Electron Transfer Mechanism in Helical Peptides

Electrochemical studies of a set of ferrocene-labeled helical peptides of increasing length were carried out by forming self-assembled monolayers (SAMs) on gold electrodes. Electron transfer (ET) rates showed a very weakly distance dependent nature that has been interpreted as a result of a dynamically controlled tunneling mechanism. Specifically, the slow equilibrium between the α- and the 3₁₀ helical conformers in a SAM has been invoked, and the rate of formation of the more conductive 3₁₀ conformer has been proposed to be related to the ET rates observed

Ferrocene–Tryptophan Conjugate: An Example of a Redox-Controlled Reversible Supramolecular Nanofiber Network

In this study, the tryptophan derivative of ferrocene-1,1′-dicarboxylic acid self-assembles in toluene to form a supramolecular nanofibrillar network structure. The ferrocene bioconjugate based nanofibers are responsive toward oxidation/reduction and show thermo and redox reversibility. Interestingly, redox-induced reversible morphological transformations between nanofiber and spheroid were observed. The self-assembly was characterized by ¹H NMR spectroscopy, FT-IR spectroscopy, UV–vis spectroscopy, circular dichroism (CD), and transmission electron microscopy (TEM)

Versatile Strategy for Biochemical, Electrochemical and Immunoarray Detection of Protein Phosphorylations

Protein kinases catalyze the phosphorylation of cellular proteins involved in the regulation of many cellular processes and have emerged as promising targets for the treatment of several diseases. Conventional assays to monitor protein kinase activity are limited because they typically rely on transfer of radioactive phosphate or phospho-specific antibodies that recognize specific substrates or sequence motifs. To overcome the limitations of conventional assays, we have developed a versatile approach based on transfer of ferrocene-phosphate that can be readily monitored using electrochemical detection or detection with antiferrocene antibodies in an immunoarray format. This assay is readily adapted to multiplex arrays and can be employed for monitoring kinase activity in complex mixtures and for kinase inhibitor profiling

Bis-amino Acid Derivatives of 1,1′-Ferrocenedicarboxylic Acid: Structural, Electrochemical, and Metal Ion Binding Studies

We report on the structural and electrochemical investigation of 1,1′-ferrocenedicarboxylic acid derivatives of tryptophan (Fc­[CO-Trp-OMe]₂ - Fc-conjugate <b>1</b>), threonine (Fc­[CO-Thr-OMe]₂ – Fc-conjugate <b>2</b>), aspartic acid (Fc­[CO-Asp-OMe]₂ - Fc-conjugate <b>3</b>) and glutamic acid (Fc­[CO-GluOMe]₂ - Fc-conjugate <b>4</b>) and their hydrolyzed analogues <b>1a</b>–<b>4a</b> respectively (Scheme ). CD and NMR spectroscopy established 1,2′-“Herrick conformation” in solution, having intramolecular interstrand hydrogen bonds for all Fc-conjugates. However, in solid state, Fc­[CO-Trp-OMe]₂ exists in “Herrick conformation” whereas Fc­[CO-Thr-OMe]₂ is present in anti conformation. In solution, the involvement of indole NH of Trp and alcoholic proton of Thr in intermolecular hydrogen bonding has been explored by temperature- and concentration-dependent NMR studies. The half-wave potentials (<i>E</i>_1/2) of ferrocene-conjugates follow the sequence <b>1 < 2 < 4 < 3</b> which is explained by the contribution of amino acid side chain functionalities toward the stability of ferrocenium ion. The CV of the Fc-conjugate <b>1/1a</b> (having Trp moiety) displays two redox processes, one of which is assigned to the Fc group, and the other being related to the indole group. The oxidation peak potential of indole was found to depend strongly on the pH of the medium. The values of diffusion coefficient (<i>D</i>) and electron transfer rate constant (<i>k</i>_sh) for all Fc-conjugates were determined from their corresponding cyclic voltammograms. In addition, metal ion interactions were studied with hydrolyzed Fc-conjugates <b>2a</b>–<b>4a</b> using CV and DPV. Upon binding to metal ions, the electrochemical changes associated with the hydrolyzed Fc-conjugates correlated to the charge density of the binding metal ion

