Bhaskar Mondal

The mechanistic coupling of N2 binding and H2 evolution through reductive elimination from the E4 intermediate constitutes a critical step in the nitrogenase catalytic cycle. Despite extensive mechanistic investigations, how the Fe centers within the FeMoco cofactor dynamically reorganize their local electronic structures to facilitate this coupled process remains unresolved. Here, using density functional theory (DFT) combined with intrinsic atomic orbital/intrinsic bond orbital (IAO/IBO) and natural bond orbital (NBO) analyses, we uncover the electronic structure evolution of FeMoco along the coupled N2-binding/H2-evolution reaction coordinate. Comparative analyses of reaction energetics, with appropriate transition states, support an “intermediate” mechanism over purely “associative” or “dissociative” pathways, revealing a key intermediate E4(H2N2) in which N2 and H2 are simultaneously coordinated to the same Fe center. Detailed molecular-orbital analyses demonstrate that the coupled reaction is enabled by highly synchronized electronic reorganization across neighboring Fe centers, which dynamically redistribute electron density to stabilize concurrent H2 release and N2 activation. In particular, cooperative Fe–Fe electronic coupling allows the reactive Fe site to adapt its local bonding interactions. These findings provide an electronic-level rationale for the functional necessity of the multinuclear FeMoco architecture and reveal how orchestrated electronic flexibility within the cluster governs the critical mechanistic step of nitrogenase catalysis.

Oxidoiron(IV) species are crucial intermediates for the functionalization of C–H bonds by biological oxidases and oxygenases, and understanding the relations between ligation, electronic structure, and reactivity of the FeIV═O unit is key to the rational design of bioinspired catalysts. Here, a series of FeIV═O complexes based on a macrocyclic tetracarbene ligand and with different anionic trans-axial ligands (Lax = CF3COO–, tBuS–, AdaS–) has been comprehensively characterized, including X-ray crystallography in two cases, as well as zero-field and applied-field 57Fe Mössbauer spectroscopy, magnetic susceptibility measurements, magnetic circular dichroism (MCD), and helium-tagging infrared photodissociation (IRPD). This establishes a detailed picture of the electronic structures of these ferryl complexes, featuring a well-separated S = 1 ground state, and the influence of the trans-ligand. The confinement to triplet-only pathways in C–H bond activations and comparison with the parent FeIV═O complex (Lax = MeCN) allow uncovering trends in hydrogen atom abstraction (HAA) ability without perturbations from different extents of two-state reactivity (TSR). Kinetic studies on HAA for C–H substrates with BDEs up to 78 kcal mol–1 show relatively minor effects of the axial ligands, with rates following an electrophilic trend in the order Lax = MeCN > CF3COO– ≈ RS–. This contrasts the antielectrophilic trend found for a related series of tetramethylcyclam (TMC)-based FeIV═O complexes for which TSR is dominant. For the present systems, temperature-dependent KIEs, in combination with computations at the DFT and ab initio levels of theory, suggest a semiclassical model with modest tunneling contributions that are most pronounced in the case of a strongly donating trans-thiolate. This study thus provides a combined experimental and computational basis for understanding intrinsic axial ligand effects and the effect of a trans-axial thiolate in S = 1 ferryl intermediates.

Adrenal glands are one of the pivotal glands of the human endocrine system. Recently, the extracellular matrix (ECM) of the adrenal capsule and cortex was explored in two fractions: outer fraction (OF) and inner fraction (IF). A significant variation in the ECM proteins' levels, including collagens, was documented. Collagen undergoes a plethora of post-translational modifications (PTMs), exhibiting crucial roles such as cell-matrix interaction and cross-linking. However, the site-specific identification and characterization of collagen PTMs remain challenging and are unknown for the human adrenal gland. By applying our in-house proteomics pipeline, we identified 1044 4-hydroxyproline (4-HyP), 205 3-hydroxyproline (3-HyP), 106 hydroxylysine (HyK), 17 galactosyl-hydroxylysine (G-HyK), and 37 glucosyl galactosyl-hydroxylysine (GG-HyK) sites from abundant collagen chains of both fractions. Although the site-specificity of collagen PTMs (3-HyP, HyK, and G/GG-HyK) across fractions is conserved, the occupancies were different in a site-specific manner. The microheterogeneity observed in K⁸⁶² of COL1A1 demonstrated that the hydroxylation level was higher in OF, while glycosylation levels were higher in IF. This points toward a change in the cross-linking of collagen I across both fractions. Furthermore, our analysis revealed that site-specific O-glycosylation, particularly GG-HyK, in basement membrane collagen-IV is higher, which may contribute to the secretion of steroids from the adrenal gland. In this study, we have annotated collagen PTMs, developed a COL1A1 PTM map, and quantitated site-specific PTMs in the human adrenal gland. Taken together, this work reveals the intratissue-specific site-specific collagen PTM heterogeneity although the ECM is stable, which is noteworthy, and lays the foundation for understanding their role in region-specific functions.

Conventional palladium-catalyzed methods for constructing biaryls and terphenyls often rely on costly arylating agents, which also face challenges including instability and the need for prefunctionalizations of arenes. Herein, we report an alternative strategy by harnessing the cyclic allyl alcohol and 1,4-epoxy naphthalene as arylating agents for the C-H arylation of arenes. This approach employs ligand design to enhance both the catalytic efficiency and regioselectivity in the C-H activation of arenes, thereby providing a sustainable, atom-economical pathway for late-stage C-H arylation. One of the key design aspects includes the strategic incorporation of steric encumbrance within a monodentate ligand framework, which accelerates reductive elimination without diminishing the efficiency of other catalytic steps. Furthermore, the density functional theory (DFT) study suggests that the extended π-electron cloud of the adjacent aryl ring of the ligand may engage in noncovalent interactions with incoming substrates to induce specific selectivity in the products. Experimental studies, along with DFT calculations, established the reaction mechanism, determined the origin of selectivity, identified the nature of the rate-determining step, and highlighted the role of dual ligands in the regioselectivity.

Proteases hydrolyze the amide bond of a polypeptide chain, which can influence protein synthesis and function. This activity has been predominantly observed in the post‐translational processing of lysyl oxidase (LOX) and lysyl‐oxidase‐like (LOXL) family proteins, which are indispensable for the remodeling of the extracellular matrix (ECM). Of the four classes, metallo‐ and serine proteases are known to catalyze the hydrolysis of pro‐LOX and pro‐LOXL family proteins into their processed forms. These LOX family proteins play instrumental roles in ECM remodeling by oxidatively deaminating the lysine and 5‐hydroxy lysine residues, primarily in collagens and elastin, thus facilitating the formation of lysyl‐derived covalent crosslinking that stabilizes the extracellular matrix assembly. Previous studies have established the presence of different proteolytic sites and the corresponding proteases for different LOX family proteins. However, the underlying mechanism of this protease‐mediated processing of the LOX family in fibrotic tissue remodeling has remained elusive. In this review, we summarize the critical role of ECM crosslinking reactions catalyzed by the LOX family and provide an overview of proteolytic sites and corresponding proteases in fibrosis. Moreover, we discuss the potential catalytic mechanisms of bone morphogenetic protein‐1, a metalloprotease, and Factor Xa, a serine protease, on LOX and LOXL2, respectively.