Multiple H-bonding Interactions Research Articles

Role of Multiple Intermolecular H-Bonding Interactions in Molecular Cluster of Hydroxyl-Functionalized Imidazolium Ionic Liquid: An Experimental, Topological, and Molecular Dynamics Study

Role of Multiple Intermolecular H-Bonding Interactions in Molecular Cluster of Hydroxyl-Functionalized Imidazolium Ionic Liquid: An Experimental, Topological, and Molecular Dynamics Study

Nanoconfinement-Enhanced Fire Safety and Mechanical Properties of Polylactic Acid with Nanocerium Metal-Organic Frameworks.

Ce-based metal-organic frameworks (Ce-MOFs) were successfully synthesized by coordinating binary acid and amino structures with cerium oxides. The quantum dot scale endows Ce-MOFs with enhanced modifiability. Additionally, the phosphorus-rich biomass phytic acid, with its numerous hydroxyl groups, strengthens the H-bond network within the system. Ce-MOFs-centered nanoconfinement can form through the multiple H-bond interactions between Ce-MOFs and polylactic acid (PLA) molecules, thereby improving the mechanical and flame-retardant properties of PLA. The PLA/CeCxOy-P-3 composite exhibited excellent fire retardancy and toxic gas suppression, with a 27.8% decrease in the peak heat release rate and a 50% reduction in the peak smoke production rate. Notably, PLA/CeCxOy-P-3 showed an 80% lower peak CO release compared with the pure PLA sample. Moreover, the incorporation of Ce-MOFs positively influenced the tensile properties of PLA, transforming it from brittle to tough. This work designed and synthesized Ce-MOFs on the quantum scale. The resulting Ce-MOFs with the anticipated structure offer a novel direction for preparing MOFs for flame retardant applications.

Mechanically customizable lignin bio-elastomers based on tailorable multiscale microstructures

The development of bio-based polyurethanes (PU) elastomers with well-designed multiscale microstructures to customize their mechanical performances was desired, but remained an ongoing challenge. Herein, a novel strategy of integrating tailorable hard-soft nanophases with multiple hydrogen bond (H-bond) interactions was proposed to construct mechanically customizable PU bio-elastomers. In this strategy, highly reactive lignin with high hydroxyl contents was employed as bio-polyol monomer reacting with hexamethylene diisocyanate (HDI) to construct carbamate bonds with a function of tailoring H-bond degree via altering the feeding ratio of lignin. The rigid lignin cooperating with HDI as hard segments self-assembled to generate phase separated nanostructures. Consequently, the integration of well-defined hard-soft nanophases and multiple H-bond interactions in the synthesized lignin-based PU (LPU) achieved excellent and customizable mechanical performances ranging from highly elastic rubber to highly stiff plastic with ultrahigh strength of 72.8 MPa and unprecedented modulus of 3.5 GPa. Impressively, rubber-like LPU elastomers could recover 90 % of initial stress even after 500 stretching cycles, indicating fascinating resilience and fatigue resistance. More especially, rigid LPU plastics with fully hard domains and dense H-bond networks could be transformed into flexible and bendable paper-like elastomers once eliminating intermolecular H-bond interactions under 60 °C hot water. Thus, this work provides a feasible and promising approach to construct supramolecular PU elastomers with excellent and customizable mechanical performances to extend their on-demand applications.

Efficient C2H2/CO2 separation in two 2-fold interpenetrating metal-organic frameworks with ultrahigh C2H2 packing density

Adsorptive separation of C2H2/CO2 is industrially important and feasible but the design of highly efficient adsorbents remains very challenging owing to the almost identical physicochemical properties of the two gases. Herein, we constructed two new microporous metal–organic frameworks (MOFs), termed CTGU-39/40 (CTGU = China Three Gorges University), with 2-fold interpenetrating frameworks and unique pore geometry/chemistry for C2H2/CO2 separation. Both the activated MOFs (CTGU-39/40-a) can preferentially adsorb C2H2 over CO2 with excellent selectivity of 8.4 and 3.3 at 298 K and 100 kPa, respectively, as verified by static adsorption measurements. More importantly, attributed to the suitable pore aperture size (∼5 Å) and abundant adsorption sites, CTGU-39-a combines exceptionally high packing density of C2H2 (0.41 g mL−1) and moderate isosteric heat of adsorption (Qst) for C2H2 (26.8 kJ mol−1), outperforming most previously reported adsorbents. Theoretical calculations reveal that the preferential adsorption of C2H2 within two MOFs is driven by cooperative adsorption behavior through multiple H-bonding and Coulomb interactions. Experimental breakthrough results further confirmed their practical separation ability and recyclability toward an equimolar C2H2/CO2 mixture at different feeding rates. This work may offer new insights into the design of advanced porous materials for C2H2 purification and potentially other gas separations.

