Mass spectrometry is one of the commonly used techniques in bioinorganic analyses to fulfill these requirements. Electrospray ionization mass spectrometry (ESI-MS) is a method of choice and the present work takes benefit of the electrochemical aspects of ESI and of the source design flexibility for studying the interactions between metal ions and biomolecules. In positive ionization mass spectrometry, an electrospray device acts as an anode, which implies oxidation reactions. Thus, transition metal electrodes can be used to generate metal ions in addition of sustaining the electrospray process. Sacrificial copper electrodes, coupled to polyimide microchips, are used first in order to study copper complexes. Firstly, it is shown that, when using different chelating agents for copper(II) and copper(I) ions, the electrodissolution of copper leads to the formation of the respective complexes that are directly analyzed in MS. This methodology offers a direct route to synthesize copper(I) complexes in aqueous solutions. Following this concept, the interactions between copper ions and peptides are investigated. On-line electrogeneration of copper-peptide complexes is achieved either by using a sacrificial copper electrode or by mixing the peptide solution with a copper(II) salt by applying a high voltage via a platinum electrode. The latter yields mainly copper(II) complexes with histidine residues or with the peptide backbone (copper(I) complexes can be also formed due to gas-phase reactions), whereas the former can generate a mixture of both copper(I) and copper(II) complexes that yields different complexation patterns. In addition, the influence of these cations on peptide fragmentation is reported. The role of copper in cysteine-containing peptide oxidation is studied with the same procedure. Using peptides containing one, two and three cysteines, we have compared the different chemical and electrochemical oxidation pathways of cysteine (RS-IIH) to cysteine (RS-IS-IR) and to sulfenic, sulfinic and sulfonic acid (RS0OH, RSIIO2H and RSIVO3H, respectively). In the absence of copper ions, intra-molecular reactions are the most abundant, whereas inter-molecular reactions are found to be enhanced by the presence of copper ions. These cations favor the formation of 2:1 (peptide : copper) complexes compared to 1:1 complexes, thus enhancing the formation of inter-molecular bridges. This study highlights the importance of the cysteines position inside a peptide in the disulfide bridges formation. The concept of sacrificial electrodes is then employed to tag molecules on-line instead of studying interactions. Phosphopeptides tagging reactions by dinuclear zinc(II) complexes are performed with a dual-channel microsprayer in ESI-MS. This device consists in a polyimide microchip with two microchannels etched on each side of the structure and connecting only at the tip of the microchip. The reaction is first studied ex situ and analyzed with a commercial electrospray source. On-chip reactions (i.e. inside the Taylor cone) are achieved with a dual-channel microsprayer both with the tag synthesized chemically before the experiments and with the tag electrogenerated by an in situ oxidation of a zinc electrode, also used to apply the electrospray current. It is thus demonstrated that mixing two solutions with different physico-chemical properties inside the Taylor cone can be used to selectively tag target molecules. Finally, a biphasic electrospray ionization (BESI) source has been built in the goal of probing interfacial complexes in MS. Based on the dual-channel microsprayer design, the two channels are filled with immiscible liquids and put in contact only within the Taylor cone. Two types of interfacial complexation reactions have been studied: the interfacial complexation of aqueous lead ions by thioether crown molecules, and the interfacial complexation of an aqueous dipeptide by dibenzo-18-crown-6 as ionophore. The mass spectra also give valuable information on the stoichiometry of the complexation reaction. BESI is also applied to the study of phospholipids, which are known to adsorb at the liquid-liquid interfaces, like their interactions with metal ions and peptides. Metal ion-phospholipid complexes show the formation of clusters containing one metal ion, where the size depends on the valence of the ion. As for the peptide-phospholipid complexes, it could be formed under these conditions and the stability of these non-covalent complexes are related to their structure. Hence, BESI sources offer new analytical opportunities in different research fields.
EPFL2009
