Holo/apo conversion two-dimensional urea PAGE for speciation of Fe3+-bound transferrin in serum

This paper presents holo/apo conversion two-dimensional urea polyacrylamide gel electrophoresis (HAC-2D urea PAGE) as a novel method for speciating Fe3+-bound transferrin (Tf) species in biological samples, with a combination of metal ion contaminant sweeping (MICS) technique and Fe3+ detection PAGE. In the HAC-2D urea MICS-PAGE approach, HAC was performed to dissociate all the Fe3+ ions bound to Tf from the Fe-Tf species, during a two-step urea PAGE. Using this method, Fe2-Tf, FeN-Tf, and FeC-Tf (holo-Tf, Fe3+-bound Tf attached to N-lobe, and Fe3+-bound Tf attached C-lobe, respectively) were completely isolated based on the difference in the higher-order structure of Tf, visible as horizontally aligned spots off the diagonal. The Fe3+ ions bound to Tf in each gel fraction were determined using PAGE with a fluorescent probe. Without the MICS technique, which electrophoretically removes all contaminant Fe3+ ions from the gel medium to ensure accurate determination of the Fe3+ concentration, it becomes challenging to precisely measure the distribution of metalloprotein species owing to the contaminants. Finally, the distribution of each Fe-bound Tf in a standard human serum sample was successfully determined by complete separation from large amounts of coexisting proteins, and the free Fe3+ concentration in the serum was estimated.

Unusually Kinetically Inert Monocationic Neptunyl Complex with a Fluorescein-Modified 1,10-Phenanthroline-2,9-dicarboxylate Ligand: Specific Separation and Detection in Gel Electrophoresis

We found a singly charged Np(V)O₂⁺ complex with unprecedented kinetic inertness in aqueous solution, one million times slower than the widely accepted fast kinetics of neptunyl complexes. An inert NpO₂⁺ complex with a fluorescent 1,10-phenanthroline-2,9-dicarboxylate derivative was found by kinetic selection using polyacrylamide gel electrophoresis (PAGE) from a small chemical library. Autoreduction from Np(VI)O₂²⁺ to Np(V)O₂⁺ via complexation was observed. A remarkably small spontaneous dissociation rate constant of 8 × 10^-6 s^-1 (half-life of 23 h) was determined using PAGE. Selective detection of Np(V)O₂⁺ was achieved in PAGE with a detection limit of 68 pmol dm^-3 (17 fg). This system was successfully applied to simulated radioactive waste samples. Our finding that electron-rich NpO₂⁺ forms a uniquely inert complex with no strong electrostatic interaction reveals a new aspect of actinide chemistry for developing a novel separation system of real radioactive material samples.

Stoichiometry between Humate Unit Molecules and Metal Ions in Supramolecular Assembly Induced by Cu2+ and Tb3+ Measured by Gel Electrophoresis Techniques

Humic acid (HA), a fraction of humic substances, can strongly complex with metal ions to form a supramolecular assembly via coordination binding and other intermolecular forces. However, determining the supramolecular size distribution and stoichiometry between small HA unit molecules constituting HA supramolecules and metal ions has proven to be challenging. Here, we investigated the changes in the size distributions of HAs induced by Cu²⁺ and Tb³⁺ ions using unique PAGE for the separation and quantification of HA complexes and metal ions bound, followed by UV-vis spectroscopy and excitation-emission matrix-parallel factor analysis. By determining the concentrations of HA and metal ions, it was possible to estimate the stoichiometry of the HA unit molecule to metal ions in supramolecular complexes. It was found that the supramolecular behaviors of Cu²⁺ and Tb³⁺ complexes with HA collected from peat (PAHA) and deep groundwater (HHA) differed. For example, two HHA unit molecules form a supramolecule via cross-linking by a Cu²⁺ ion in the case of Cu²⁺-HHA. Our results suggest that this supramolecular stoichiometry is related to the abundance of sulfur atoms in the elemental composition of HHA. Our experimental results and analysis provide new insights into HA supramolecules formed via metal complexation.

A terbium(iii) luminescent ATCUN-based peptide sensor for selective and reversible detection of copper(ii) in biological media

Detection of copper(ii) in biological media via time-delayed luminescence by a selective and reversible terbium(iii)-luminescent peptide sensor with pM affinity.

Advanced Gel Electrophoresis Techniques Reveal Heterogeneity of Humic Acids Based on Molecular Weight Distributions of Kinetically Inert Cu2+-Humate Complexes

Humic acids (HAs) play important roles for the fate of metal ions in the environment. Most chemical speciation models involving HAs assume heterogeneous metal ion binding. However, these models also assume that the binding affinities of metal ions with HAs are the same regardless of the molecular weight (MW) ranges of the HAs involved. Here, we develop new polyacrylamide gel electrophoresis (PAGE) techniques to investigate the MW distributions of HAs with strongly complexed Cu²⁺ ions. By combining contaminant metal-free and high-resolution PAGE for HAs, this work was able to provide accurate MW distributions for the complexed metal ions. The MW distribution of Cu²⁺ binding ability per quantity of HA indicates that strong metal-binding moieties in HAs are heterogeneous in terms of MW. Coupling of the PAGE techniques with UV-vis and excitation-emission matrix (EEM) spectrometry-parallel factor analysis (PARAFAC) methods revealed new insights into kinetically inert interactions between HAs and Cu²⁺ ions. By this method, we found that the protein-like fluorescence components in the high- and low-MW regions cooperatively responded through Cu²⁺ binding. Thus, the advanced gel electrophoresis techniques developed herein are able to shed new light on the heterogeneity of metal binding affinities of HAs in terms of MW.