Chemometric guided isolation of new triterpenoid saponins as acetylcholinesterase inhibitors from seeds of Achyranthes bidentata Blume

Achyranthes bidentata Blume (Amaranthaceae) is an annual or perennial herb widely used as ethnomedicine in Traditional Chinese Medicine for treating fever, cold, ulcers, mensural pain, dementia, and osteoporosis. In the current study, UPLC-IM-Q-TOF-MS/MS-based chemometric approach was adopted for the tentative identification of fifty-six compounds in the extract and fractions of A.bidentata seeds. Further, the chemometric-guided isolation led to the isolation of two previously undescribed oleanane-type triterpenoid saponins, named achyranosides A-B (27 and 30), along with three known compounds (31, 44, and 23) from water fraction of A. bidentata seeds. The structures of new compounds were elucidated based on the detailed analysis of NMR, HR-ESI-MS, FT-IR spectral data, and GC-FID techniques. The isolated compounds in vitro acetylcholinesterase inhibitory activity revealed the promising activity of chikusetsusaponin IVa (23) (IC50 = 63.7 μM) with mixed type of AChE inhibition in enzyme kinetic studies. Additionally, in silico binding free energy of isolated compounds disclosed the greater stability of enzyme-ligand complex owing to underlying multiple H-bond interactions. Overall, the study demonstrates the effectiveness of a chemometric-guided approach for the phytochemical exploration and isolation of new oleanane-type triterpenoid saponins from A. bidentata seeds.

Structure and molecular interactions in 1‑ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ion pair conformers: abinitio DFT based study

Density functional theory (DFT) calculations with Grimme’s B97D functional including dispersion are employed to provide both structural and electronic insight into the multifold interactions occurring in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid [EMI][TFSI], which is currently being targeted for applications in next-generation Li-ion battery electrolytes. The geometric structure, interaction energy and natural bond orbital analysis of the ion-pair conformers of [EMI][TFSI] show differences from the conventional H-bonds. A total of six stable [EMI][TFSI] ion pair conformers were observed. Our results show that a doubly ionic H-bond exists between [EMI]+ and [TFSI]− ion pair conformers. The greater the number of multiple H-bond type interactions, the greater the absolute value of the interaction energy. Moreover, results from the NBO analysis show that, for [EMI][TFSI] ion pair conformers, charge transfer occurs mainly from the lone pairs of oxygen and nitrogen atom to the σ-type anti-bonding orbital of the C–H and π-type anti-bonding orbitals of N–C bonds. This was evident from the values of the stabilization energy E(2) associated with each electron delocalization from the donor to acceptor orbitals. There exist large numbers of multiple concomitant hydrogen bonds, but the values of the stabilization energy E(2) are generally small (E(2)n→\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ o$$\\end{document}σ* < 3 kcal/mol) for the individual H-bond interactions. The shorter the C–H···O and/or C–H···N bond is, the larger charge transfer, and the larger the stabilization energy E(2) associated with electron delocalization from donor to acceptor. Generally, the [EMI][TFSI] ion pair conformers tend to form multiple but bent H bonds, reducing the strength of the individual H bonds from a potential (linear) maximum.

Multiple H-Bonding Cross-Linked Supramolecular Solid-Solid Phase Change Materials for Thermal Energy Storage and Management.

