Resonance Raman and surface-enhanced resonance Raman studies of polymer-modified electrodes which mimic heme enzymes

Abstract

Iron-5,10,15,20-tetraphenylporphyrin (FeTPP) has been incorporated into films of a coordinating hydrogel polymer support medium, poly(γ-ethyl-L-glutamate) (PEG) functionalised with imidazole pendant arms (PEG-Im), and studied insitu on silver electrodes using a combination of both resonance Raman (RR) and surface-enhanced resonance Raman (SERR) spectroscopy. The SERR spectra give information on the portion of the film close to the electrode surface while RR spectra probe the "‘bulk’' of the film. At open-circuit potentials the RR spectra are characteristic of the expected low-spin Fe^III(TPP)(PEG-Im)₂ complex formed by axial ligation but the SERR spectra show that, at the electrode surface, the complex is composed primarily of Fe^III(TPP)(PEG-Im). The reasons for the difference have been investigated by systematic RR and SERR studies of both PEG-Im and a more inert polymer support based on simple PEG, which does not carry any potentially ligating imidazole pendant arms. On application of a reducing potential (-400 mV vs. SSCE) only partial reduction is observed at the surface of the PEG-Im films. However, RR spectra of the reduced films show complete and reversible conversion to the expected Fe^II(TPP)(PEG-Im)₂ complexes so that the low electrochemical activity near the surface does not prevent efficient electron transport from the electrode surface right through the thickness of the doped polymer layer. There are striking similarities between the properties of this model system, which contains multiple randomly oriented iron porphyrins which are bis-axially coordinated by imidazoles in a solvent accessible poly(amino acid) matrix, and those of cytochrome c₃, which is a tetraheme protein of low molecular weight. In cyt c₃ the Fe^III hemes, which are bis-coordinated by histidine residues, lie close to each other and again show efficient inter-heme electron transfer. The structure of the synthetic PEG-Im film model appears to be sufficiently close in structure to the enzyme that it also reproduces the main features of its behaviour.