title: Radical pathway in catecholase activity with zinc-based model complexes of compartmental ligands.
abstract: Four dinuclear and three mononuclear Zn(II) complexes of phenol-based compartmental ligands (HL(1)-HL(7)) have been synthesized with the aim to investigate the viability of a radical pathway in catecholase activity. The complexes have been characterized by routine physicochemical studies as well as  X-ray single-crystal structure analysis: [Zn(2)(H(2)L(1))(OH)(H(2)O)(NO(3))](NO(3))(3) (1), [Zn(2)L(2)Cl(3)] (2), [Zn(2)L(3)Cl(3)] (3), [Zn(2)(L(4))(2)(CH(3)COO)(2)] (4), [Zn(HL(5))Cl(2)] (5), [Zn(HL(6))Cl(2)] (6), and [Zn(HL(7))Cl(2)] (7) [L(1)-L(3) and L(5)-L(7) = 2,6-bis(R-iminomethyl)-4-methylphenolato, where R= N-ethylpiperazine for L(1), R = 2-(N-ethyl)pyridine for L(2), R = N-ethylpyrrolidine for L(3), R = N-methylbenzene for L(5), R = 2-(N-methyl)thiophene for L(6), R = 2-(N-ethyl)thiophene for L(7), and L(4) = 2-formyl-4-methyl-6-N-methylbenzene-iminomethyl-phenolato]. Catecholase-like activity of the complexes has been investigated in methanol medium by UV-vis spectrophotometric study using 3,5-di-tert-butylcatechol as model substrate. All  complexes are highly active in catalyzing the aerobic oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylbenzoquinone (3,5-DTBQ). Conversion of 3,5-DTBC to 3,5-DTBQ catalyzed by mononuclear complexes (5-7) is observed to proceed via  formation of two enzyme-substrate adducts, ES1 and ES2, detected spectroscopically, a finding reported for the first time in any Zn(II) complex catalyzed oxidation of catechol. On the other hand, no such enzyme-substrate adduct has been identified, and  3,5-DTBC to 3,5-DTBQ conversion is observed to be catalyzed by the dinuclear complexes (1-4) very smoothly. EPR experiment suggests  generation of radicals in the presence of 3,5-DTBC, and that finding has been strengthened by cyclic voltammetric study. Thus, it may be proposed that the radical pathway is probably responsible for conversion of 3,5-DTBC to 3,5-DTBQ promoted by  complexes of redox-innocent Zn(II) ion. The ligand-centered radical generation has further been verified by density functional theory calculation.
authors.affiliation: Department of Chemistry, University of Calcutta, 92, A. P. C. Road, Kolkata 700 009, India.
authors.forename: Averi
authors.initials: A
authors.lastname: Guha
authors.forename: Tanmay
authors.initials: T
authors.lastname: Chattopadhyay
authors.forename: Nanda Dulal
authors.initials: ND
authors.lastname: Paul
authors.forename: Madhuparna
authors.initials: M
authors.lastname: Mukherjee
authors.forename: Somen
authors.initials: S
authors.lastname: Goswami
authors.forename: Tapan Kumar
authors.initials: TK
authors.lastname: Mondal
authors.forename: Ennio
authors.initials: E
authors.lastname: Zangrando
authors.forename: Debasis
authors.initials: D
authors.lastname: Das
mesh_terms: D001555:Benzene Derivatives / Q000737:chemistry; D016227:Benzoquinones / Q000737:chemistry; D040761:Biomimetic Materials / Q000737:chemistry; D002384:Catalysis; D004156:Catechol Oxidase / Q000737:chemistry; D002396:Catechols / Q000737:chemistry*; D056831:Coordination Complexes / Q000138:chemical synthesis*; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry*; D007700:Kinetics; D008024:Ligands; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010879:Piperazines / Q000737:chemistry; D011725:Pyridines / Q000737:chemistry; D011789:Quantum Theory; D013053:Spectrophotometry; D013876:Thiophenes / Q000737:chemistry; D015032:Zinc / Q000737:chemistry*
chemical_list: D001555:Benzene Derivatives; D016227:Benzoquinones; D002396:Catechols; D056831:Coordination Complexes; D005609:Free Radicals; D008024:Ligands; D010879:Piperazines; D011725:Pyridines; D013876:Thiophenes; D004156:Catechol Oxidase; D015032:Zinc