Solid-solid phase change materials (SSPCMs) are considered among the most promising candidates for thermal energy storage and management. However, the application of SSPCMs is consistently hindered by the canonical trade-off between high TES capacity and mechanical robustness. In addition, they suffer from poor recyclability due to chemical cross-linking. Herein, a straightforward but effective strategy for fabricating supramolecular SSPCMs with high latent heat and mechanical strength is proposed. The supramolecular polymer employs multiple H-bonding interactions as robust physical cross-links. This enables SSPCM with a high enthalpy of phase transition (142.5 J g-1 ), strong mechanical strength (36.9MPa), and sound shape stability (maintaining shape integrity at 120 °C) even with a high content of phase change component (97 wt%). When SSPCM is utilized to regulate the operating temperature of lithium-ion batteries, it significantly diminishes the battery working temperature by 23 °C at a discharge rate of 3 C. The robust thermal management capability enabled through solid-solid phase change provides practical opportunities for applications in fast discharging and high-power batteries. Overall, this study presents a feasible strategy for designing linear SSPCMs with high latent heat and exceptional mechanical strength for thermal management.

A strong, biodegradable, and closed-loop recyclable bamboo-based plastic substitute enabled by polyimine covalent adaptable networks

Environmental pollution caused by nondegradable fossil-based plastics is one of the biggest global concerns. Bamboo, which is a renewable and biodegradable biomass resource, is a promising material for developing green plastic substitutes. However, the processing (e.g., thermal processing) of bamboo into plastic substitutes remains a considerable challenge because bamboo is a stable multicomponent structure crosslinked via strong hydrogen bond (H-bond) and covalent-bond interactions, which results in its non-thermoplastic characteristic. Herein, bamboo powders (BPs) were integrated into polyimine (PI) covalent adaptable networks (CANs) using a simple heat-pressing approach, which helped us successfully fabricate a thermally processable composite film (BP/PI). The heat-driven dynamic network exchange characteristic of PI also provides BP/PI with excellent reshaping, rehealing, and remolding capabilities. The multiple H-bond interactions between BPs and PI provide BP/PI with excellent tensile strength (45.2 MPa) and flexural strength (121.9 MPa). BP/PI also exhibits remarkable water and organic solvent resistance, chemical degradability, closed-loop recyclability, and biodegradability. This attractive BP/PI material represents a high-performance and ecofriendly plastic substitute.

Porous Covalent Organic Framework Based Hydrogen-Bond Nanotrap for the Precise Recognition and Separation of Gold.

Utilizing weak interactions to effectively recover and separate precious metals in solution is of great importance but the practice remains a challenge. Herein, we report a novel strategy to achieve precise recognition and separation of gold by regulating the hydrogen-bond (H-bond) nanotrap within the pore of covalent organic frameworks (COFs). It is found that both COF-HNU25 and COF-HNU26 can efficiently capture AuIII with fast kinetics, high selectivity, and uptake capacity. In particular, the COF-HNU25 with the high density of H-bond nanotraps exhibits an excellent gold uptake capacity of 1725 mg g-1 , which is significantly higher than that (219 mg g-1 ) of its isostructural COF (COF-42) without H-bond nanostrap in the pores. Importantly, the uptake capacity is strongly correlated to the number of H-bonds between phenolic OH in the COF and [AuCl4 ]- in water, and multiple H-bond interactions are the key driving force for the excellent gold recovery and reusability of the adsorbent.

Highly scalable acid-base resistant Cu-Prussian blue metal-organic framework for C2H2/C2H4, biogas, and flue gas separations

One of the emerging problems plaguing the chemical industry today is the selective capture and separation of gases from their mixtures in an efficient and cost-effective manner. MOFs are new-age physisorbent materials extensively investigated for various gas mixture separations (such as biogas, flue gas, olefin/paraffin etc.); however, face the challenge of separations in a realistic environment. Here, we have investigated one of the Prussian blue analogues (Cu-PSB) to explore its potential as energy-efficient gas separation material. Cu-PSB can easily be scaled up at room temperature from water and is highly robust under harsh acidic, basic environments (pH = 1–11, 6 M HCl, 18 M H2SO4), exhibiting excellent separation of C2H2/C2H4, biogas (CO2:CH4 = 50:50), and flue gas (CO2:N2 = 15:85) mixtures. The IAST selectivity at ambient conditions (295 K, 50:50 mixture) could reach up to 5.2 for C2H2/C2H4, 14.7 for CO2/CH4, and 60.5 for the CO2/N2 (15:85 mixture). Such high C2H2 and CO2 uptake capacity and separation selectivity could be attributed to the synergistic effect of open CuII sites and the multiple H-bonding interactions within the functional pore channels of optimal pore size. Further, breakthrough simulation confirmed the complete separations from their binary mixtures, thus proving to be highly useful for the C2H2/C2H4 separation and CO2 capture from the bio and post-combustion flue gas mixtures. Cu-PSB was found to be even a more robust framework than ZIF-8 and UiO-66 MOFs.

