Aqueous Spectroscopy and Redox Properties of Carboxylate-Bound Titanium

Toward the development of a molecular toolkit for the microbial remediation of per-and polyfluoroalkyl substances

Per- and polyfluoroalkyl substances (PFAS) are highly fluorinated synthetic organic compounds that have been used extensively in various industries owing to their unique properties. The PFAS family encompasses diverse classes, with only a fraction being commercially relevant. These substances are found in the environment, including in water sources, soil, and wildlife, leading to human exposure and fueling concerns about potential human health impacts. Although PFAS degradation is challenging, biodegradation offers a promising, eco-friendly solution. Biodegradation has been effective for a variety of organic contaminants but is yet to be successful for PFAS due to a paucity of identified microbial species capable of transforming these compounds. Recent studies have investigated PFAS biotransformation and fluoride release; however, the number of specific microorganisms and enzymes with demonstrable activity with PFAS remains limited. This review discusses enzymes that could be used in PFAS metabolism, including haloacid dehalogenases, reductive dehalogenases, cytochromes P450, alkane and butane monooxygenases, peroxidases, laccases, desulfonases, and the mechanisms of microbial resistance to intracellular fluoride. Finally, we emphasize the potential of enzyme and microbial engineering to advance PFAS degradation strategies and provide insights for future research in this field.

Cellular fate and performance of group IV metal organic framework radioenhancers

Nano-sized metal organic frameworks (nanoMOFs) have gained increasing importance in biomedicine due to their tunable properties. In addition to their use as carriers in drug delivery, nanoMOFs containing hafnium have been successfully employed as radio-enhancers augmenting damage caused by X-ray irradiation in tumor tissue. While results are encouraging, there is little mechanistic understanding available, and the biological fate of these radio-enhancer nanoparticles remains largely unexplored. Here, we synthesized a selection of group IV metal-based (Hf, Ti, Ti/Zr) nanoMOFs and investigated their cell compatibility and radio-enhancement performance in direct comparison to the corresponding metal oxides. We report surprising radio-enhancement performance of Ti-containing nanoMOFs reaching dose modifying ratios of 3.84 in human sarcoma cells and no relevant dose modification in healthy human fibroblasts. These Ti-based nanoMOFs even outperformed previously reported Hf-based nanoMOFs as well as equimolar group IV metal oxides in direct benchmarking experiments. While group IV nanoMOFs were well-tolerated by cells in the absence of irradiation, the nanoMOFs partially dissolved in lysosomal buffer conditions showing distinctly different chemical stability compared to widely researched group IV oxides (TiO2, ZrO2, and HfO2). Taken together, this study illustrates the promising potential of Ti-based nanoMOFs for radio-enhancement and provides insight into the intracellular fate and stability of group IV nanoMOFs.

A General Design Strategy Enabling the Synthesis of Hydrolysis-Resistant, Water-Stable Titanium(IV) Complexes

Despite its prevalence in the environment, the chemistry of the Ti⁴⁺ ion has long been relegated to organic solutions or hydrolyzed TiO₂ polymorphs. A knowledge gap in stabilizing molecular Ti⁴⁺ species in aqueous environments has prevented the use of this ion for various applications such as radioimaging, design of water-compatible metal-organic frameworks (MOFs), and aqueous-phase catalysis applications. Herein, we show a thorough thermodynamic screening of bidentate chelators with Ti⁴⁺ in aqueous solution, as well as computational and structural analyses of key compounds. In addition, the hexadentate analogues of catechol (benzene-1,2-diol) and deferiprone (3-hydroxy-1,2-dimethyl-4(1H)-pyridone), TREN-CAM and THP^Me respectively, were assessed for chelation of the ⁴⁵ Ti isotope (t_1/2 =3.08 h, β⁺ =85 %, E_β+ =439 keV) towards positron emission tomography (PET) imaging applications. Both were found to have excellent capacity for kit-formulation, and [⁴⁵ Ti]Ti-TREN-CAM was found to have remarkable stability in vivo.