Bis-amino Acid Derivatives of 1,1′-Ferrocenedicarboxylic Acid: Structural, Electrochemical, and Metal Ion Binding Studies

We report on the structural and electrochemical investigation of 1,1′-ferrocenedicarboxylic acid derivatives of tryptophan (Fc­[CO-Trp-OMe]₂ - Fc-conjugate <b>1</b>), threonine (Fc­[CO-Thr-OMe]₂ – Fc-conjugate <b>2</b>), aspartic acid (Fc­[CO-Asp-OMe]₂ - Fc-conjugate <b>3</b>) and glutamic acid (Fc­[CO-GluOMe]₂ - Fc-conjugate <b>4</b>) and their hydrolyzed analogues <b>1a</b>–<b>4a</b> respectively (Scheme ). CD and NMR spectroscopy established 1,2′-“Herrick conformation” in solution, having intramolecular interstrand hydrogen bonds for all Fc-conjugates. However, in solid state, Fc­[CO-Trp-OMe]₂ exists in “Herrick conformation” whereas Fc­[CO-Thr-OMe]₂ is present in anti conformation. In solution, the involvement of indole NH of Trp and alcoholic proton of Thr in intermolecular hydrogen bonding has been explored by temperature- and concentration-dependent NMR studies. The half-wave potentials (<i>E</i>_1/2) of ferrocene-conjugates follow the sequence <b>1 < 2 < 4 < 3</b> which is explained by the contribution of amino acid side chain functionalities toward the stability of ferrocenium ion. The CV of the Fc-conjugate <b>1/1a</b> (having Trp moiety) displays two redox processes, one of which is assigned to the Fc group, and the other being related to the indole group. The oxidation peak potential of indole was found to depend strongly on the pH of the medium. The values of diffusion coefficient (<i>D</i>) and electron transfer rate constant (<i>k</i>_sh) for all Fc-conjugates were determined from their corresponding cyclic voltammograms. In addition, metal ion interactions were studied with hydrolyzed Fc-conjugates <b>2a</b>–<b>4a</b> using CV and DPV. Upon binding to metal ions, the electrochemical changes associated with the hydrolyzed Fc-conjugates correlated to the charge density of the binding metal ion

Investigation of the Utility of Complementary Electrochemical Detection Techniques to Examine the in Vitro Affinity of Bacterial Flagellins for a Toll-Like Receptor 5 Biosensor

An initial investigation of the fabrication of a novel biosensor utilizing toll-like receptor 5 (TLR5) has been conducted. The detection assay using this sensor platform has been carried out using two complementary electrochemical techniques. The electrochemical properties of the modified bare gold surface following TLR5 immobilization were characterized. The electrochemical response to changes in the sensor film resistance and electron charge-transfer permittivity triggered by independent exposures to flagellins from Salmonella typhimurium (S. typhimurium) and Bacillus subtilis (B. subtilis) were examined and observed. The quantified film resistance data gathered using electrochemical impedance spectroscopy (EIS) over a macroscopic scale are in significant agreement with the corresponding electron charge-transfer permittivity measured locally by scanning electrochemical microscopy (SECM). Unlike other sensors that exploit pathogen recognition elements, TLR5 biosensors have the potential to carry out broad-spectrum detection of flagellated bacterial pathogens in near real time. This broad-spectrum detection platform is a significant step toward the development of fast, inexpensive clinical tools for early warning diagnoses and immediate on-site treatment

Surface Plasmon Resonance Imaging of Amyloid‑β Aggregation Kinetics in the Presence of Epigallocatechin Gallate and Metals

A number of human protein misfolding disorders, including Alzheimer’s disease (AD), are closely related to the accumulation of β-sheet-rich amyloid fibrils or aggregates. Neuronal toxicity in AD has been linked to the interactions of amyloid-β (Aβ) with metals, especially Zn²⁺, Cu²⁺, and Fe³⁺, which leads to the production of reactive oxygen species. Nucleation-dependent Aβ aggregation, or “seeding”, is thought to propagate fibril formation. In this surface plasmon resonance imaging (SPRi) study, we have shown that the fibril seeds formed with the incubation of Aβ in the presence of metals are better at promoting monomer elongation compared to Aβ alone or in the presence of a well-described polyphenol, (-)-epigallocatechin-3-gallate (EGCG). This is a novel attempt to simultaneously monitor the effects of multiple modulators on fibril elongation using a single chip. EGCG was shown in transmission electron microscopy (TEM) and thioflavin T (ThT) studies to promote the formation of off-pathway, highly stable unstructured oligomers, supporting the SPRi results. These findings suggest that SPRi provides a promising platform as a screening tool for small molecules that can affect the aggregation pathways in neurodegenerative diseases

