Experimental and theoretical investigation of the electronic structures of ferrocene-peptide conjugates
Wilks, Regan G.
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A combined experimental and theoretical examination of the electronic structures of a number of modified helical peptides, specifically ferrocene-(proline)x-benzyl ester (referred to as Fc-Proₓ-OBz), is presented. Soft X-ray absorption (XAS) and emission (XES) spectroscopy using synchrotron radiation are used to probe the unoccupied and occupied partial densities of electronic states, respectively. The identities of the finalstate orbitals corresponding to all near-edge features in the XAS spectra of all elements in the molecules are determined by comparison with published results for other systems, as well as density functional theory (DFT) spectral simulations. Experimental evidence of the splitting of the Fc 3e₂ᵤ orbital due to the presence of a substituent is observed for the first time. The 3e₂ᵤ orbital is also found to contribute to a delocalized orbital that extends throughout the peptide, suggesting a possible electron transfer pathway. Analysis of the Fe 2p XAS spectrum leads to the reassignment of the origin of the prominent 711.4 eV resonance to transitions into hybridized orbitals involving Fe 4s levels and the Ϭ* orbitals associated with the Fc C-H bonds. Previous studies incorrectly described this feature. Comparison of the DFT-simulated ground-state DOS of Fc-Pro₂-OBz and Fc-Pro₃-OBz shows a major reorganization of the orbitals of the benzyl ester, which acts as the reduction centre in redox reactions. In Fc-Pro₃-OBz, the orbitals closely resemble those of a gaseous substituted benzene ring and were found at a lower energy than the equivalent orbitals of Fc-Pro₂-OBz. This shift is proposed as a possible explanation for the decrease in redox potential observed upon the addition of a third proline residue to the peptide.