Evaluation of electron transfer rates in peptide films: Simplified calculation and theory

Controlled self-assembling structures of ferrocene-dipeptide conjugates composed of Ala-Pro-NHCH2CH2SH chain

Bioorganometallic ferrocene-dipeptide conjugates with the Ala-Pro-cysteamine chain, Fc-L-Ala-L-Pro-NHCH₂CH₂SH (2) and Fc-L-Ala-D-Pro-NHCH₂CH₂SH (4) (Fc=ferrocenoyl), were prepared by the reduction of the ferrocene-dipeptide conjugates, Fc-L-Ala-L-Pro-cystamine-L-Pro-L-Ala-Fc (1) or Fc-L-Ala-D-Pro-cystamine-D-Pro-L-Ala-Fc (3), respectively. Control of the self-assembling structures of the ferrocene-dipeptide conjugates was demonstrated by changing the chirality of the amino acid. The molecular structure of 2 composed of the L-Ala-L-Pro-NHCH₂CH₂SH chain confirmed the formation of intramolecular hydrogen bond of N-H⋯N pattern between the NH of cysteamine moiety and the nitrogen of Pro moiety. Furthermore, intermolecular hydrogen bonds between NH (Ala) and CO (Pro of another molecule) and between NH (cysteamine) and CO (the ferrocenoyl moiety of another molecule) were formed, wherein each molecule is connected to four neighboring molecules by continuous intermolecular hydrogen bonds to form the hydrogen-bonded molecular assembling structure. On the contrary, the left-handed helical assembly through an intermolecular hydrogen-bonding network of 15-membered intermolecularly hydrogen-bonded ring between NH (Ala) and CO (the ferrocenoyl moiety of another molecule) and between NH (the cysteamine moiety of another molecule) and CO (Ala) was observed in the crystal packing of 4 composed of the L-Ala-D-Pro-NHCH₂CH₂SH chain.

A modular electrochemical peptide-based sensor for antibody detection

We report a modular electrochemical peptide-based sensor targeting the anti-deamidated gliadin peptide (DGP) antibody. A recognition peptide, here DGP, is grafted onto a supporting peptide bearing a redox label. The fabricated peptide-based sensor supports the detection of the target antibody (anti-DGP antibody) in the nanomolar range.

Kinetic dispersion in redox-active dithiocarbamate monolayers

Dithiocarbamates (dtcs) have been implicated as important gold-binding groups in molecular electronics. Dtcs have two alkane branches connected at a single anchoring point that has a bidentate resonance structure. Forming readily in situ by the combination of secondary amines and CS(2), dtcs adsorb quickly onto gold surfaces. Electroactive self-assembled monolayers (eSAMs) were prepared by the coadsorption of ferrocene dialkyldithiocarbamates (Fc dtcs) with diluent dtcs on gold electrodes. Short and long alkane chains were used (11 and 16 methylene groups, respectively), and a polar ester group was incorporated. Cyclic voltammetry (CV) shows that the electrochemistry is quasi-reversible. At high surface coverage, the peak separations and full widths at half-maximum for Fc dtcs deviate from theoretical values and are analogous to that of ferrocene alkane thiols on gold at high surface coverage. Importantly, these features do not change at low Fc dtc surface coverage as observed for ferrocene alkane thiols. Ferrocene dtcs were used to label monolayer defect sites and to demonstrate the exchange of surface-bound dtcs with solution dtcs. Finally, the rate of electron transfer was analyzed using Tafel plots and ac voltammetric methods. The results for both techniques are consistent with a kinetically disperse population of redox sites. The length of the diluent alkane chain appears to have an effect on the distribution of electron-transfer rates, likely because of the eSAM structure. This work indicates that structurally, Fc dtc eSAMs are fundamentally different from alkane thiol SAMs on gold.

Electron transfer through a self-assembled monolayer of a double-helix peptide with linking the terminals by ferrocene

A unique molecular structure, a double-helix peptide, was self-assembled on gold, and the electron transfer through the monolayer was studied. The double-helix peptide consists of two 9mer 3(10)-helical peptide chains having a disulfide group at each N terminal and being linked by a ferrocene dicarboxylic acid between the C terminals. Each helical peptide chain has three naphthyl groups in a linear arrangement along the helix. The monolayer properties and the electron transfer from the ferrocene unit to gold were studied with reference peptides with a similar double helix but without naphthyl groups, a single helix with a dicarboxylic ferrocene unit, and a single helix with a monocarboxylic ferrocene unit. It was demonstrated that the naphthyl groups on the side chains had no effect on electron transfer, and the electron-transfer rate in the double-helix monolayer was not promoted, despite the two electron pathways in the molecule. We propose that in the double-helix monolayer, molecular motions are suppressed, possibly by its rigid structure tethered by the two linkers on gold to cancel out acceleration effects of the 2-fold electron pathways and the ferrocene substitution number. The factors that affect the electron-transfer reaction across the helical peptide SAMs are discussed in depth.

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.