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

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- = Cl- 1, NCS- 2, CH3COO- 3, ClO4 - 4] 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

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

Synthesis, structure, spectra and redox of Cu(II) complexes of chelating bis(benzimidazole)-thioether ligands as models for electron transfer blue copper proteins

The tridentate ligand 1,5-bis(benzimidazol-2-yl)-3-thiapentane (L1) with N2S donor set forms the complex [Cu(L1)(H2O)Cl]Cl 1a and the linear quadridentate ligand 1,8-bis(benzimidazol-2-yl)-3,6-dithiaoctane (L2) with N2S2 donor set forms the complexes [Cu(L2)](ClO4)2·2H2O 2a and [Cu(L2)(NO3)]NO32b. The linear pentadentate ligand 1,11-bis(pyrid-2-yl)-3,6,9-trithiaundecane (L3) with N2S3 donor set forms the complex [Cu(L3)](ClO4)23. The perchlorate complex [Cu(L4)](ClO4)2·2CH3CN 4 of the pentadentate ligand, N,N-bis(benzimidazol-2-ylmethylthioethyl)methylamine (L4) with N3S2 donor set has also been isolated. In 1a Cu(II) is coordinated to the two benzimidazole nitrogens and thioether sulfur of the ligand L1, a chloride ion and a water molecule. The coordination geometry around copper is intermediate between trigonal bipyramidal and square pyramidal geometries and is better described as trigonal bipyramidal distorted square based pyramidal (TBDSBP) with the sulfur and nitrogen atoms and the chloride ion in the equatorial positions and the oxygen of water in the apical position. The coordination geometry around copper(II) in 2b is best described as trigonal bipyramidal, with both the thioether sulfur atoms [Cu-S(1), 2.529(5) and Cu-S(2), 2.438(6) Å] and one of the oxygen atoms of the nitrate ion [Cu-O(1), 2.066(13) Å] constituting the trigonal plane and both the benzimidazole nitrogens [Cu-N, 1.985(14) and 1.953(13) Å] occupying the axial positions. The bulky benzimidazole moieties of the ligand prevent the other nitrate ion from coordinating and favours trigonal bipyramidal geometry in spite of the presence of two six-membered chelate rings. In 4 the coordination plane of Cu(II) is comprised of two benzimidazole nitrogens, one thioether sulfur and N-methyl substituted amine nitrogen atom with the other thioether sulfur atom coordinated axially. The coordination geometry is best described as trigonal bipyramidal distorted square based pyramidal (TBDSBP). The ligand field and EPR spectra of 1a, 2a and 2b are consistent with trigonal bipyramidal geometry in the solid state, whereas two ligand field bands in solution and an axial EPR spectrum in frozen solution were observed suggesting a change in coordination geometry to a square-based one on dissolution. The complexes 3 and 4 exhibit only one ligand field band in the solid state and axial EPR spectrum consistent with a square based geometry. All the complexes exhibit an intense S(σ ) → Cu(II) CT band in the range 330-380 nm and a high positive CuII/CuI redox potential

Axial versus equatorial coordination of thioether sulfur: mixed ligand copper(ii) complexes of 2-pyridyl-n-(2'-methylthiophenyl)-methyleneimine with bidentate diimine ligands

