Metallated Schiff-Base Macromolecules as Alternative Metalloprotein Electron Transfer Intermediates

Abstract

In the construction of biosensors, enzymes function as mediators converting biological signals generated by specific biological processes, into electrochemical signals. The ideology of bio-sensor design is retention of electron transfer activity of the enzyme utilizing superior interfacial architecture. In this work a Schiff-base macromolecule has been synthesized by reflux of 2, 3-diaminonaphthalene and pyrrole-2-carboxaldehyde starting materials. The Schiff-base ligand was subsequently complexed with FeCl2∙4H2O under reflux, to produce the Fe-Schiff-base complex. The Schiff-base ligand and Fe-Schiff-base complex were characterized using nuclear magnetic resonance (NMR) spectroscopy, Ultra Violet/Visible (UV/Vis) spectroscopy, Fourier transfer infrared resonance (FTIR) and electron energy loss spectroscopy (EELS) to confirm the structure of the synthesis products. NMR spectroscopy confirmed the imide linkage of Schiff-base formation as two symmetrical peaks at 8.1 and 7.7 ppm respectively. Comparison of starting materials and product spectra by UV/Vis spectroscopy confirmed the disappearance of the diaminonaphthalene peak at 250 nm as evidence of complete conversion to product. FTIR spectroscopy of the Schiff-base ligand confirmed the formation of the imine bond at 1595 cm-1. EELS spectra comparing FeCl2∙4H2O and the Fe-Schiff-base complex, showed good agreement in the energy loss profiles associated with changes to the electronic arrangement of Fe d-orbitals. EDS clearly identified a spectral band for Fe (7 - 8 eV) in the Fe-Schiff-base complex. Electrochemical evaluation of the Fe-Schiff-base complex was compared to the electrochemical signature of denatured cytochrome-C using cyclic voltammetry and square wave voltammetry. The Fe2+/Fe3+ quasi-reversible behavior for iron in the metallated complex was observed at -0.430 V vs. Ag/AgCl, which is consistent with reference values for iron in macromolecular structures.