Combined Spectroscopic and Computational Analysis of the Vibrational Properties of Vitamin B₁₂ in its Co³⁺, Co²⁺, and Co¹⁺ Oxidation States

While the geometric and electronic structures of vitamin B₁₂ (cyanocobalamin, CNCbl) and its reduced derivatives Co²⁺cobalamin (Co²⁺Cbl) and Co¹⁺cobalamin (Co¹⁺Cbl^–) are now reasonably well established, their vibrational properties, in particular their resonance Raman (rR) spectra, have remained quite poorly understood. The goal of this study was to establish definitive assignments of the corrin-based vibrational modes that dominate the rR spectra of vitamin B₁₂ in its Co³⁺, Co²⁺, and Co¹⁺ oxidation states. rR spectra were collected for all three species with laser excitation in resonance with the most intense corrin-based π → π* transitions. These experimental data were used to validate the computed vibrational frequencies, eigenvector compositions, and relative rR intensities of the normal modes of interest as obtained by density functional theory (DFT) calculations. Importantly, the computational methodology employed in this study successfully reproduces the experimental observation that the frequencies and rR excitation profiles of the corrin-based vibrational modes vary significantly as a function of the cobalt oxidation state. Our DFT results suggest that this variation reflects large differences in the degree of mixing between the occupied Co 3d orbitals and empty corrin π* orbitals in CNCbl, Co²⁺Cbl, and Co¹⁺Cbl^–. As a result, vibrations mainly involving stretching of conjugated C–C and C–N bonds oriented along one axis of the corrin ring may, in fact, couple to a perpendicularly polarized electronic transition. This unusual coupling between electronic transitions and vibrational motions of corrinoids greatly complicates an assignment of the corrin-based normal modes of vibrations on the basis of their rR excitation profiles

Spectroscopic Characterization of Active-Site Variants of the PduO-type ATP:Corrinoid Adenosyltransferase from <i>Lactobacillus reuteri</i>: Insights into the Mechanism of Four-Coordinate Co(II)corrinoid Formation

The PduO-type adenosine 5′-triphosphate (ATP):corrinoid adenosyltransferase from <i>Lactobacillus reuteri</i> (<i>Lr</i>PduO) catalyzes the transfer of the adenosyl-group of ATP to Co¹⁺cobalamin (Cbl) and Co¹⁺cobinamide (Cbi) substrates to synthesize adenosylcobalamin (AdoCbl) and adenosylcobinamide (AdoCbi⁺), respectively. Previous studies revealed that to overcome the thermodynamically challenging Co²⁺ → Co¹⁺ reduction, the enzyme drastically weakens the axial ligand–Co²⁺ bond so as to generate effectively four-coordinate (4c) Co²⁺corrinoid species. To explore how <i>Lr</i>PduO generates these unusual 4c species, we have used magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) spectroscopic techniques. The effects of active-site amino acid substitutions on the relative yield of formation of 4c Co²⁺corrinoid species were examined by performing eight single-amino acid substitutions at seven residues that are involved in ATP-binding, an intersubunit salt bridge, and the hydrophobic region surrounding the bound corrin ring. A quantitative analysis of our MCD and EPR spectra indicates that the entire hydrophobic pocket below the corrin ring, and not just residue F112, is critical for the removal of the axial ligand from the cobalt center of the Co²⁺corrinoids. Our data also show that a higher level of coordination among several <i>Lr</i>PduO amino acid residues is required to exclude the dimethylbenzimidazole moiety of Co­(II)­Cbl from the active site than to remove the water molecule from Co­(II)­Cbi⁺. Thus, the hydrophilic interactions around and above the corrin ring are more critical to form 4c Co­(II)­Cbl than 4c Co­(II)­Cbi⁺. Finally, when ATP analogues were used as cosubstrate, only “unactivated” five-coordinate (5c) Co­(II)­Cbl was observed, disclosing an unexpectedly large role of the ATP-induced active-site conformational changes with respect to the formation of 4c Co­(II)­Cbl. Collectively, our results indicate that the level of control exerted by <i>Lr</i>PduO over the timing for the formation of the 4c Co²⁺corrinoid intermediates is even more exquisite than previously anticipated