Assignments of 19F NMR resonances and exploration of dynamics in a long-chain flavodoxin

Flavodoxin is a small protein that employs a non-covalently bound flavin to mediate single-electron transfer at low potentials. The long-chain flavodoxins possess a long surface loop that is proposed to interact with partner proteins. We have incorporated ¹⁹F-labeled tyrosine in long-chain flavodoxin from Rhodopseudomonas palustris to gain a probe of possible loop dynamics, exploiting the presence of a Tyr in the long loop in addition to Tyr residues near the flavin. We report ¹⁹F resonance assignments for all four Tyrs, and demonstration of a pair of resonances in slow exchange, both corresponding to a Tyr adjacent to the flavin. We also provide evidence for dynamics affecting the Tyr in the long loop. Thus, we show that ¹⁹F NMR of ¹⁹F-Tyr labeled flavodoxin holds promise for monitoring possible changes in conformation upon binding to partner proteins.

Efficient removal of mercury (II) from water by use of cryogels and comparison to commercial adsorbents under environmentally relevant conditions

Mercury is a toxic element, which can be found in air, water and soil in several inorganic and organic forms. Mercury pollution comes from a variety of industrial sources, including vinyl-chloride, pulp and paper, fertilizers and pharmaceuticals industry, gold mining and cement production. Gels have increasingly attracted the interest over the past decades and one of the investigated applications is the fast removal of organic substances, metals and other cations and anions from water. In this work, two types of cryogels were synthesized at sub-zero temperature by free-radical polymerization technique, characterized by using a set of complimentary methods and used for the removal of mercury from aqueous solutions of different chemistry. Kinetics and equilibrium studies were performed in ultra-pure water solutions in order to study the mechanisms in the presence nitrate and chloride ions. The cryogels exhibited excellent efficiency towards mercury removal from all model solutions. Moreover, the cryogels were tested in different water matrixes (tap, river and sea water) and compared to commercial adsorbents (activated carbon, strong acid resin and zeolite Y). Cryogels were able to remove mercury much faster than commercial adsorbents with the exception of seawater where activated carbon was superior.

Natural and Engineered Electron Transfer of Nitrogenase

As the only enzyme currently known to reduce dinitrogen (N2) to ammonia (NH3), nitrogenase is of significant interest for bio-inspired catalyst design and for new biotechnologies aiming to produce NH3 from N2. In order to reduce N2, nitrogenase must also hydrolyze at least 16 equivalents of adenosine triphosphate (MgATP), representing the consumption of a significant quantity of energy available to biological systems. Here, we review natural and engineered electron transfer pathways to nitrogenase, including strategies to redirect or redistribute electron flow in vivo towards NH3 production. Further, we also review strategies to artificially reduce nitrogenase in vitro, where MgATP hydrolysis is necessary for turnover, in addition to strategies that are capable of bypassing the requirement of MgATP hydrolysis to achieve MgATP-independent N2 reduction.

Study of arsenic (III) removal by monolayer protected silver nanoadsorbent and its execution on prokaryotic system

This work deals with the removal of arsenic by nanoadsorbent from aqueous environment that is subsequently applied to the biological system for the evaluation of its efficiency. We started our aspiration by the modification of carboxylate functionalized silver nanoparticle (nanoadsorbent) fabrication process. Batch mode arsenic uptake study by the nanoadsorbent was conducted considering several altering parameters and the reactants in addition to products were evaluated by several analytical techniques for the interpretation of the interaction mechanism. It was found nanoadsorbent, Ag@MSA is an efficient system for the exclusion of arsenic (III) from the aqueous system and due to the alteration in the ratio of silver content and protective agent, the characteristic profile of silver nanoparticles with an average diameter of 15 nm also became changed in respect of reported results. Here the low pH range (4-6) favors the interaction between nanoparticle and arsenic and it was found that the interaction was chemical in nature through adsorption or complex formation with surface carboxylate groups of the protecting agent (MSA). Following the interaction, a successful removal of arsenic (III) was achieved at a percentage of 94.16 with an initial concentration of 45 mg/L and equilibrium time of 60 min. Hence nanoparticles were executed against the toxic effect of arsenic in E. coli, an important gut microbe of higher animals, for the restoration of bacterial growth in arsenic pre-removed media. In this context the validation of this removal efficiency against arsenic induced toxicity was proved through several morphological studies, degree of oxidative damages and other biochemical attributes.

