Reorganization energies of ferrocene-peptide monolayers

To deepen understanding of the electron transfer through a peptide backbone, we have investigated a series of noncyclic and cyclic ferrocene-peptide (Fc-peptide) cystamine conjugates immobilized on the gold microelectrode. Interaction of the ferrocenium group with BF4-, ClO4-, and PF6- as counterions was explored and the electron-transfer rates and reorganization energies were determined by variable temperature cyclic voltammetry. The highest reorganization energy was observed for the BF4- counterion, which has the weakest ability to associate with the ferrocenium cation. In addition, the more rigid cyclic Fc-peptide conjugates have a smaller reorganization energy ranging from 0.3 to 0.5 eV compared to less rigid noncyclic Fc-peptide cystamine conjugates which have higher reorganization energies in the range of 0.5-1.0 eV, which suggests that the dynamic properties of the conjugate play a role in mediating electron transfer in these systems.     

Electrochemical "signal-on" reporter for amyloid aggregates

The synthesis and characterization of four new Ferrocene (Fc) bioconjugates, bearing a podant (Lys)-Leu-Val-Phe-Phe motif, namely the hydrophobic sequence of amyloid-β-peptides (Aβ), is reported. The Fc-peptide conjugates are characterized by a reversible redox activity and the ability to undergo hydrophobic and hydrogen bonding interactions. Biomolecular interactions between Fc-bioconjugates with Aβ(12-28) fragments were studied by circular dichroism (CD), transmission electron microscopy (TEM), and electrochemistry. All four Fc-peptides interacted favourable with Aβ(12-28) and prevented fibril formation, the extent of which depended on the length of the peptide and the nature of the C-terminal group. The aggregates obtained for the Aβ(12-28)/Fc-peptide mixtures range from short fibrils to spherical aggregates. We demonstrated that in solution the peptide sequence and peptide charge affect the biomolecular interactions. Fc-peptide interactions with immobilized Aβ(12-28)-Cys films on Au surfaces were detected by measuring the current response of the Fc redox process. The formal redox potential, E(0), at ~440 (10) mV and i(pc)/i(pa) at 0.9 were observed characteristic for the monosubstituted Fc-derivative undergoing a one-electron redox process. On the surface, methyl ester-protected Fc-peptides (1 and 3) interacted only weakly with Aβ(12-28)-Cys films, giving rise to minimal redox activity. In contrast, charged Fc-peptides (2 and 4) gave a significant electrochemical readout following the interaction with Aβ(12-28)-Cys films. Interestingly, the Fc-peptide charge dictates the surface-assisted interactions, while hydrophobic and ionic effects contribute to the overall solution behaviour of the Fc-bioconjugates with Aβ(12-28).     

Optimizing electrode-attached redox-peptide systems for kinetic characterization of protease action on immobilized substrates. Observation of dissimilar behavior of trypsin and thrombin enzymes

In this work, we experimentally address the issue of optimizing gold electrode attached ferrocene (Fc)-peptide systems for kinetic measurements of protease action. Considering human α-thrombin and bovine trypsin as proteases of interest, we show that the recurring problem of incomplete cleavage of the peptide layer by these enzymes can be solved by using ultraflat template-stripped gold, instead of polished polycrystalline gold, as the Fc-peptide bearing electrode material. We describe how these fragile surfaces can be mounted in a rotating disk configuration so that enzyme mass transfer no longer limits the overall measured cleavage kinetics. Finally, we demonstrate that, once the system has been optimized, in situ real-time cyclic voltammetry monitoring of the protease action can yield high-quality kinetic data, showing no sign of interfering effects. The cleavage progress curves then closely match the Langmuirian variation expected for a kinetically controlled surface process. Global fit of the progress curves yield accurate values of the peptide cleavage rate for both trypsin and thrombin. It is shown that, whereas trypsin action on the surface-attached peptide closely follows Michaelis-Menten kinetics, thrombin displays a specific and unexpected behavior characterized by a nearly enzyme-concentration-independent cleavage rate in the subnanomolar enzyme concentration range. The reason for this behavior has still to be clarified, but its occurrence may limit the sensitivity of thrombin sensors based on Fc-peptide layers.     

Interaction of ferrocenoyl-dipeptides with 3-aminopyrazole derivatives: beta-sheet models? A synthetic, spectroscopic, structural, and electrochemical study

The use of 3-aminopyrazole derivatives as beta-sheet templates is investigated using a series of ferrocenoyl (Fc)-dipeptides (Fc-Gly(2)-OEt, Fc-Ala(2)-OBzl, Fc-Leu-Phe-OMe, Fc-Val-Phe-OMe, Fc-Phe(2)-OMe, Fc-Leu(2)-OMe, Fc-Val(2)-OMe). The synthesis and full characterization are reported. The solid-state structures of Fc-Gly(2)-OMe and Fc-Leu-Phe-OMe show extensive hydrogen bonding of the podand peptide substituents, resulting in the formation of supramolecular Fc-dipeptide assemblies. For Fc-Gly(2)-OMe, this can be described as a parallel beta-sheet, whereas intermolecular interactions in Fc-Leu-Phe-OMe result in the formation of supramolecular helical structures. The saturation titrations of Fc-dipeptides with 3-amino-5-methylpyrazole (3-AMP) and 3-trifluoroacetylamido-5-methylpyrazole (3-TFAc-AMP) show a 1:1 interaction of the Fc-peptide with the aminopyrazole derivatives. IR measurements in solution confirm binding to the top face of the Fc-dipeptide and the involvement of the Fc-C=O and the ester C=O groups in establishing H-bonding interactions with the 3-TFAc-AMP. However, binding constants in chloroform are low and range from 8 to 27 M(-1), which correspond to binding energies of 5-7 kJ mol(-1). In higher polarity solvents, such as acetonitrile or acetone, the binding constants are below 5 M(-1), emphasizing the limited utility of 3-AMP derivatives as beta-sheet templates. Electrochemical measurements confirm the weak interactions between the various Fc-dipeptides and 3-TFAc-AMP. Typical shifts in the redox potential of the Fc moiety are in the range 0-20 mV. Attempts to modify 3-AMP at the 3-position by carbodiimide coupling with amino acid derivatives and, thus, enhance the binding to the Fc-peptides resulted in 2-amino acid substituted 3-AMP derivatives. Substitution at the 2-position blocks the binding site, and no interactions with Fc-dipeptides are observed.