Design, Synthesis And Enzyme-Mimetic Catalysis Of New Cyclophosphazene Polydentate Metal Complexes

Enzyme mimetic systems are associated with the design and synthesis of compounds by chemistry methods to imitate the structure and/or functions of enzymes and serve as artificial enzyme systems. These chemicals usually have unique catalytic quality of nature enzyme, but can be more stable than enzyme, which makes them more widely applicable. Hence, it is very important to investigate these highly efficient artificial enzyme reagents. In the last several years, many metal complexes have been developed as artificial enzymes. Among them, multi-nuclear complexes are particularly attractive due to their potential cooperative effects between/among the metal centers.In this dissertation, the design and synthesis of a series of novel cyclotriphosphazene polydentate ligands with different pendant heterocyclic metal-binding ligands are discussed. These macromolecular ligands can effectively bind metal ions and serve as the mimic enzyme catalystsand their catalytic efficiency and mechanisms of action are further investigated.Section One:The design and synthesis of ligands(1) Five polydentate cyclotriphosphazene ligands (L1-L5) were designed and synthesized through the steps of nucleophilic substitution, reduction, bromination and functionalized with metal-binding heterocycles. These dendritic chemicals contain six active heterocyclic arms as potential functional models for mimic enzymes catalyses. The general synthetic procedures of the ligands are shown in Scheme 1. (2) Three multidentate ligands (L6, L7, and L8 in Scheme 2) with two, four, or six metal chelating'arms' attached to a cyclotriphosphazene core have been synthesized according to the method above. Scheme 2. Structures of the New Ligands L6-L8 used in this studyAll these new target compounds were fully characterized by 1H NMR,31P NMR, 13C NMR, IR and ESI-MS.Section Two:Metal complexes of these new ligands have been prepared and utilized as potential functional models of metalloenzymes for the investigation of the hydrolysis of phosphoesters and polyphenol oxidation. The reactions of interest are:(1) Hydrolytic cleavage of DNA plasmid.Firstly, the metal complexes of the five polydentate cyclotriphosphazene ligands (L1-L5) are investigated as artificial nucleases. The cleavage of pUC19 DNA in the presence of a metal complex revealed that these new class of compounds were indeed as effective catalysts under optimum conditions. Especially, the complexes of the ligand L2 with imidazole arms have superior reactivity comparing to the other hexanuclear analogous.(2) Hydrolyses of PhosphoestersThe metal complexes (Mx-L2, x=1,2, or 3; M=ZnⅡ, CuⅡ, or CoⅡ) of the imidazole-containing cyclophosphazene were prepared and used as artificial phospho-esterase models toward hydrolysis of the model phosphomonoester p-nitrophenyl-phosphate (NPP) and phosphodiester bis(p-nitrophenyl)phosphate (BNPP) in 75% DMSO buffer solution at pH=7-11 and 37℃under pseudo-first-order reaction conditions. The hydrolysis of BNPP and NPP by the metal complex Cu3L2 exhibits enzyme-like saturation kinetics with first-order rate constants kcat (1.4±0.2)×10-5 and (6.8±0.2)×10-4 s-1,respectively, which are ca.6200～560 fold higher than that of the unanalyzed reaction. The complex Cu3L2 exhibits a tremendous selectivity toward NPP, showing nearly a stoichiometric binding with 1:1 (substrate:complex). An intramolecular dinuclear pathway also was proposed.(3) Oxidation of catechol type of substratesThree multinuclear Complexes (Cu2-L6, Cu4-L7, and Cu6-L8) with two, four, or six metal centers have been prepared with the aim to investigate their nuclearity effect, which as potential functional models for polyphenol oxidase and related dinuclear enzymes. These new complexes show significant oxidase activities toward several catechol-containing substrates in methanol-HEPES buffer solution at pH 7.0 and follow enzyme-like pre-equilibrium kinetic patterns. These three complexes exhibit similar kcat and Km values per Cu(Ⅱ) center, i.e., (17.5±1.2)×10-3 s-1 and 2.8±0.3 mM, suggesting similar mechanistic pathways. The kcat values for catechol oxidation by these three complexes increase with the increasing of metal centers, which are about 8,18, and 24 times higher than that of the mononuclear Cu(cyclen) complex (4.2×10-3 s-1) under the same conditions, indicating the significance of multinuclear center in oxidative catalysis. The cooperativity among the metal centers also was confirmed by the presence of magnetic interaction interaction revealed with EPR experiments.The complex Cu2L6 exhibits a good selectivity toward catechol and the mechanism Job plot revealed a intramolecular dinuclear pathway, whereas the simple complex Cu(cyclen) exhibits a dimolecular dinuclear pathway.