Fueling a Hot Debate on the Application of TiO2 Nanoparticles in Sunscreen

Titanium is one of the most abundant elements in the earth's crust and while there are many examples of its bioactive properties and use by living organisms, there are few studies that have probed its biochemical reactivity in physiological environments. In the cosmetic industry, TiO2 nanoparticles are widely used. They are often incorporated in sunscreens as inorganic physical sun blockers, taking advantage of their semiconducting property, which facilitates absorbing ultraviolet (UV) radiation. Sunscreens are formulated to protect human skin from the redox activity of the TiO2 nanoparticles (NPs) and are mass-marketed as safe for people and the environment. By closely examining the biological use of TiO2 and the influence of biomolecules on its stability and solubility, we reassess the reactivity of the material in the presence and absence of UV energy. We also consider the alarming impact that TiO2 NP seepage into bodies of water can cause to the environment and aquatic life, and the effect that it can have on human skin and health, in general, especially if it penetrates into the human body and the bloodstream.

Exploring titanium(IV) chemical proximity to iron(III) to elucidate a function for Ti(IV) in the human body

Despite its natural abundance and widespread use as food, paint additive, and in bone implants, no specific biological function of titanium is known in the human body. High concentrations of Ti(IV) could result in cellular toxicity, however, the absence of Ti toxicity in the blood of patients with titanium bone implants indicates the presence of one or more biological mechanisms to mitigate toxicity. Similar to Fe(III), Ti(IV) in blood binds to the iron transport protein serum transferrin (sTf), which gives credence to the possibility of its cellular uptake mechanism by transferrin-directed endocytosis. However, once inside the cell, how sTf bound Ti(IV) is released into the cytoplasm, utilized, or stored remain largely unknown. To explain the molecular mechanisms involved in Ti use in cells we have drawn parallels with those for Fe(III). Based on its chemical similarities with Fe(III), we compare the biological coordination chemistry of Fe(III) and Ti(IV) and hypothesize that Ti(IV) can bind to similar intracellular biomolecules. The comparable ligand affinity profiles suggest that at high Ti(IV) concentrations, Ti(IV) could compete with Fe(III) to bind to biomolecules and would inhibit Fe bioavailability. At the typical Ti concentrations in the body, Ti might exist as a labile pool of Ti(IV) in cells, similar to Fe. Ti could exhibit different types of properties that would determine its cellular functions. We predict some of these functions to mimic those of Fe in the cell and others to be specific to Ti. Bone and cellular speciation and localization studies hint toward various intracellular targets of Ti like phosphoproteins, DNA, ribonucleotide reductase, and ferritin. However, to decipher the exact mechanisms of how Ti might mediate these roles, development of innovative and more sensitive methods are required to track this difficult to trace metal in vivo.

