• Mallayan Palaniandavar

      Articles written in Journal of Chemical Sciences

    • Iron(III) complexes of phenolate ligands as models for catechol dioxygenases

      Mallayan Palaniandavar Rathinam Viswanathan

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      Catechol 1,2-dioxygenase (CTD) and protocatechuate 3,4-dioxygenase (PCD) enzymes catalyse the oxidative cleavage of catechols tocis, cis-muconic acids with the incoporation of molecular oxygen. In our laboratory two series of iron(III) complexes of linear tridentate and tripodal tetradentate phenolate ligands have been characterised using IR, UV-Vis and EPR spectral and electrochemical techniques. The X-ray crystal structure of a few of the complexes have been determined. The interactions of the complexes with a variety of monodentate and bidentate heterocyclic bases as well as phenols have been investigated. The interactions with catecholate anions reveal changes in the phenolate-to-iron(III) charge transfer band, which are remarkably similar to catechol dioxygenase-substrate complexes. The redox behaviour of the complexes and their 1:1 adducts with 3,5-di-t-butylcatechol (H2DBC) has been investigated. All the complexes catalyse the oxidative cleavage of H2DBC by molecular oxygen to yieldcis,cis-muconic anhydride. The structure, redox and catalytic activities of the iron(III) complexes have been discussedvis-a-vis those of the enzymes.

    • Models for the active site in galactose oxidase: Structure, spectra and redox of copper(II) complexes of certain phenolate ligands

      Mathrubootham Vaidyanathan Mallayan Palaniandavar

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      Galactose oxidase (GOase) is a fungal enzyme which is unusual among metalloenzymes in appearing to catalyse the two electron oxidation of primary alcohols to aldehydes and H2O2. The crystal structure of the enzyme reveals that the coordination geometry of mononuclear copper(II) ion is square pyramidal, with two histidine imidazoles, a tyrosinate, and either H2O (pH 7.0) or acetate (from buffer,pH 4-5) in the equatorial sites and a tyrosinate ligand weakly bound in the axial position. This paper summarizes the results of our studies on the structure, spectral and redox properties of certain novel models for the active site of the inactive form of GOase. The monophenolato Cu(II) complexes of the type [Cu(L1)X][H(L1) = 2-(bis(pyrid-2-ylmethyl)aminomethyl)-4-nitrophenol and X = Cl1, NCS2, CH3COO3, ClO44] reveal a distorted square pyramidal geometry around Cu(II) with an unusual axial coordination of phenolate moiety. The coordination geometry of 3 is reminiscent of the active site of GOase with an axial phenolate and equatorial CH3COO ligands. All the present complexes exhibit several electronic and EPR spectral features which are also similar to the enzyme. Further, to establish the structural and spectroscopic consequences of the coordination of two tyrosinates in GOase enzyme, we studied the monomeric copper(II) complexes containing two phenolates and imidazole/pyridine donors as closer structural models for GOase. N,N-dimethylethylenediamine and N,N’-dimethylethylenediamine have been used as starting materials to obtain a variety of 2,4-disubstituted phenolate ligands. The X-ray crystal structures of the complexes [Cu(L5)(py)], (8) [H2(L5) = N,N-dimethyl-N’,N’-bis(2-hydroxy-4-nitrobenzyl) ethylenediamine, py = pyridine] and [Cu(L8)(H2O)] (11), [H2(L8) = N,N’-dimethyl-N,N’-bis(2-hydroxy-4-nitrobenzyl)ethylenediamine] reveal distorted square pyramidal geometries around Cu(II) with the axial tertiary amine nitrogen and water coordination respectively. Interestingly, for the latter complex there are two different molecules present in the same unit cell containing the methyl groups of the ethylenediamine fragmentcis to each other in one molecule andtrans to each other in the other. The ligand field and EPR spectra of the model complexes reveal square-based geometries even in solution. The electrochemical and chemical means of generating novel radical species of the model complexes, analogous to the active form of the enzyme is presently under investigation.

