Engineering uranyl-chelating peptides from NikR for electrochemical peptide-based sensing applications

Coordination Chemistry of Uranyl Ions with Surface-Immobilized Peptides: An XPS Study

The coordination chemistry of uranyl ions with surface immobilized peptides was studied using X-ray photoemission spectroscopy (XPS). All the peptides in the study were modified using a six-carbon alkanethiol as a linker on a gold substrate with methylene blue as the redox label. The X-ray photoemission spectra reveal that each modified peptide interacts differently with the uranyl ion. For all the modified peptides, the XPS spectra were taken in both the absence and presence of the uranium, and their comparison reveals that the interaction depends on the chemical group present in the peptides. The XPS results show that, among all the modified peptides in the current study, the (arginine)₉ (R9) modified peptide showed the largest response to uranium. In the order of response to uranium, the second largest response was shown by the modified (arginine)₆ (R6) peptide followed by the modified (lysine)₆ (K6) peptide. Other modified peptides, (alanine)₆ (A6), (glutamic acid)₆ (E6) and (serine)₆ (S6), did not show any response to uranium.

Threonine Phosphorylation of an Electrochemical Peptide-Based Sensor to Achieve Improved Uranyl Ion Binding Affinity

We have successfully designed a uranyl ion (U(VI)-specific peptide and used it in the fabrication of an electrochemical sensor. The 12-amino acid peptide sequence, (n) DKDGDGYIpTAAE (c), originates from calmodulin, a Ca(II)-binding protein, and contains a phosphothreonine that enhances the sequence's affinity for U(VI) over Ca(II). The sensing mechanism of this U(VI) sensor is similar to other electrochemical peptide-based sensors, which relies on the change in the flexibility of the peptide probe upon interacting with the target. The sensor was systematically characterized using alternating current voltammetry (ACV) and cyclic voltammetry. Its limit of detection was 50 nM, which is lower than the United States Environmental Protection Agency maximum contaminant level for uranium. The signal saturation time was ~40 min. In addition, it showed minimal cross-reactivity when tested against nine different metal ions, including Ca(II), Mg(II), Pb(II), Hg(II), Cu(II), Fe(II), Zn(II), Cd(II), and Cr(VI). Its reusability and ability to function in diluted aquifer and drinking water samples were further confirmed and validated. The response of the sensor fabricated with the same peptide sequence but with a nonphosphorylated threonine was also analyzed, substantiating the positive effects of threonine phosphorylation on U(VI) binding. This study places emphasis on strategic utilization of non-standard amino acids in the design of metal ion-chelating peptides, which will further diversify the types of peptide recognition elements available for metal ion sensing applications.

Hydrophobic core formation and secondary structure elements in uranyl(VI)-binding peptides

Cyclic peptides as well as a modified EF-hand motif of calmodulin have been newly designed to achieve high affinity towards uranyl(VI). Cyclic peptides may be engineered to bind uranyl(VI) to its backbone under acidic conditions, which may enhance its selectivity. For the modified EF-hand motif of calmodulin, strong electrostatic interactions between uranyl(VI) and negatively charged side chains play an important role in achieving high affinity; however, it is also essential to have a secondary structure element and formation of hydrophobic cores in the metal-bound state of the peptide.

Dual-signal amplification electrochemical sensing for the sensitive detection of uranyl ion based on gold nanoparticles and hybridization chain reaction-assisted synthesis of silver nanoclusters

Herein, a dual-signal amplification electrochemical sensing has been proposed for the ultrasensitive detection of uranyl ions (UO₂²⁺) by integration of gold nanoparticles (AuNPs) and hybridization chain reaction (HCR)-assisted synthesis of silver nanoclusters (AgNCs). In this sensing platform, AuNPs are used as an ideal signal amplification carrier, aiming at increasing the loads of UO₂²⁺-specific DNAzyme on the gold electrode. In the presence of UO₂²⁺, UO₂²⁺-specific DNAzyme can be activated, leading to the cleavage of substrate strands (S-DNA). Then, HCR is triggered to produce long dsDNA through hybridization the probe with the ssDNA on the electrode surface. As a result, an amplified electrochemical response can be detected by inserting a large amount of AgNCs generated in situ using dsDNA as template. Featured with amplification efficiency, good specificity and high sensitivity, the strategy could quantitatively detect UO₂²⁺ down to 6.2 pM with a linear calibration range from 20 pM to 5000 pM. The proposed sensing platform has been also successfully demonstrated the practical application of detecting UO₂²⁺, indicating that the developed method has the potential applications and can open up a new avenue for highly sensitive detection of UO₂²⁺ in environmental monitoring.

Uranyl Binding to Proteins and Structural-Functional Impacts

The widespread use of uranium for civilian purposes causes a worldwide concern of its threat to human health due to the long-lived radioactivity of uranium and the high toxicity of uranyl ion (UO₂²⁺). Although uranyl–protein/DNA interactions have been known for decades, fewer advances are made in understanding their structural-functional impacts. Instead of focusing only on the structural information, this article aims to review the recent advances in understanding the binding of uranyl to proteins in either potential, native, or artificial metal-binding sites, and the structural-functional impacts of uranyl–protein interactions, such as inducing conformational changes and disrupting protein-protein/DNA/ligand interactions. Photo-induced protein/DNA cleavages, as well as other impacts, are also highlighted. These advances shed light on the structure-function relationship of proteins, especially for metalloproteins, as impacted by uranyl–protein interactions. It is desired to seek approaches for biological remediation of uranyl ions, and ultimately make a full use of the double-edged sword of uranium.