Porphyrin tripod as a monomeric building block for guest-induced reversible supramolecular polymerisation

Reversible supramolecular polymerisation and depolymerisation of biomacromolecules are common and fundamental phenomena in biological systems, which can be controlled by the selective modification of biomacromolecules through molecular recognition. Herein, a porphyrin tripod (DPZnT) connected through a triazole bridge was prepared as a monomeric building block for guest-induced supramolecular polymerisation. Although the lone pair electrons in triazolic nitrogen potentially bind to the zinc porphyrin units through axial ligation, the intrinsic steric hindrance suppressed the coordination of the triazole bridge to the porphyrin unit in DPZnT. Therefore, DPZnT formed spherical nanoparticles through π-π interactions. The addition of 1,3,5-tris(pyridine-4-yl)benzene (Py3B) caused the guest-induced fibrous supramolecular polymerisation of DPZnT by forming a 1:1 host-guest complex, which was further assembled into a fibrous polymer. Furthermore, addition of Cl− to DPZnT induced the transformation of spherical nanoparticles to fibrous supramolecular polymers. The fibrous supramolecular polymers of DPZnT obtained by adding Py3B or Cl− were depolymerised to their original spherical particles after adding Cu(ClO4)2 or AgNO3, respectively.

Effects of Axial Solvent Coordination to Dirhodium Complexes on Reactivity and Selectivity in C–H Insertion Reactions: A Computational Study

Density functional theory calculations were used to systematically explore the effects of axial ligation by solvent molecules on the reactivity and selectivity of dirhodium tetracarboxylates with diazo compounds in the context of C–H insertion into propane. Insertions on three types of diazo compounds—acceptor/acceptor, donor/acceptor, and donor/donor—promoted by dirhodium tetraformate were tested with and without axial solvent ligation for no surrounding solvent, dichloromethane, isopropanol, and acetonitrile. Magnitudes, origins, and consequences of structural and electronic changes arising from axial ligation were characterized. The results suggest that axial ligation affects barriers for N2 extrusion and C–H insertion, the former to a larger extent.

Effects of Axial Solvent Coordination to Dirhodium Complexes on Reactivity and Selectivity in C–H Insertion Reactions: A Computational Study

Density functional theory calculations were used to systematically explore the effects of axial ligation by solvent molecules on the reactivity and selectivity of dirhodium tetracarboxylates with diazo compounds in the context of C–H insertion into propane. Insertions on three types of diazo compounds—acceptor/acceptor, donor/acceptor, and donor/donor—promoted by dirhodium tetraformate were tested with and without axial solvent ligation for no surrounding solvent, dichloromethane, isopropanol, and acetonitrile. Magnitudes, origins, and consequences of structural and electronic changes arising from axial ligation were characterized. The results suggest that axial ligation affects barriers for N2 extrusion and C–H insertion, the former to a larger extent.

Recent advances in the structural diversity of reaction centers

Photosynthetic reaction centers (RC) catalyze the conversion of light to chemical energy that supports life on Earth, but they exhibit substantial diversity among different phyla. This is exemplified in a recent structure of the RC from an anoxygenic green sulfur bacterium (GsbRC) which has characteristics that may challenge the canonical view of RC classification. The GsbRC structure is analyzed and compared with other RCs, and the observations reveal important but unstudied research directions that are vital for disentangling RC evolution and diversity. Namely, (1) common themes of electron donation implicate a Ca2+ site whose role is unknown; (2) a previously unidentified lipid molecule with unclear functional significance is involved in the axial ligation of a cofactor in the electron transfer chain; (3) the GsbRC features surprising structural similarities with the distantly-related photosystem II; and (4) a structural basis for energy quenching in the GsbRC can be gleaned that exemplifies the importance of how exposure to oxygen has shaped the evolution of RCs. The analysis highlights these novel avenues of research that are critical for revealing evolutionary relationships that underpin the great diversity observed in extant RCs.