Detection of the Lipopeptide Pam3CSK4 Using a Hybridized Toll-like Receptor Electrochemical Sensor

Electrochemical detection of Pam3CSK4, a synthetic triacylated lipopeptide that mimics the structural moieties of its natural Gram negative bacterial pathogen-associated molecular pattern (PAMP) counterpart, has been achieved using hybridized toll-like receptors (TLR) combining TLR1 and TLR2 onto a single sensor surface. These sensors represent the first hybridized TLR sensors. The limit of detection for Pam3CSK4 attained was 7.5 μg/mL, which is within the same order of magnitude for that of the more labor-intensive and time-consuming cell-assay technique, 2.0 μg/mL. The results gathered in these electrochemical experiments show that sensors fabricated by immobilizing a mixture of cooperative TLR1 and -2 generate higher responses when exposed to the analyte in comparison to the control sensors fabricated using pure TLR1 or -2 standalone. A PAMP selectivity test was carried out in line with our inspiration from the mammalian innate immune response. TLRs1–5 as standalone biorecognition elements and the hybridized “TLR1 and 2” sensor surface were investigated, understanding the known TLR-PAMP interactions, through the exploitation of this electrochemical sensor fabrication technique. The experimental result is consistent with observations from previously published <i>in vivo</i> and <i>in vitro</i> studies, and it is the first demonstration of the simultaneous evaluation of electrochemical responses from multiple, unique fabricated TLR sensor surfaces against the same analyte

Ferrocene-Modified Phospholipid: An Innovative Precursor for Redox-Triggered Drug Delivery Vesicles Selective to Cancer Cells

Controlled payload release is one of the key elements in the creation of a reliable drug delivery system. We report the discovery of a drug delivery vessel able to transport chemotherapeutic agents to target cancer cells and selectively trigger their release using the electrochemical activity of a ferrocene-modified phospholipid. Supported by <i>in vitro</i> assays, the competitive advantages of this discovery are (i) the simple one step scalability of the synthetic process, (ii) the stable encapsulation of toxic drugs (doxorubicin) during transport, and (iii) the selective redox triggering of the liposomes to harness their cytotoxic payload at the cancer site. Specifically, the redox-modified giant unilamellar vesicle and liposomes were characterized using advanced methods such as scanning electrochemical microscopy (SECM), transmission electron microscopy (TEM), dynamic light scattering (DLS), and fluorescent imaging

Synthesis and Surface Investigations of N-Substituted 2,5-Dithio-7-azabicyclo[2.2.1]heptanes on Gold Surfaces

The reaction of various primary amines and 2,5-dihydroxy-1,4-dithiane in the presence of a catalytic amount of Mg­(II) in distilled water provided a series of N-substituted 2,5-dithia-7-azabicyclo[2.2.1]­heptanes. The adsorption profiles of the sulfur-containing heterocycles on gold surfaces have been explored by time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS), and electrochemistry. SIMS data indicated that these novel bicyclic sulfides interact with gold surfaces favorably, independent of the N-substitution, with minimal fragmentation. An XPS study revealed the three component core levels of S 2p with binding energies at 161, 162, and 163 eV, indicating a combination of the bound and unbound sulfur species. Using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), we found the efficient adsorption of heterocycles onto gold and the formation of densely packed films for alkyl and phenyl analogues. However, the adsorption and film packing properties were greatly compromised by an N<i>-</i>pyridyl substitution. The findings indicate that the surface behavior of N-substituted 2,5-dithia-7-azabicyclo[2.2.1]­heptanes varies with respect to the N-substitution and the nature of the substituent, suggesting that the adsorption profiles and the film packing of bicyclic sulfides on gold surfaces are highly dependent on the binding interface and the molecular orientation