Long-Range Electron Transfer over 4 nm Governed by an Inelastic Hopping Mechanism in Self-Assembled Monolayers of Helical Peptides

Well-ordered self-assembled monolayers (SAMs) were prepared on gold from helical peptides carrying a ferrocene (Fc) moiety at the N- or C-terminal end, and long-range electron transfer (ET) from Fc to gold was investigated. Electrochemical studies revealed that an inelastic hopping mechanism dominated over the superexchange mechanism in the ET reactions in the present SAMs and the dipole moment of the helix accelerated the ET reactions probably due to the lowering of the barrier height between the gold surface and peptide layer

Determination of Single Molecule Conductances of Alkanedithiols by Conducting-Atomic Force Microscopy with Large Gold Nanoparticles

Single molecular conductances of alkanethiols with different lengths were determined by conducting-atomic force microscopy with large gold nanoparticles (5.4 nm diameter) as the contact. Because of the large size of the nanoparticles, the Coulomb blockade effect is small and the quantized level effect is negligible. These effects were the problems with small nanoparticles (1.5 nm) in analysis of the current−voltage data. Therefore, the measurements with large nanoparticles gave the reasonable conductances and reasonable distance dependence that agree well with the scanning tunneling microscopy repeated break-junction results

Electric Field Effect of Helical Peptide Dipole in Self-Assembled Monolayers on Electronic Structure of Oligo(Phenyleneethynylene)

A series of novel linear conjugates of helical peptides and oligo(phenyleneethynylene) (OPE) were synthesized and studied on the dipole effect of the helical peptide moieties on the electronic structure of the OPE. The helical peptides have a generalized formulation of multiple repeats of l-alanine and α-aminoisobutyric acid, (Ala-Aib)n or m, which are connected to OPE in series of (Ala-Aib)n-OPE-(Ala-Aib)m abbreviated by 2nOPE2m, (n, m) = (4, 0), (4, 4), (8, 0), (8, 4), (8, 8). The conjugates having one or two hexadecapeptides formed well-packed and vertically oriented self-assembled monolayers on gold as revealed by infrared-reflection absorption spectroscopy, cyclic voltammetry, and ellipsometry. Absorption spectra of the OPE moiety in the SAMs showed a bathochromic shift of ca. 25 nm from a reference conjugate of 11-mercaptoundecanoic acid and OPE (C11OPE). The shift is consistent with the density functional theory calculations, showing that an external electric field directed along the molecular axis diminishes the highest occupied molecular orbital−lowest unoccupied molecular orbital gap of OPE. We thus conclude that the electric field generated by the peptide dipoles effectively modulate the electronic structure of the OPE moiety in the SAMs

Fabrication of Langmuir–Blodgett Film of a Fullerene Derivative with a Cyclic Peptide as an Anchor

A novel cyclic octapeptide carrying a fullerene unit and poly(ethylene glycol) at the side chain (cyclo8-C60 + PEG) was synthesized, and its monolayer formation at the air/water interface and on a substrate was studied. Surface pressure–area per molecule isotherms indicated that cyclo8-C60 + PEG formed a stable monolayer at the air/water interface. The cyclo8-C60 + PEG monolayers prepared from various spreading volumes (i.e., from various initial areas per molecule) overlapped nicely on a single curve, suggesting that the molecules were uniformly dispersed on the surface without aggregation of the fullerene units. The uniform dispersibility is due to the scaffold effect of the cyclic peptide unit to keep the fullerene units away from each other. The formed monolayer could be quantitatively transferred onto a solid substrate. UV–vis absorption spectroscopy of the Langmuir–Blodgett (LB) monolayer showed that the electronic structure of the fullerene unit was not affected by the formation of the monolayer. Cyclic voltammetry of the LB monolayer in an aqueous solution containing redox species indicated that the LB monolayer was densely packed. Furthermore, reversible redox peaks attributed to the one-electron reduction of the fullerene unit were observed, showing that the redox property of the fullerene unit was also retained in the monolayer. It is thus concluded that the cyclic peptide is a good candidate as a scaffold for stable monolayer formation at the air/water interface and for intact immobilization of the fullerene moiety onto a substrate

pH-Controlled Switching of Photocurrent Direction by Self-Assembled Monolayer of Helical Peptides

A novel molecular system, where the photocurrent direction can be reversibly switched by changing the pH of the solution, was prepared on gold from helical peptides carrying a photosensitizer and a carboxyl group at the terminal. Upon photoexcitation of the photosensitizer in an aqueous solution containing an electron donor and acceptor at pH 10, the monolayer generated an anodic photocurrent due to enhancement of the dipole moment by a carboxylate anion, while it generated an opposite cathodic photocurrent at pH 3.