A novel hexanuclear titanium(iv)-oxo-iminodiacetate cluster with a Ti6O9 core: single-crystal structure and photocatalytic activities

A new family of hexanuclear titanium(iv)-oxo-carboxylate cluster K7H[Ti6O9(ida)6]Cl2·13H2O {Ti6O9} has been synthesized via the H2O2-assisted reaction between TiCl4 and iminodiacetate ligands. This cluster was fully characterized by single-crystal X-ray diffraction and a wide range of analytical methods, including FT-IR, UV/vis spectroscopy as well as electrochemistry and thermogravimetric analysis. As a new type of carboxylate substituted Ti-oxo-cluster, the structural motif of the {Ti6O9} cluster consists of one symmetric {Ti6O6} hexagonal prism with two staggered triangular {Ti3O3} subunits linked by three μ2-O bridges. The {Ti6O9} polyanions are linked by K(+) cations to form a novel 3D architecture. The structural information and stability of the {Ti6O9} polyanion in aqueous solution were thoroughly investigated by solid-state/solution NMR, ESI-MS spectroscopy. Moreover, this Ti-oxo cluster exhibits remarkable potential as a visible-light homogeneous photocatalyst for degradation of rhodamine B (RhB). Finally, a proposed peroxotitanium(iv)-mediated photocatalytic pathway involved is illustrated by spectroscopic data.

Titanium Imido Complexes via Displacement of -SiMe3 and C-H Bond Activation in a Ti(III) Amido Complex, Promoted by a Cyclic (Alkyl)(Amino) Carbene (cAAC)

A strong σ-donating cyclic (alkyl)(amino) carbene (cAAC) triggers rearrangement of the silyl(aryl) amido ligand -N(SiMe3)Dipp (Dipp = 2,6-diisopropylphenyl) in the coordination sphere of titanium(III) to afford a novel zwitterionic titanium imido complex with a TiCH2SiMe2[cAAC] linkage. Reduction of this species produces a new DippN=Ti imido complex containing a cAAC-centered radical species, characterized by single crystal diffraction analysis and electron paramagnetic resonance spectroscopy.

Contemplating a role for titanium in organisms

Titanium is the ninth most abundant element in the Earth's crust and some organisms sequester it avidly, though no essential biological role has yet been recognized. This Minireview addresses how the properties of titanium, especially in an oxic aqueous environment, might make a biological role difficult to recognize. It further considers how new -omic technologies might overcome the limitations of the past and help to reveal a specific role for this metal. While studies with well established model organisms have their rightful place, organisms that are known avid binders or sequesterers of titanium should be promising places to investigate a biological role.

Nickel-Substituted Rubredoxin as a Minimal Enzyme Model for Hydrogenase

A simple, functional mimic of [NiFe] hydrogenases based on a nickel-substituted rubredoxin (NiRd) protein is reported. NiRd is capable of light-initiated and solution-phase hydrogen production and demonstrates high electrocatalytic activity using protein film voltammetry. The catalytic voltammograms are modeled using analytical expressions developed for hydrogenase enzymes, revealing maximum turnover frequencies of approximately 20-100 s(-1) at 4 °C with an overpotential of 540 mV. These rates are directly comparable to those observed for [NiFe] hydrogenases under similar conditions. Like the native enzymes, the proton reduction activity of NiRd is strongly inhibited by carbon monoxide. This engineered rubredoxin-based enzyme is chemically and thermally robust, easily accessible, and highly tunable. These results have implications for understanding the enzymatic mechanisms of native hydrogenases, and, using NiRd as a scaffold, it will be possible to optimize this catalyst for application in sustainable fuel generation.

Spectroscopic investigations of nano-sized titanium(IV) complexes containing electron-rich oxygen-based ligands

A convenient method has been explored to synthesize some nano-sized, mixed-ligand complexes of titanium(IV) with the general formula [Ti(acac)Cl(2-n)(L)(n)(OOCC(15)H(31))] (where Hacac=acetylacetone, HL=dibenzoylmethane or benzoylacetone and n=1 or 2). They have been synthesized by stepwise substitutions of chloride ions from titanium(IV) chloride with straight chain carboxylic acid and β-diketones. These were characterized by elemental analyses, molecular weight determinations, spectral (electronic, FTIR, (1)H NMR and powder XRD) and TEM studies. Conductance measurements indicated their non-conducting nature which may behave like insulators. Bidentate chelating nature of carboxylate and β-diketones anions in the complexes was established by their infrared spectra. LMCT bands were observed in the electronic spectra. Molecular weight determinations indicated mononuclear nature of the complexes. Powder XRD and transmission electron microscopy (TEM) studies indicated the particles of these were lying in the nano-size range. The complexes exhibited high resistance to hydrolysis. On the basis of these studies, coordination number 7 or 8 is assigned for titanium in the synthesized complexes.