The synthesis, structure and spectral and redox properties of the copper(II) complexes [Cu(pmtpm)Cl2] (1) and [Cu(pmtpm)2](ClO4)2 (6), where pmtpm is the linear tridentate ligand 2-pyridyl-N-(2'-methylthiophenyl)methyleneimine containing a thioether and two pyridine donors, are described. Also, the mixed ligand complexes [Cu(pmtpm)(diimine)](ClO4)2 (2-5), where the diimine is 2,2'-bipyridine (bpy) (2), 1,10-phenanthroline (phen) (3), 2,9-dimethyl-1,10-phenanthroline (2,9-dmp) (4) or dipyrido-[3,2-d:2',3'-f]-quinoxaline (dpq) (5), have been isolated and studied. The X-ray crystal structures of the complexes 1, [Cu(pmtpm)(2,9-dmp)](ClO4)2 4 and 6 have been successfully determined. The complex 1 possesses a trigonal bipyramidal distorted square based pyramidal (TBDSBP) coordination geometry in which three corners of the square plane are occupied by two nitrogens and thioether s of pmtpm ligand and the remaining equatorial and the axial positions by two chloride ions. The complex 4 contains a CuN4S chromophore also with a TBDSBP coordination geometry in which two nitrogens and the thioether sulfur of pmtpm ligand occupy three corners of the square plane. One of the two nitrogens of 2,9-dmp ligand is equatorially coordinated and the other axially to copper. On the other hand, the complex 6 is found to possess a square based pyramidal distorted trigonal bipyramidal (SPDTBP) coordination geometry. The CuN2S trigonal plane in it is comprised of the pyridine and imine nitrogens and the thioether sulfur of the pmtpm ligand. The pyridine nitrogens of the ligand occupy the axial positions and the second thioether sulfur remains uncoordinated. On long standing in acetonitrile solution the mixed ligand complexes 2 and 3 undergo ligand disproportionation to provide the corresponding bis-complexes of bpy and phen, respectively, and 6. The electronic and EPR spectral parameters and the positive redox potential of complex 4 are consistent with the equatorial location of the thioether sulfur in the square-based coordination geometry around copper(II). On the other hand, the higher g|| and lower A|| values and lower E1/2 values for the complexes 2, 3 and 5 are consistent with the axial coordination of the thioether sulfur. Also, the Cu(II)/Cu(I) redox potentials increase with increase in number of aromatic rings of the diimine ligand. The steric and electronic effects of the bidentate diimine ligands in orienting the thioether coordination to axial or equatorial position are discussed

Carboxylate mixed ligand complexes of copper(II) with some chelating carbonyl and imine ligands and their reactivity

The reaction of CuL2 where L is ethylacetoacetate (EAA),o-hydroxyacetophenone (OHAP) ando-hydroxycrotonophenone (OHCP), with trichloroacetic acid (TCA) in chloroform yielded mixed ligand complexes of the type CuL(TCA). They have sub-normal magnetic moments and their ligand field spectra suggest a square-planar configuration. Their i.r. spectra indicate bidentate chelating carboxylate groups. The mixed ligand complexes CuL(OXINE), where LH is salicylaldehyde (SAL) or salicylaldimine (SALIMINE), have been also obtained. The treatment of CuL(OXINE) where L is SAL, SALIMINE, ACAC or salicylaldehydephenylhydrazone (SALPHN) with TCA yielded Cu(OXINE)(TCA). On reacting Cu(OHAP)(SAL) and Cu(SAL)(ACAC) with TCA, Cu(SAL)(TCA) and Cu(ACAC)(TCA), respectively, were produced. The displacement of coordinated L from CuL(TCA) by other bidentate chelating ligands in benzene has been also studied. Thus, EAA in Cu(EAA)(TCA) was replaced by OHAP, and SAL in Cu(SAL)(TCA) by OHCP. The reactions reveal the following orders of Cu-O strength: TCA > OXINE > SALIMINE > ACAC > OHCP > SAL > OHAP > EAA and OXINE > SALPHN

Models for enzyme-copper-nucleic acid interaction <SUP>1</SUP>H NMR line broadening study of the interaction of some copper complexes with cytidine

Sodium (salicylideneglycylglycinato) copper(II), bis(2,9-dimethyl-1, 10-phenanthroline)copper (II), and (1,5-bis (benzimidazol-2-yl)-3-thiapentane) copper(II) were interacted with cytidine in D2O. For all the complexes, the broadening of the1H signals of cytidine is found to be in the order H(5)&gt;H(6)&gt;H(1'); this suggests the binding of N (3) and O2 sites of cytidine to copper. The possibility that the ribose moiety is also involved in coordination cannot be ruled out. The complete broadening of the H(5) signals for the last complex above is because of the coordinative unsaturation of the complex. It has been concluded that steric factors and coordinative unsaturation in athe complexes affect the extent of interaction with cytidine

Mononuclear non-heme iron(III) complexes as functional models for catechol dioxygenases