Outstanding Enhancement in the Axial Coordination Ability of the Highly Rigid Cofacial Cyclic Metalloporphyrin Dimer

<div>Copper- and nickel-porphyrin complexes show extremely weak axial coordination ability without any electron-withdrawing groups. Herein, we report axial ligation on Cu^II- and Ni^II-porphyrins in a highly rigid cofacial porphyrin dimer with a bidentate ligand, 1,4-diazabicyclo[2.2.2]octane (DABCO). To the best of our knowledge, this is the first report on the use of Cu^II- and Ni^II-porphyrins for coordination-induced guest binding of porphyrin-based host molecules without the help of other metal ions. The high rigidity of the dimer induces guest binding through the cooperative effect of weak axial ligation. The results showed that Cu^II- and Zn^II-complexes bind one DABCO molecule inside their cavities, whereas the Ni^II-complex binds two additional DABCO molecules outside to form a stable 6-coordinate paramagnetic Ni^II-complexes. <br></div>

Outstanding Enhancement in the Axial Coordination Ability of the Highly Rigid Cofacial Cyclic Metalloporphyrin Dimer

<div>Copper- and nickel-porphyrin complexes show extremely weak axial coordination ability without any electron-withdrawing groups. Herein, we report axial ligation on Cu^II- and Ni^II-porphyrins in a highly rigid cofacial porphyrin dimer with a bidentate ligand, 1,4-diazabicyclo[2.2.2]octane (DABCO). To the best of our knowledge, this is the first report on the use of Cu^II- and Ni^II-porphyrins for coordination-induced guest binding of porphyrin-based host molecules without the help of other metal ions. The high rigidity of the dimer induces guest binding through the cooperative effect of weak axial ligation. The results showed that Cu^II- and Zn^II-complexes bind one DABCO molecule inside their cavities, whereas the Ni^II-complex binds two additional DABCO molecules outside to form a stable 6-coordinate paramagnetic Ni^II-complexes. <br></div>

Local Electric Fields as a Natural Switch of Heme-Iron Protein Reactivity

Heme-iron oxidoreductases operating through the high-valent Fe^IVO intermediates perform crucial and complicated transformations, such as oxidations of unreactive saturated hydrocarbons. These enzymes share the same Fe coordination, only differing by the axial ligation, e.g., Cys in P450 oxygenases, Tyr in catalases, and His in peroxidases. By examining ~200 heme-iron proteins, we show that the protein hosts exert highly specific intramolecular electric fields on the active sites, and there is a strong correlation between the direction and magnitude of this field and the protein function. In all heme proteins, the field is preferentially aligned with the Fe‒O bond (<b><i>F_z</i></b>). The Cys-ligated P450 oxygenases have the highest average <b><i>F_z</i></b> of 28.5 MV cm^-1, i.e., most enhancing the oxyl-radical character of the oxo group, and consistent with the ability of these proteins to activate strong C‒H bonds. In contrast, in Tyr-ligated proteins, the average <b><i>F_z</i></b> is only 3.0 MV cm^-1, apparently suppressing single-electron off-pathway oxidations, and in His-ligated proteins, <b><i>F_z</i></b> is –8.7 MV cm^-1. The operational field range is given by the trade-off between the low reactivity of the Fe^IVO Compound I at the more negative <b><i>F_z</i></b>, and the low selectivity at the more positive <b><i>F_z</i></b>. Consequently, a heme-iron site placed in the field characteristic of another heme-iron protein class loses its canonical function, and gains an adverse one. Thus, electric fields produced by the protein scaffolds, together with the nature of the axial ligand, control all heme-iron chemistry.

Local Electric Fields as a Natural Switch of Heme-Iron Protein Reactivity

Heme-iron oxidoreductases operating through the high-valent Fe^IVO intermediates perform crucial and complicated transformations, such as oxidations of unreactive saturated hydrocarbons. These enzymes share the same Fe coordination, only differing by the axial ligation, e.g., Cys in P450 oxygenases, Tyr in catalases, and His in peroxidases. By examining ~200 heme-iron proteins, we show that the protein hosts exert highly specific intramolecular electric fields on the active sites, and there is a strong correlation between the direction and magnitude of this field and the protein function. In all heme proteins, the field is preferentially aligned with the Fe‒O bond (<b><i>F_z</i></b>). The Cys-ligated P450 oxygenases have the highest average <b><i>F_z</i></b> of 28.5 MV cm^-1, i.e., most enhancing the oxyl-radical character of the oxo group, and consistent with the ability of these proteins to activate strong C‒H bonds. In contrast, in Tyr-ligated proteins, the average <b><i>F_z</i></b> is only 3.0 MV cm^-1, apparently suppressing single-electron off-pathway oxidations, and in His-ligated proteins, <b><i>F_z</i></b> is –8.7 MV cm^-1. The operational field range is given by the trade-off between the low reactivity of the Fe^IVO Compound I at the more negative <b><i>F_z</i></b>, and the low selectivity at the more positive <b><i>F_z</i></b>. Consequently, a heme-iron site placed in the field characteristic of another heme-iron protein class loses its canonical function, and gains an adverse one. Thus, electric fields produced by the protein scaffolds, together with the nature of the axial ligand, control all heme-iron chemistry.