Spectroscopic studies on some fluorescent mixed-ligand titanium(IV) complexes

A novel route to synthesize some titanium(IV) complexes containing acetylacetone, straight chain carboxylic acid and hydroxycarboxylic acid ligands has been investigated. Complexes with the general formula [Ti(acac)Cl(2-n)(OOCR*)(n)(OOCC(15)H(31))] (where Hacac=acetylacetone, R*COOH=hydroxycarboxylic acids and n=1 or 2) have been isolated and characterized. Molecular weight determinations indicated mononuclear nature of the complexes. LMCT bands were observed in the electronic spectra. Infrared spectra suggested bidentate nature of the ligands. Fluorescent behaviour of the complexes was noticed on the basis of fluorescence spectra. Powder XRD indicated them to be semi-crystalline having the crystallite size in 136-185 nm range. Transmission electron microscopy (TEM) indicated spherical particles of ~ 200 nm diameter. On the basis of physico-chemical studies, it is suggested that titanium is having coordination number 7 or 8 in these complexes.

A practical silver nanoparticle-based adsorbent for the removal of Hg2+ from water

In this work, we describe the use of silver nanoparticles of 9 ± 2 and 20 ± 5 nm core diameter, protected by mercaptosuccinic acid (MSA) and supported on activated alumina for the removal of mercuric ions present in contaminated waters, at room temperature (28 ± 1 °C). These two nanoparticle samples were prepared by using two Ag:MSA ratios 1:6 and 1:3, respectively, during synthesis and were loaded on alumina at 0.5 and 0.3% by weight. The mechanism of interaction of silver nanoparticles with Hg(2+) ions was studied using various analytical techniques such as ultraviolet-visible spectroscopy (UV-vis), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), dynamic light scattering (DLS), inductively coupled plasma-optical emission spectrometry (ICP-OES), energy dispersive analysis of X-rays (EDAX), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Interactions of the metal ion with the metal core, the surface head group and the monolayer functionality were investigated. A high removal ability of 0.8 g of mercury per gram of Ag@MSA was achieved in the case of 1:6 Ag@MSA. These two materials show better uptake capacity of Hg(2+) in the pH range of 5-6. The ease of synthesis of the nanomaterial by wet chemistry, capability to load on suitable substrates to create stable materials and affordable cost will make it possible to use this approach in field applications, especially for the treatment of Hg(2+) contaminated waters.

Nano-sized, quaternary titanium(IV) metal-organic frameworks with multidentate ligands

Some mononuclear nano-sized, quaternary titanium(IV) complexes having the general formula [Ti(acac)(OOCR)2(SB)] (where Hacac=acetylacetone, R=C15H31 or C17H35, HSB=Schiff bases) have been synthesized using different multidentate ligands. These were characterized by elemental analyses, molecular weight determinations and spectral (FTIR, 1H NMR and powder XRD) studies. Conductance measurement indicated their non-conducting nature which may behave like insulators. Structural parameters like the values of limiting indices h, k, l, cell constants a, b, c, angles α, β, γ and particle size are calculated from powder XRD data for complex 1 which indicated nano-sized triclinic system in them. Bidentate chelating nature of acetylacetone, carboxylate and Schiff base anions in the complexes was established by their infrared spectra. Molecular weight determinations confirmed mononuclear nature of the complexes. On the basis of physico-chemical studies, coordination number 8 was assigned for titanium(IV) in the complexes. Transmission electron microscopy (TEM) and the selected area electron diffraction (SAED) studies indicated spherical particles with poor crystallinity.