Advances in Bifunctional Squaramide-Catalyzed Asymmetric Sulfa-Michael Addition: A Decade Update

AbstractThe application of bifunctional organocatalysts in organic chemistry has advanced remarkably owing to their high stability to moisture and air, relatively low toxicity, ease of handling, and recoverability. Among chiral bifunctional organocatalysts, squaramides have emerged as a privileged catalyst in recent years. This review article presents a comprehensive report on fine-tunable bifunctional chiral squaramide-catalyzed sulfa-Michael addition, which is based upon synchronous interplay of synergistic ‘electrophilic–nucleophilic’ dual activation strategies via multiple H-bonding interactions for the construction of both common and complex molecular entities bearing multiple stereocenters. Mechanistic discussions are kept brief, but significant understandings have been recorded. The contribution of squaramide catalyst to the construction of C–S bonds via sulfa-Michael addition has been applied in medicinal, natural, and industrial chemistry. Attention is focused on summarizing the progress made in chiral squaramide-catalyzed asymmetric sulfa-Michael addition and subsequent cascade/domino reaction sequences between 2011 and 2022.1 Introduction2 Quinine-Squaramide Organocatalysis3 Iminophosphorane-Squaramide Organocatalysis4 Chinchona-Squaramide Organocatalysis5 trans-1,2-Diaminocyclohexane-Squaramide Organocatalysis6 Conclusion

Design, synthesis, in vitro antiproliferative evaluation and GSK-3β kinase inhibition of a new series of pyrimidin-4-one based amide conjugates

A new series of novel amide conjugates of pyrimidin-4-one and aromatic/heteroaromatic /secondary cyclic amines has been synthesized and their in vitro antiproliferative activities against a panel of 60 human cancer cell lines of nine different cancer types were tested at NCI. Among the synthesized compounds, compound (4i) showed significant anti-proliferative activity. Compound (4i) displayed most potent activity against the breast tumor cell line T-47D and CNS tumor cell line SNB-75 exhibiting a growth of 1.93 % and 14.63 %, respectively. ADMET studies of the synthesized compounds were also performed and they were found to exhibit good drug like properties. Compound (4i) was found to exhibit potential inhibitory effect over GSK-3β with IC50 value of 71 nM. The molecular docking studies revealed that (4i) showed good binding affinity to GSK-3β and revealed multiple H-bonding and p-cation interactions with important amino acid residues on the receptor site. Compound (4i) may thus serve as a potential candidate for further development of novel anticancer therapeutics.

Bioguided Isolation of Cyclopenin Analogues as Potential SARS-CoV-2 Mpro Inhibitors from Penicillium citrinum TDPEF34.

SARS-CoV-2 virus mutations might increase its virulence, and thus the severity and duration of the ongoing pandemic. Global drug discovery campaigns have successfully developed several vaccines to reduce the number of infections by the virus. However, finding a small molecule pharmaceutical that is effective in inhibiting SARS-CoV-2 remains a challenge. Natural products are the origin of many currently used pharmaceuticals and, for this reason, a library of in-house fungal extracts were screened to assess their potential to inhibit the main viral protease Mpro in vitro. The extract of Penicillium citrinum, TDPEF34, showed potential inhibition and was further analysed to identify potential Mpro inhibitors. Following bio-guided isolation, a series of benzodiazepine alkaloids cyclopenins with good-to-moderate activity against SARS-CoV-2 Mpro were identified. The mode of enzyme inhibition of these compounds was predicted by docking and molecular dynamic simulation. Compounds 1 (isolated as two conformers of S- and R-isomers), 2, and 4 were found to have promising in vitro inhibitory activity towards Mpro, with an IC50 values range of 0.36–0.89 µM comparable to the positive control GC376. The in silico investigation revealed compounds to achieve stable binding with the enzyme active site through multiple H-bonding and hydrophobic interactions. Additionally, the isolated compounds showed very good drug-likeness and ADMET properties. Our findings could be utilized in further in vitro and in vivo investigations to produce anti-SARS-CoV-2 drug candidates. These findings also provide critical structural information that could be used in the future for designing potent Mpro inhibitors.

