Click-generated triazole based ferrocene-carbohydrate bioconjugates: a highly selective multisignalling probe for Cu(II) ions

Two Cu2+-specific colorimetric sensors, based on ferrocene-carbohydrate bioconjugates, 2, C46H56O20N6Fe and 3, C28H33O10N3Fe were designed and synthesized in good yields. Both the compounds, 2 and 3, behave as very selective and sensitive chromogenic and electrochemical chemosensor for Cu2+ ion in aqueous environment (CH3CN/H2O (2:8, v/v). The analytical detection limit (ADL) for receptor 2 was 7.5 × 10−7 M. The considerable changes in their absorption spectra of 2 and 3 are accompanied by the appearance of a new low energy (LE) peak at 630 nm (2: ε = 1600 M−1 cm−1 and 3: 822 M−1 cm−1). This is further accompanied by a strong colour change from yellow to dark green that allows the prospective for ‘naked eye’ detection of Cu2+ ion

A highly selective redox, chromogenic, and fluorescent chemosensor for Hg²⁺ in aqueous solution based on ferrocene–glycine bioconjugates

The synthesis, electrochemical, optical, and metal-cation-sensing properties of ferrocene–glycine conjugates C₃₀H₃₈O₈N₈Fe (2) and C₂₀H₂₄O₄N₄Fe (3) have been documented. Both compounds 2 and 3 behave as very selective redox (ΔE_1/2 = 217 mV for 2 and ΔE_1/2 = 160 mV for 3), chromogenic, and fluorescent chemosensors for Hg²⁺ cations in an aqueous environment. The considerable changes in their absorption spectra are accompanied by the appearance of a new low-energy peak at 630 nm (2, ε = 1600 M^–1cm^–1; 3, ε = 822 M^–1cm^–1). This is also accompanied by a strong color change from yellow to purple, which allows a prospective for the “naked eye” detection of Hg²⁺ cations. These chemosensors present immense brightness and fluorescence enhancement (chelation-enhanced fluorescence = 91 for 2 and 42 for 3) following Hg²⁺ coordination within the limit of detection for Hg²⁺ at 7.5 parts per billion

The “Gln-Type” Thiol Dioxygenase from <i>Azotobacter vinelandii</i> Is a 3‑Mercaptopropionic Acid Dioxygenase

Cysteine dioxygenase (CDO) is a non-heme iron enzyme that catalyzes the O₂-dependent oxidation of l-cysteine to produce cysteinesulfinic acid. Bacterial CDOs have been subdivided as either “Arg-type” or “Gln-type” on the basis of the identity of conserved active site residues. To date, “Gln-type” enzymes remain largely uncharacterized. It was recently noted that the “Gln-type” enzymes are more homologous with another thiol dioxygenase [3-mercaptopropionate dioxygenase (MDO)] identified in <i>Variovorax paradoxus</i>, suggesting that enzymes of the “Gln-type” subclass are in fact MDOs. In this work, a putative “Gln-type” thiol dioxygenase from <i>Azotobacter vinelandii</i> (<i>Av</i>) was purified to homogeneity and characterized. Steady-state assays were performed using three substrates [3-mercaptopropionic acid (<i><b>3mpa</b></i>), l-cysteine (<i><b>cys</b></i>), and cysteamine (<i><b>ca</b></i>)]. Despite comparable maximal velocities, the “Gln-type” <i>Av</i> enzyme exhibited a specificity for <i><b>3mpa</b></i> (<i>k</i>_cat/<i>K</i>_M = 72000 M^–1 s^–1) nearly 2 orders of magnitude greater than those for <i><b>cys</b></i> (110 M^–1 s^–1) and <i><b>ca</b></i> (11 M^–1 s^–1). Supporting X-band electron paramagnetic resonance (EPR) studies were performed using nitric oxide (NO) as a surrogate for O₂ binding to confirm obligate-ordered addition of substrate prior to NO. Stoichimetric addition of NO to solutions of <i><b>3mpa</b></i>-bound enzyme quantitatively yields an iron-nitrosyl species (<i>Av</i> <b>ES</b>-NO) with EPR features consistent with a mononuclear (<i>S</i> = ³/₂) {FeNO}⁷ site. Conversely, two distinct substrate-bound conformations were observed in <i>Av</i> <b>ES</b>-NO samples prepared with <i><b>cys</b></i> and <i><b>ca</b></i>, suggesting heterogeneous binding within the enzymatic active site. Analytical EPR simulations are provided to establish the relative binding affinity for each substrate (<i><b>3map</b></i> > <i><b>cys</b></i> > <i><b>ca</b></i>). Both kinetic and spectroscopic results presented here are consistent with <i><b>3mpa</b></i> being the preferred substrate for this enzyme

Structure of 3-mercaptopropionic acid dioxygenase with a substrate analog reveals bidentate substrate binding at the iron center.

Thiol dioxygenases are a subset of nonheme iron oxygenases that catalyze the formation of sulfinic acids from sulfhydryl-containing substrates and dioxygen. Among this class, cysteine dioxygenases (CDOs) and 3-mercaptopropionic acid dioxygenases (3MDOs) are the best characterized, and the mode of substrate binding for CDOs is well understood. However, the manner in which 3-mercaptopropionic acid (3MPA) coordinates to the nonheme iron site in 3MDO remains a matter of debate. A model for bidentate 3MPA coordination at the 3MDO Fe-site has been proposed on the basis of computational docking, whereas steady-state kinetics and EPR spectroscopic measurements suggest a thiolate-only coordination of the substrate. To address this gap in knowledge, we determined the structure of Azobacter vinelandii 3MDO (Av3MDO) in complex with the substrate analog and competitive inhibitor, 3-hydroxypropionic acid (3HPA). The structure together with DFT computational modeling demonstrates that 3HPA and 3MPA associate with iron as chelate complexes with the substrate-carboxylate group forming an additional interaction with Arg168 and the thiol bound at the same position as in CDO. A chloride ligand was bound to iron in the coordination site assigned as the O2-binding site. Supporting HYSCORE spectroscopic experiments were performed on the (3MPA/NO)-bound Av3MDO iron nitrosyl (S&nbsp;= 3/2) site. In combination with spectroscopic simulations and optimized DFT models, this work provides an experimentally verified model of the Av3MDO enzyme-substrate complex, effectively resolving a debate in the literature regarding the preferred substrate-binding denticity. These results elegantly explain the observed 3MDO substrate specificity, but leave unanswered questions regarding the mechanism of substrate-gated reactivity with dioxygen