Efficient Photocurrent Generation by Self-Assembled Monolayers Composed of 3₁₀-Helical Peptides Carrying Linearly Spaced Naphthyl Groups at the Side Chains

Self-assembled monolayers (SAMs) were prepared on a gold substrate from a 310-helical peptide carrying three naphthyl groups at the side chain (SSN3B) or from the reference peptides carrying no or one naphthyl group. The 310-helical conformation of SSN3B in solution was confirmed by 1H NMR spectroscopy and geometry optimization. Cyclic voltammetry and infrared absorption−reflection spectroscopy showed vertical molecular orientation and a well-packed structure in the SSN3B SAM. Anodic photocurrent was successfully generated by the SSN3B SAM in the presence of triethanolamine, and the current intensity was found to be much larger than those by the other SAMs from peptides carrying one naphthyl group. It was therefore concluded that the linearly spaced naphthyl groups along the helical axis act as photosensitizer and electron-hopping site to promote photocurrent generation remarkably

Vertical and Directional Insertion of Helical Peptide into Lipid Bilayer Membrane

A novel helical hexadecapeptide carrying a poly(ethylene glycol) (PEG) chain at the N terminal was synthesized. The N and C terminals of the compound are labeled with a fluorescein isothiocyanate (FITC) group and an N-ethylcarbazolyl group (ECz), respectively. An octapeptide carrying the same groups and a hexadecapeptide without a PEG chain were also synthesized and used as control. A mixture of the peptide and dimyristoylphosphatidylcholine was sonicated in a buffer to prepare the liposome. The orientation as well as direction of the helical segment in the lipid bilayer were analyzed by quenching experiments of the FITC and the ECz fluorescence. The results clearly indicated that the helical segment of the peptide penetrated into the lipid bilayer with vertical orientation in both the gel and liquid crystalline states of the lipid bilayer. Notably, the bulky N terminal was left behind in the outer aqueous phase of liposome, meaning that the C terminal of the peptide points to the inner aqueous phase of liposome. The insertion mode of the helical peptide into a bilayer membrane is therefore well-regulated in terms of the orientation and the directionality by designing the balance between the PEG chain and the helix length. The methodology presented here will initiate a way to construct artificial functional molecular systems that can induce vectorial transport phenomena as seen in biological systems

Donor−Sensitizer−Acceptor Triad System for Photoenergy Migration, Photoenergy Transfer, and Electron Transfer in a Bilayer Membrane

A novel photoinduced electron-transfer system was constructed by using a photoenergy-harvesting bilayer membrane composed of two amphiphiles:  one having an antenna group (an N-ethylcarbazolyl (ECz) group) and the other having a photoenergy-accepting group (anthryl group) and an electron-accepting group (viologen group). Photoenergy migration among ECz groups occurs in the membrane, and the photoenergy is transferred efficiently to the anthryl group. The excited ECz group reduces the viologen group through the excitation of the anthryl group with a quantum yield of 0.67 in the presence of 3 mol % acceptor. The photoinduced electron-transfer process was simulated successfully to determine the number of the excitation migration steps between ECz groups and the electron-transfer rate from the excited ECz group to the viologen group. Interestingly, the transient absorption spectroscopy revealed that the photooxidized chromophore decays faster than the reduced viologen, which has a lifetime longer than a millisecond, suggesting the electron donation to the oxidized chromophore by amide groups in the amphiphile molecules. This membrane is regarded as a donor-sensitizer-acceptor triad system, in which the sensitizer is coupled with two-dimensional array of photoharvesting chromophores, resulted in a more efficient electron transport system than a donor−acceptor diad system

Ultra-Long-Range Electron Transfer through a Self-Assembled Monolayer on Gold Composed of 120-Å-Long α-Helices

Electron transfer through α-helices has attracted much attention from the viewpoints of their contributions to efficient long-range electron transfer occurring in biological systems and their utility as molecular-electronics elements. In this study, we synthesized a long 80mer helical peptide carrying a redox-active ferrocene unit at the terminal and immobilized the helical peptide on a gold surface. The molecular length is calculated to be 134 Å, in which the helix accounts for 120 Å. The preparation conditions of the self-assembled monolayers were intentionally changed to obtain monolayers with different physical states to study the correlation between molecular motions and electron transfer. Ellipsometry and infrared spectroscopy showed that the helical peptide forms a self-assembled monolayer with vertical orientation. Electrochemical measurements revealed that an electron is transferred from the ferrocene unit to gold through the monolayer composed of this long helical peptide, and the experimental data are well explained by theoretical results calculated under the assumption that electron transfer occurs by a unique hopping mechanism with the amide groups as hopping sites. Furthermore, we have observed a unique dependence of electron transfer on the monolayer packing, suggesting the importance of structural fluctuations of peptides on the electron transfer controlled by the hopping mechanism

NiCl₂(PMe₃)₂-Catalyzed Borylation of Aryl Chlorides

The cross-coupling of aryl chlorides and bis(pinacolato)diboron was achieved using NiCl2(PMe3)2 catalyst in the presence of metal 2,2,2-trifluoroethoxide. The catalyst smoothly provided the desired products regardless of a variety of functional groups and substituted positions