A thin-layer electrochemical flow cell coupled on-line with electrospray-mass spectrometry for the study of biological redox reactions

摘要

A three electrode thin-layer, flow-by electrode cell was coupled on-line with electrospray-mass spectrometry (i.e., EC/ES-MS) for the study of biological redox reactions. The cell made use of a commercially available 6.0 mm diameter, offset glassy carbon disk working electrode, a Ag/AgCl reference electrode, and a working/counter electrode spacing gasket (16 mu m), along with a home-built PEEK counter electrode block with a Pt foil (7.0mm wide) counter electrode (cell volume ca. 1.1 mu L). Off-line hydrodynamic voltammetry experiments and electrolytic conversion efficiency measurements using chronocoulometry were used to characterize the performance of the cell. On-line EC/ES-MS conversion efficiency was found consistent with the off-line results and the EC/ES-MS response time was measured as 5.1 s at a flow rate of 30 mu L/min and 2.4 s at 62 mu L/min. The use of this hybrid system for the study of the products of biologically relevant redox reactions was demonstrated using the oxidation of dopamine in aqueous CH3OH (pH 4.0) as a test case. Tandem mass spectrometry experiments provided evidence to conclude that the structure of the two major dopamine oxidation products observed in the ES mass spectra were 5,6-dihydroxyindoline and 5,6-hydroxyindole. Reaction of oxidized dopamine with the surrogate biogenic nucleophile benzene thiol resulted in the formation of mono-, di-, and tri-benzene thiol addition products. These spectra also indicated that the reaction of oxidized dopamine with benzene thiol was significantly faster than the intermolecular cyclization reaction of this intermediate, which normally produces 5,6-dihydroxyindoline. [References: 37]