27 Chapter 2 An Interest in Thiolate Coordination and Hydrogen Bonding

Biochem. 2009; 103: 906–911. 28 The Gray group has been studying electron transfer in protein systems for the past three decades. During this time a vast amount of information has been collected concerning the nature of the protein matrix and its ability to facilitate such charge transfer reactions. This led to the development of techniques for the covalent attachment of photosensitizers to metalloproteins, and later to the development of compounds consisting of sensitizers linked to substrates (dubbed “wires”) in order to promote interactions between the photosensitizer and the metal active site buried deep within the protein. A Ru-diimine wire, [(4,4’,5,5’-tetramethylbipyridine)2Ru(F9bp)] 2+ (tmRu-F9bp, where F9bp is 4-methyl-4’-methylperfluorobiphenylbipyridine), binds tightly to the oxidase domain of inducible nitric oxide synthase (iNOSoxy). The binding of tmRu-F9bp is independent of tetrahydrobiopterin, arginine, and imidazole, indicating that the wire resides on the surface of the enzyme, distant from the active-site heme. Photoreduction of an imidazole-bound active-site heme iron in the enzyme-wire conjugate (kET = 2(1) × 10

Azole assisted C-H bond activation promoted by an osmium-polyhydride: Discerning between N and NH

This is an open access article published under a Creative Commons Attribution (CC-BY) License.The capacity of the hexahydride complex OsH6(PiPr3)2 (1) to discern between the nitrogen atom and the NH unit in the azole assisted aryl C−H bond activation has been investigated. Complex 1 reacts with 2-phenylimidazole to give OsH3{κ2-C,N-(C6H4-imidazole)}(PiPr3)2 (2), which has been characterized by X-ray diffraction analysis. The structure proves the higher affinity of the metal center for the N atom in the presence of the NH unit, which remains unchanged, and reveals that in the solid state the molecules of this complex form infinite chains by means of intermolecular asymmetric 3-center bifurcated dihydrogen bonds. In solution, 1HDOSY NMR experiments suggest that the association degree decreases as the temperature increases. The fused six-membered ring of benzimidazole weakens the NH bond, enhancing its reactivity. As a consequence, complex 1 cannot discern between the N atom and the NH unit of 2-phenylbenzimidazole. Thus, the treatment of 1 with this substrate leads to a mixture of OsH3{κ2-C,N-(C6H4-benzimidazole)}(PiPr3)2 (3) and the dinuclear species (PiPr3)2H3Os(C6H4-benzimidazolate)OsH(η2-H2)(PiPr3)2 (4). The latter is the result of a N-assisted ortho-C−H bond activation of the phenyl group promoted by 0.5 equiv of 1 and the N−H bond activation promoted by the remaining 0.5 equiv of hexahydride 1 along with the agostic coordination of the remaining ortho-C−H bond to the metal center of the unsaturated fragment OsH(η 2-H2)(PiPr3)2. The comparison of the redox properties of 3 and 4 suggests that the interaction between the metal centers in the dinuclear compound is negligible. The replacement of the NH group of the azoles by a sulfur atom does not modify the behavior of the substrates. Thus, the reactions of 1 with 2-phenylthiazole and 2-phenylbenzothiazole afford OsH3{κ 2-C,N-(C6H4-thiazole)}(PiPr3)2 (5) and OsH3{κ2-C,N-(C6H4-benzothiazole)}(PiPr3)2 (6). In turn, complexes 2, 3, 5, and 6 are phosphorescent.Financial support from the Spanish MINECO and FEDER (Projects CTQ2013-46459-C2-01-P to M.A.S., CTQ2014-52799-P to M.A.E., CTQ2013-44303-P to I.F., CTQ2014-54071-P to A.L., and CTQ2014-51912-REDC, the DGA (E35), and the European Social Fund (FSE) is acknowledged.Peer Reviewe

Conjugates of Heme-Thiolate Enzymes with Photoactive Metal-Diimine Wires

Heme-thiolate enzymes, notably cytochromes P450 and nitric oxide synthases, use dioxygen to oxygenatesubstrates. Photoactive metal-diimine molecular wires that are capable of effecting rapid redox state changesat buried active sites have been developed to generate intermediates in the catalytic cycles of these enzymes.Wires that feature a photoactive head group tethered to an active-site ligand bind P450CAM and induciblenitric oxide synthase (iNOS) primarily by hydrophobic interactions. The wire-binding specificity of eachenzyme is critically dependent on the structural flexibility of the protein. P450CAM:wire conjugates canadopt open or partially open conformations, thereby accommodating a wide range of wires, whereas onlylong wires with smaller [Re(CO)_3(bpy)Im]^+ head groupsare able to bind tightly in the rigid active-site channel of iNOS. Dansyl-terminated molecular wires functionas highly sensitive and isoform specific fluorescent sensors for P450CAM

Conjugates of Heme-Thiolate Enzymes with Photoactive Metal-Diimine Wires

Heme-thiolate enzymes, notably cytochromes P450 and nitric oxide synthases, use dioxygen to oxygenatesubstrates. Photoactive metal-diimine molecular wires that are capable of effecting rapid redox state changesat buried active sites have been developed to generate intermediates in the catalytic cycles of these enzymes.Wires that feature a photoactive head group tethered to an active-site ligand bind P450CAM and induciblenitric oxide synthase (iNOS) primarily by hydrophobic interactions. The wire-binding specificity of eachenzyme is critically dependent on the structural flexibility of the protein. P450CAM:wire conjugates canadopt open or partially open conformations, thereby accommodating a wide range of wires, whereas onlylong wires with smaller [Re(CO)_3(bpy)Im]^+ head groupsare able to bind tightly in the rigid active-site channel of iNOS. Dansyl-terminated molecular wires functionas highly sensitive and isoform specific fluorescent sensors for P450CAM