Chiral Phosphoric Acid-Catalyzed Remote Control of Axial Chirality at Boron-Carbon Bond.

The previously elusive catalytic enantioselective construction of axially chiral B-aryl-1,2-azaborines with a C-B stereogenic axis has been realized through a chiral phosphoric acid-catalyzed desymmetrization strategy reported herein. The electrophilic aromatic substitution reaction of 3,5-disubsituted phenols with diazodicarboxamides could afford these axially chiral structures in good efficiency with excellent enantiocontrol. The efficient long-range stereochemical control is achieved by multiple well-defined H-bonding interactions between chiral phosphoric acid and both substrates. Meanwhile, the reaction duration could be markedly shortened with weakly acidic N-H in 1,2-azaborine acting as H-bond donor. The scalability of the reaction and facile cleavage of the N-N bond in the product further demonstrated the practicality of this method.

Switchable mechanoresponsive luminescence from traditional triphenylamine-thiophene carbaldehyde luminogens

Two donor-π-acceptor molecules (TPA-TC and 2Br-TPA-TC) with the common backbone of triphenylamine-thiophene carbaldehyde are used to explore the mechanoresponsive luminescence (MRL) properties. Both of them show remarkable solvatochromic effects, and their emission colors in solution could be changed from blue to yellow depending on the solvent polarity. Density function theory calculations indicate that the energy band gap of 2Br-TPA-TC is slightly larger than that of TPA-TC, which could explain their UV/Vis and fluorescence behaviors. It is noted that the green emission of TPA-TC and 2Br-TPA-TC is turned into deeper yellow-green after grinding and recovered via heating, and the two-color emission cycles are reversible. The MRL behaviors associated with the morphology transformation between crystalline and amorphous states could be monitored by powder X-ray diffraction spetra. Single-crystal structural analyses for TPA-TC and 2Br-TPA-TC help to understand the MRL mechanisms, in which the twisted molecules are loosely packed with the affixation of multiple H-bonding interactions and they are susceptible to grinding-induced photoluminescence changes.

Oxidopolyborate anions templated by transition-metal complex cations: Self-assembled syntheses and structural studies (XRD) of [Co(NH3)6]2[B4O5(OH)4]3·11H2O, [Ni(phen)3][B7O9(OH)5].9.5H2O and [Zn(dac)2(H2O)2][B7O9(OH)5]·H2O

The new oxidopolyborate salts [Co(NH3)6]2[B4O5(OH)4]3·11H2O (1), [Ni(phen)3][B7O9(OH)5].9.5H2O (2), and [Zn(dac)2(H2O)2][B7O9(OH)5]·H2O (3), were crystallized, in moderate yields, from aqueous solutions containing B(OH)3 and the hydroxide salt of the corresponding cationic metal complex (prepared in situ) in moderate yields. Complexes 1–3, characterized by spectroscopic (NMR, IR) and thermal (TGA) methods and by single-crystal XRD studies, were obtained through cation-templated self-assembly reactions. Compound 1 contains insular tetraborate(2-) anions (4-1:2Δ + 2T) and 2 and 3 both contain insular heptaborate(2-) anions (‘O+’ isomer, 6:(3Δ + 3T) + Δ). The X-ray structures show that in all three compounds the polyborate anions are H-bonded together into extended networks, and there are multiple cation-anion H-bond interactions present in 1 and 3. Compound 2 has cation-cation π-stacking interactions. These interactions are likely to be responsible for templating the observed structures. Thermal decomposition of 1–3 in air yielded amorphous anhydrous borates of composition CoB6O10.5, NiB7O11.5 and ZnB7O11.5, respectively.

Bifunctional Phosphonium Salt Directed Enantioselective Formal [4 + 1] Annulation of Hydroxyl-Substituted para-Quinone Methides with α-Halogenated Ketones.

A highly diastereo- and enantioselective [4 + 1] cycloaddition of para-quinone methides to α-halogenated ketones was realized by bifunctional phosphonium salt catalysis, furnishing functionalized 2,3-dihydrobenzofurans in high yields and excellent stereoselectivities (>20:1 dr and up to >99.9% ee). Mechanistic observations suggested that the reaction underwent a cascade intermolecular substitution/intramolecular 1,6-addition process. DFT calculations revealed the presence of multiple H-bonding interactions between the catalyst and the enolate intermediate in the stereodetermining transition states.