    • Iron(III) complexes of certain tetradentate phenolate ligands as functional models for catechol dioxygenases

      Mallayan Palaniandavar Marappan Velusamy Ramasamy Mayilmurugan

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      Catechol 1,2-dioxygenase (CTD) and protocatechuate 3,4-dioxygenase (PCD) are bacterial non-heme iron enzymes, which catalyse the oxidative cleavage of catechols tocis, cis-muconic acids with the incorporation of molecular oxygen via a mechanism involving a high-spin ferric centre. The iron(III) complexes of tripodal phenolate ligands containing N3O and N2O2 donor sets represent the metal binding region of the iron proteins. In our laboratory iron(III) complexes of mono- and bisphenolate ligands have been studied successfully as structural and functional models for the intradiol-cleaving catechol dioxygenase enzymes. The single crystal X-ray crystal structures of four of the complexes have been determined. One of thebis-phenolato complexes contains a FeN2O2Cl chromophore with a novel trigonal bipyramidal coordination geometry. The Fe-O-C bond angle of 136.1‡ observed for one of the iron(III) complex of a monophenolate ligand is very similar to that in the enzymes. The importance of the nearby sterically demanding coordinated -NMe2 group has been established and implies similar stereochemical constraints from the other ligated amino acid moieties in the 3,4-PCD enzymes, the enzyme activity of which is traced to the difference in the equatorial and axial Fe-O(tyrosinate) bonds (Fe-O-C, 133, 148‡). The nature of heterocyclic rings of the ligands and the methyl substituents on them regulate the electronic spectral features, FeIII/FeII redox potentials and catechol cleavage activity of the complexes. Upon interacting with catecholate anions, two catecholate to iron(III) charge transfer bands appear and the low energy band is similar to that of catechol dioxygenase-substrate complex. Four of the complexes catalyze the oxidative cleavage of H2DBC by molecular oxygen to yield intradiol cleavage products. Remarkably, the more basic N-methylimidazole ring in one of the complexes facilitates the rate-determining productreleasing phase of the catalytic reaction. The present study provides support to the novel substrate activation mechanism proposed for the intradiol-cleavage enzymes.

    • Mononuclear non-heme iron(III) complexes of linear and tripodal tridentate ligands as functional models for catechol dioxygenases: Effect of 𝑁-alkyl substitution on regioselectivity and reaction rate

      Mallayan Palaniandavar Kusalendiran Visvaganesan

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      Catechol dioxygenases are responsible for the last step in the biodegradation of aromatic molecules in the environment. The iron(II) active site in the extradiol-cleaving enzymes cleaves the C-C bond adjacent to the hydroxyl group, while the iron(III) active site in the intradiol-cleaving enzymes cleaves the C-C bond in between two hydroxyl groups. A series of mononuclear iron(III) complexes of the type [Fe(L)Cl3], where L is the linear 𝑁-alkyl substituted bis(pyrid-2-ylmethyl)amine, 𝑁-alkyl substituted 𝑁-(pyrid-2-ylmethyl)ethylenediamine, linear tridentate 3N ligands containing imidazolyl moieties and tripodal ligands containing pyrazolyl moieties have been isolated and studied as structural and functional models for catechol dioxygenase enzymes. All the complexes catalyse the cleavage of catechols using molecular oxygen to afford both intra- and extradiol cleavage products. The rate of oxygenation depends on the solvent and the Lewis acidity of iron(III) center as modified by the sterically demanding 𝑁-alkyl groups. Also, our studies reveal that stereo-electronic factors like the Lewis acidity of the iron(III) center and the steric demand of ligands, as regulated by the 𝑁-alkyl substituents, determine the regioselectivity and the rate of dioxygenation. In sharp contrast to all these complexes, the pyrazole-containing tripodal ligand complexes yield mainly the oxidized product benzoquinone.

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