Design Strategy toward Recyclable and Highly Efficient Heterogeneous Catalysts for the Hydrogenation of CO₂ to Formate

One bottleneck in the realization of CO₂ conversion into value-added compounds is the lack of catalysts with both excellent activity and recyclability. Herein, a catalyst is designed for the hydrogenation of CO₂ to formate to boost up these features by considering the leaching pathway of previously reported heterogenized catalyst; the design strategy incorporates oxyanionic ligand(s) in the coordination sphere to provide a pathway for both preventing the deleterious interactions and assisting the heterolysis of H₂. The tailored heterogenized catalyst, [bpy-CTF-Ru­(acac)₂]­Cl, demonstrated excellent recyclability over consecutive runs with a highest turnover frequency of 22 700 h^–1, and produced a highest formate concentration of 1.8 M in 3 h. This work is significant in elucidating new principles for the development of industrially viable hydrogenation catalysts

Overcoming the Interfacial Photocatalytic Degradation of Nonfullerene Acceptor-Based Organic Photovoltaics by Introducing a UV-A-Insensitive Titanium Suboxide Layer

Although recent dramatic advances in power conversion efficiencies (PCEs) have resulted in values over 19%, the poor photostability of organic photovoltaics (OPVs) has been a serious bottleneck to their commercialization. The photocatalytic effect, which is caused by incident ultraviolet-A (UV-A, 320–400 nm) light in the most commonly used zinc oxide (ZnOX) electron transport layer (ETL), significantly deteriorates the photostability of OPVs. In this work, we develop a new and facile method to enhance the photostability of nonfullerene acceptor-based OPVs by introducing UV-A-insensitive titanium suboxide (TiOX) ETL. Through an in-depth analysis of mass information at the interface between the ETL and photoactive layer, we confirm that the UV-A-insensitive TiOX suppresses the photocatalytic effect. The resulting device employing the TiOX ETL shows excellent photostability, obtaining 80% of the initial PCE for up to 200 h under 1 sun illumination, which is 10 times longer than that of the conventional ZnOX system (19 h)

Mechanistic Insights into Tunable Metal-Mediated Hydrolysis of Amyloid‑β Peptides

An amyloidogenic peptide, amyloid-β (Aβ), has been implicated as a contributor to the neurotoxicity of Alzheimer’s disease (AD) that continues to present a major socioeconomic burden for our society. Recently, the use of metal complexes capable of cleaving peptides has arisen as an efficient tactic for amyloid management; unfortunately, little has been reported to pursue this strategy. Herein, we report a novel approach to validate the hydrolytic cleavage of divalent metal complexes toward two major isoforms of Aβ (Aβ₄₀ and Aβ₄₂) and tune their proteolytic activity based on the choice of metal centers (M = Co, Ni, Cu, and Zn) which could be correlated to their anti-amyloidogenic properties. Such metal-dependent tunability was facilitated employing a tetra-<i>N</i>-methylated cyclam (TMC) ligand that imparts unique geometric and stereochemical control, which has not been available in previous systems. Co­(II)­(TMC) was identified to noticeably cleave Aβ peptides and control their aggregation, reporting the first Co­(II) complex for such reactivities to the best of our knowledge. Through detailed mechanistic investigations by biochemical, spectroscopic, mass spectrometric, and computational studies, the critical importance of the coordination environment and acidity of the aqua-bound complexes in promoting amide hydrolysis was verified. The biological applicability of Co­(II)­(TMC) was also illustrated via its potential blood-brain barrier permeability, relatively low cytotoxicity, regulatory capability against toxicity induced by both Aβ₄₀ and Aβ₄₂ in living cells, proteolytic activity with Aβ peptides under biologically relevant conditions, and inertness toward cleavage of structured proteins. Overall, our approaches and findings on reactivities of divalent metal complexes toward Aβ, along with the mechanistic insights, demonstrate the feasibility of utilizing such metal complexes for amyloid control