Controlled Thermoreversible Formation of Supramolecular Hydrogels Based on Poly(vinyl alcohol) and Natural Phenolic Compounds.

Supramolecular hydrogels have promising applications in a wide variety of fields including 3D bioprinting, sensors and actuators, biomedicine, and controlled drug delivery. This communication reports the facile reversible thermotriggered formation of novel pH-responsive supramolecular hydrogels based on poly(vinyl alcohol) (PVA) bonded via dynamic H-bridge with small phenolic biomolecules. PVA and phenolic compounds form a clear solution when they are physically mixed in water at high temperature, but a fast gelation is produced at room temperature through multiple strong H-bonding interactions. The structure and type of functional groups of different phenolic molecules allow preparing hydrogels with tailor-made viscoelastic properties, controlled low phase transition temperature, and pH-dependent swelling behavior. This combination makes these supramolecular networks very interesting candidates to be used in 3D bioprinting and topical drug delivery of thermolabile biomolecules.

High-Performance Polymeric Materials through Hydrogen-Bond Cross-Linking.

It has always been critical to develop high-performance polymeric materials with exceptional mechanical strength and toughness, thermal stability, and even healable properties for meeting performance requirements in industry. Conventional chemical cross-linking leads to enhanced mechanical strength and thermostability at the expense of extensibility due to mutually exclusive mechanisms. Such major challenges have recently been addressed by using noncovalent cross-linking of reversible multiple hydrogen-bonds (H-bonds) that widely exist in biological materials, such as silk and muscle. Recent decades have witnessed the development of many tailor-made high-performance H-bond cross-linked polymeric materials. Here, recent advances in H-bond cross-linking strategies are reviewed for creating high-performance polymeric materials. H-bond cross-linking of polymers can be realized via i) self-association of interchain multiple H-bonding interactions or specific H-bond cross-linking motifs, such as 2-ureido-4-pyrimidone units with self-complementary quadruple H-bonds and ii) addition of external cross-linkers, including small molecules, nanoparticles, and polymer aggregates. The resultant cross-linked polymers normally exhibit tunable high strength, large extensibility, improved thermostability, and healable capability. Such performance portfolios enable these advanced polymers to find many significant cutting-edge applications. Major challenges facing existing H-bond cross-linking strategies are discussed, and some promising approaches for designing H-bond cross-linked polymeric materials in the future are also proposed.

Geometric and Electronic Structure Contributions to O-O Cleavage and the Resultant Intermediate Generated in Heme-Copper Oxidases.

This study investigates the mechanism of O-O bond cleavage in heme-copper oxidase (HCO) enzymes, combining experimental and computational insights from enzyme intermediates and synthetic models. It is determined that HCOs undergo a proton-initiated O-O cleavage mechanism where a single water molecule in the active site enables proton transfer (PT) from the cross-linked tyrosine to a peroxo ligand bridging the heme FeIII and CuII, and multiple H-bonding interactions lower the tyrosine p Ka. Due to sterics within the active site, the proton must either transfer initially to the O(Fe) (a high-energy intermediate), or from another residue over a ∼10 Å distance to reach the O(Cu) atom directly. While the distance between the H+ donor (Tyr) and acceptor (O(Cu)) results in a barrier to PT, this separation is critical for the low barrier to O-O cleavage as it enhances backbonding from Fe into the O22- σ* orbital. Thus, PT from Tyr precedes O-O elongation and is rate-limiting, consistent with available kinetic data. The electron transfers from tyrosinate after the barrier via a superexchange pathway provided by the cross-link, generating intermediate PM. PM is evaluated using available experimental data. The geometric structure contains an FeIV═O that is H-bonded to the CuII-OH. The electronic structure is a singlet, where the FeIV and CuII are antiferromagnetically coupled through the H-bond between the oxo(Fe) and hydroxo(Cu) ligands, while the CuII and Tyr• are ferromagnetically coupled due their delocalization into orthogonal magnetic orbitals on the cross-linked His residue. These findings provide critical insights into the mechanism of efficient O2 reduction in HCOs, and the nature of the PM intermediate that couples this reaction to proton pumping.