RESONANCE RAMAN AND SURFACE-ENHANCED RESONANCE RAMAN STUDIES OF POLYMER-MODIFIED ELECTRODES WHICH MIMIC HEME ENZYMES

Iron-5,10,15,20-tetraphenylporphyrin (FeTPP) has been incorporated int o films of a coordinating hydrogel polymer support medium, poly(gamma- ethyl-L-glutamate) (PEG) functionalised with imidazole pendant arms (P EG-Im), and studied in situ on silver electrodes using a combination o f both resonance Raman (RR) and surface-enhanced resonance Raman (SERR ) spectroscopy. The SERR spectra give information on the portion of th e film close to the electrode surface while RR spectra probe the ''bul k'' of the film. At open-circuit potentials the RR spectra are charact eristic of the expected low-spin Fe-III(TPP)(PEG-Im), complex formed b y axial ligation but the SERR spectra show that, at the electrode surf ace, the complex is composed primarily of Fe-III(TPP)(PEG-Im). The rea sons for the difference have been investigated by systematic RR and SE RR studies of both PEG-Im and a more inert polymer support based on si mple PEG, which does not carry any potentially ligating imidazole pend ant arms. On application of a reducing potential (-400 mV vs. SSCE) on ly partial reduction is observed at the surface of the PEG-Im films. H owever, RR spectra of the reduced films show complete and reversible c onversion to the expected Fe-II(TPP)(PEG-Im), complexes so that the lo w electrochemical activity near the surface does not prevent efficient electron transport from the electrode surface right through the thick ness of the doped polymer layer. There are striking similarities betwe en the properties of this model system, which contains multiple random ly oriented iron porphyrins which are bis-axially coordinated by imida zoles in a solvent accessible poly(amino acid) matrix, and those of cy tochrome c(3), which is a tetraheme protein of low molecular weight. I n cyt c(3) the Fe-III hemes, which are bis-coordinated by histidine re sidues, lie close to each other and again show efficient inter-heme el ectron transfer. The structure of the synthetic PEG-Im film model appe ars to be sufficiently close in structure to the enzyme that it also r eproduces the main features of its behaviour.