Two-Dimensional Polyacrylamide Gel Electrophoresis for Metalloprotein Analysis Based on Differential Chemical Structure Recognition by CBB Dye

In an effort to develop an analytical method capable of finding new metalloproteins, this is the first report of a new diagonal gel electrophoresis method to isolate and identify metalloproteins, based on the molecular recognition of holo- and apo-metalloproteins (metalbound and -free forms, respectively) by CBB G-250 dye and employing metal ion contaminant sweeping-blue native-polyacrylamide gel electrophoresis (MICS-BN-PAGE). The difference in electrophoretic mobilities between holo- and apo-forms was exaggerated as a result of interactions between the metalloproteins and the dye with no metal ion dissociation. The different binding modes of proteins with CBB G-250 dye, primarily related to hydrogen bonding, were confirmed by capillary zone electrophoresis (CZE) and molecular docking simulations. Due to in-gel holo/apo conversion between the first and second dimensions of PAGE, holo-metalloproteins in the original sample were completely isolated as spots off the diagonal line in the second dimension of PAGE. To prove the high efficiency of this method for metalloprotein analysis, we successfully identified a copper-binding protein from a total bacterial soluble extract for the first time.

ICPMS analysis of proteins separated by Native-PAGE: Evaluation of metaloprotein profiles in human synovial fluid with acute and chronic arthritis

The role of trace elements bound to proteins in the etiology and pathogenesis of rheumatoid arthritis (RA) remains unclear. In this sense, the identification and detection of metalloproteins has a strong and growing interest. Metalloprotein studies are currently carried out by polyacrylamide gel electrophoresis (PAGE) associated to inductively coupled plasma mass spectrometry (ICPMS), and despite that complete information can be obtained for metals such as Fe, Cu and Zn, difficulties due to poor sensitivity for other trace elements such as Sn, As, etc, are currently faced. In the present work, a simple and fast method for the determination of trace metals bound to synovial fluid (SF) proteins was optimized. Proteins from SF (long and short-term RA) were separated in ten fractions by native PAGE, then dissolved in nitric acid and peroxide hydrogen, and analyzed by ICPMS. Fifteen metals were determined in each separated protein fraction (band). Adequate calibration of proteins molecular weight allowed stablishing which protein type were bound to different metals.

c-Type Cytochrome Assembly Is a Key Target of Copper Toxicity within the Bacterial Periplasm

In the absence of a tight control of copper entrance into cells, bacteria have evolved different systems to control copper concentration within the cytoplasm and the periplasm. Central to these systems, the Cu⁺ ATPase CopA plays a major role in copper tolerance and translocates copper from the cytoplasm to the periplasm. The fate of copper in the periplasm varies among species. Copper can be sequestered, oxidized, or released outside the cells. Here we describe the identification of CopI, a periplasmic protein present in many proteobacteria, and show its requirement for copper tolerance in Rubrivivax gelatinosus. The ΔcopI mutant is more susceptible to copper than the Cu⁺ ATPase copA mutant. CopI is induced by copper, localized in the periplasm and could bind copper. Interestingly, copper affects cytochrome c membrane complexes (cbb₃ oxidase and photosystem) in both ΔcopI and copA-null mutants, but the causes are different. In the copA mutant, heme and chlorophyll synthesis are affected, whereas in ΔcopI mutant, the decrease is a consequence of impaired cytochrome c assembly. This impact on c-type cytochromes would contribute also to the copper toxicity in the periplasm of the wild-type cells when they are exposed to high copper concentrations.

Copper is an essential cation required as a cofactor in enzymes involved in vital processes such as respiration, photosynthesis, free radical scavenging, and pathogenesis. However, copper is highly toxic and has been implicated in disorders in all organisms, including humans, because it can catalyze the production of toxic reactive oxygen species and targets various biosynthesis pathways. Identifying copper targets, provides insights into copper toxicity and homeostatic mechanisms for copper tolerance. In this work, we describe for the first time a direct effect of excess copper on cytochrome c assembly. We show that excess copper specifically affects periplasmic and membrane cytochromes c, thus suggesting that the copper toxicity targets c-type cytochrome biogenesis.

Highly sensitive visual detection of copper (II) using water-soluble azide-functionalized gold nanoparticles and silver enhancement

A high-sensitive method for the visual detection of copper ions in aqueous solution is developed. The method is based on copper ion-catalyzed 'click' reaction between the water-soluble azide-functionalized gold nanoparticles (AuNPs) and alkyne-modified glass slide. The PEG linker was employed as a stabilizing component along with the terminal azide group to keep the AuNPs stably dispersed in water without the assistance of any organic solvent. In the presence of copper ions, the AuNPs are 'clicked' on the slide, and the darkness of the AuNPs in the sample spot is promoted by silver enhancement process. Only a tiny amount of sample (10 μl) is needed with the detectable concentration down to 62 pM by the commonly used flatbed scanner, which is 2-3 orders of magnitude lower than those in previous reports. The selectivity relative to other potentially interfering ions and the applicability in real samples, human serum and tap water, have also been evaluated. Our method has a good potential in point-of-use applications and environment surveys.