NRVS Studies of the Peroxide Shunt Intermediate in a Rieske Dioxygenase and Its Relation to the Native Fe^II O₂ Reaction

The Rieske dioxygenases are a major subclass of mononuclear nonheme iron enzymes that play an important role in bioremediation. Recently, a high-spin Fe^III–(hydro)­peroxy intermediate (BZDOp) has been trapped in the peroxide shunt reaction of benzoate 1,2-dioxygenase. Defining the structure of this intermediate is essential to understanding the reactivity of these enzymes. Nuclear resonance vibrational spectroscopy (NRVS) is a recently developed synchrotron technique that is ideal for obtaining vibrational, and thus structural, information on Fe sites, as it gives complete information on all vibrational normal modes containing Fe displacement. In this study, we present NRVS data on BZDOp and assign its structure using these data coupled to experimentally calibrated density functional theory calculations. From this NRVS structure, we define the mechanism for the peroxide shunt reaction. The relevance of the peroxide shunt to the native Fe^II/O₂ reaction is evaluated. For the native Fe^II/O₂ reaction, an Fe^III–superoxo intermediate is found to react directly with substrate. This process, while uphill thermodynamically, is found to be driven by the highly favorable thermodynamics of proton-coupled electron transfer with an electron provided by the Rieske [2Fe-2S] center at a later step in the reaction. These results offer important insight into the relative reactivities of Fe^III–superoxo and Fe^III–hydroperoxo species in nonheme Fe biochemistry

Geometric and Electronic Structure of the Mn(IV)Fe(III) Cofactor in Class Ic Ribonucleotide Reductase: Correlation to the Class Ia Binuclear Non-Heme Iron Enzyme

The class Ic ribonucleotide reductase (RNR) from <i>Chlamydia trachomatis</i> (<i>Ct</i>) utilizes a Mn/Fe heterobinuclear cofactor, rather than the Fe/Fe cofactor found in the β (R2) subunit of the class Ia enzymes, to react with O₂. This reaction produces a stable Mn^IVFe^III cofactor that initiates a radical, which transfers to the adjacent α (R1) subunit and reacts with the substrate. We have studied the Mn^IVFe^III cofactor using nuclear resonance vibrational spectroscopy (NRVS) and absorption (Abs)/circular dichroism (CD)/magnetic CD (MCD)/variable temperature, variable field (VTVH) MCD spectroscopies to obtain detailed insight into its geometric/electronic structure and to correlate structure with reactivity; NRVS focuses on the Fe^III, whereas MCD reflects the spin-allowed transitions mostly on the Mn^IV. We have evaluated 18 systematically varied structures. Comparison of the simulated NRVS spectra to the experimental data shows that the cofactor has one carboxylate bridge, with Mn^IV at the site proximal to Phe₁₂₇. Abs/CD/MCD/VTVH MCD data exhibit 12 transitions that are assigned as d–d and oxo and OH^– to metal charge-transfer (CT) transitions. Assignments are based on MCD/Abs intensity ratios, transition energies, polarizations, and derivative-shaped pseudo-A term CT transitions. Correlating these results with TD-DFT calculations defines the Mn^IVFe^III cofactor as having a μ-oxo, μ-hydroxo core and a terminal hydroxo ligand on the Mn^IV. From DFT calculations, the Mn^IV at site 1 is necessary to tune the redox potential to a value similar to that of the tyrosine radical in class Ia RNR, and the OH^– terminal ligand on this Mn^IV provides a high proton affinity that could gate radical translocation to the α (R1) subunit