A review of recent trends in electrospray ionisation-mass spectrometry for the analysis of metal-organic ligand complexes

Electrospray ionisation-mass spectrometry (ESI-MS) is used in a wide variety of fields to examine the formation, stoichiometry and speciation of complexes involving metals and organic ligands. This article reviews the literature in this area over the past 5 years, examining trends in ESI-MS use and novel applications that enhance the scope of the technique. ESI-MS can provide direct information on changes in speciation with metal:ligand ratio and pH, identify metal oxidation state directly and allow insight into competitive interactions in ternary systems. However, both the instrumental set-up and artefacts in the electrospraying process can affect the species distribution observed, and changes in solution chemistry can affect the relative ion intensity of species. Therefore, ESI-MS data is at its most powerful when corroborated by data from other experimental techniques, such as pH potentiometry. The challenges in interpreting direct ESI-MS data quantitatively are discussed in detail, with reference to differences in the ion intensities of species, signal suppression and quantifying species distributions. The use of HPLC-ESI-MS is also reviewed, highlighting challenges and applications. Overall, the need for more standard reporting of quality assurance data is discussed, to strengthen the applications of ESI-MS to metal-organic ligand complexes further.

H135A controls the redox activity of the Sco copper center. Kinetic and spectroscopic studies of the His135Ala variant of Bacillus subtilis Sco

Sco-like proteins contain copper bound by two cysteines and a histidine residue. Although their function is still incompletely understood, there is a clear involvement with the assembly of cytochrome oxidases that contain the Cu(A) center in subunit 2, possibly mediating the transfer of copper into the Cu(A) binuclear site. We are investigating the reaction chemistry of BSco, the homologue from Bacillus subtilis. Our studies have revealed that BSco behaves more like a redox protein than a metallochaperone. The essential H135 residue that coordinates copper plays a role in stabilizing the Cu(II) rather than the Cu(I) form. When H135 is mutated to alanine, the oxidation rate of both hydrogen peroxide and one-electron outer-sphere reductants increases by 3 orders of magnitude, suggestive of a redox switch mechanism between the His-on and His-off conformational states of the protein. Imidazole binds to the H135A protein, restoring the N superhyperfine coupling in the EPR, but is unable to rescue the redox properties of wild-type Sco. These findings reveal a unique role for H135 in Sco function. We propose a hypothesis that electron transfer from Sco to the maturing oxidase may be essential for proper maturation and/or protection from oxidative damage during the assembly process. The findings also suggest that interaction of Sco with its protein partner(s) may perturb the Cu(II)-H135 interaction and thus induce a sensitive redox activity to the protein.

Binding of catechols to mononuclear titanium(IV) and to 1- and 5-nm TiO2 nanoparticles

The binding of catechol derivatives (LH 2 = catechol, 4-methyl catechol, 4-t-butyl catechol, and dopamine) to 1- and 4.7-nm TiO2 nanoparticles in aqueous, pH 3.5 suspensions has been characterized by UV-vis spectroscopy. The binding constants derived from Benesi-Hildebrand plots are (2-4) x 10(3) M(-1) for the 1-nm nanoparticles and (0.4-1) x 10(4)M(-1) for the 4.7-nm particles. Ti(IV)L3 complexes were prepared from the same catechols. The L = methyl catechol, and dopamine complexes are reported for the first time. The TiL3 reduction potentials are not very sensitive to the nature of the catechol nor evidently are the binding constants to TiO2 nanoparticles. The intense (epsilon > or = 10(3) M(-1)cm(-1)), about 400-nm, ligand-to-metal charge-transfer (LMCT) absorptions of the nanoparticle complexes are compared with those of the TiL 3 complexes (epsilon approximately 10(4)M(-1) cm(-1)) which lie in the same spectral region. The nanoparticle colors are attributed (as are the colors of the Ti(IV)L3 complexes) to the tails of the about 400-nm LMCT bands.