This article provides an overview of our work on mononuclear iron(III) complexes of phenolate and non-phenolate ligands as structural and functional models for the intradiol-cleaving non-heme catechol 1,2-dioxygenase (CTD) and protocatechuate 3,4-dioxygenase (PCD) enzymes. All the complexes are cis-facially coordinated to iron(III) with a distorted octahedral geometry. The iron(III) complexes of linear tridentate 3N ligands and tetradentate tripodal phenolate ligands possess octahedral geometries with cis-coordination positions available for bidentate coordination of catechols. In two of these complexes with sterically demanding -NMe2 pendant, the Fe-O-C bond angle is around 135.7&#176; , which is close to those (Fe-O-C, 133&#176; , 148&#176; ) in 3,4-PCD enzyme. Also, interestingly, one of the bis-phenolate complexes displays trigonal bipyramidal coordination geometry as in the enzymes. The efficiency of the complexes to catalyze the intradiol-cleavage of 3,5-di-tert-butylcatechol (H2DBC) could be illustrated not only on the basis of Lewis acidity of the iron(III) center alone, but also by assuming that product release is the rate-determining phase of the catalytic reaction

E.p.r. studies of the effect of extensive conjugation of coordinated C=O and C=N in some copper(II) complexes

E.p.r. studies indicate the delocalisation of the unpaired electron in the CuII complex of 2'-hydroxychalcone into the benzene ring. The 2-hydroxycrotonophenone complex has decreased covalency in the M-O &#963; -bond. The e.p.r. parameters of these complexes in pyridine solution show that their adducts are formed. A good correlation of magnetic properties with bonding characteristics has been obtained. The &#963; -bond covalency parameter shows a linear relationship with Niso values which follow the order: ethylenediamine &gt; N-Me &gt; N-alkyl &gt; N-H in the chalconeimine complexes

Chiral discrimination in the binding of tris(phenanthroline)ruthenium(II) to calf thymus DNA: an electrochemical study

The binding of &#916;-, &#923;-, and rac-[Ru(phen)3]2+ (phen = 1,10-phenanthroline) and &#916;-, &#923;-, and rac-[Ru(bpy)3]2+ (bpy = 2,2'-bipyridyl) with calf thymus DNA has been examined by cyclic and differential pulse voltammetric techniques to obtain structural insight into the noncovalent binding of the enantiomers to DNA. The insignificant shift in RuII/RuIII peak potentials on the addition of DNA suggests that both the oxidized and reduced forms bind to DNA to the same extent. Interestingly, DNA selectively decreases the peak currents of &#916;-[Ru(phen)3]2+ but not those of the &#923;-enantiomer; rac-[Ru(phen)3]2+ exhibits an intermediate behavior, thus suggesting that the &#916;-form exhibits significant selectivity for B-DNA. The binding constants (K2+) and binding site sizes (s) have been determined from the decrease in the peak currents. The binding constant (K2+) of &#916;-[Ru(phen)3]2+ is on the order of 104 M-1 which is less than that for proven intercalators. In contrast, the electrochemical behavior of all three forms of [Ru(bpy)3]2+ remains almost unaffected in the presence of DNA, suggesting that the complexes might reside on the hydrophilic coat of the DNA helix

Influence of chelate-ring size and number of sulfur-donor atoms on spectra and redox behaviour of copper(II) bis(benzimidazolyl) tetra- and penta-thloether complexes

The linear quadridentate R(CH2)mSCH2CH2S(CH2)nSCH2CH2S(CH2)mR, where m= 1, n= 2 (L1), or 3 (L2), m= 2 n= 2 (L3) or 3 (L4), and the pentadentate RCH2(SCH2CH2)4SCH2R (L5)(R = benzimidazol-2-yl) compounds formed 1:1 copper(II) perchlorate complexes. Some of the quadridentate compounds also formed complexes of the type CuLX2(X = Cl-, NO3- or BF4-). All the complexes exhibited an absorption band around 30 000 cm-1 originating from a S(&#963; )&#8594; CuII charge-transfer transition. In solution the ClO4- and BF4- salts of [CuL1]2+ exhibit only one ligand-field band (14 800 cm-1) while the other complexes show two bands (&#8776; 11 000, 15 000-16 000 cm-1). The polycrystalline EPR spectra of the former complexes are axial while those of the other complexes are rhombic. The cryogenic solution EPR spectra of the former complexes differ from those of the other complexes by exhibiting comparatively low gII values and well resolved nitrogen superhyperfine structures. All these spectral features suggest a unique 'folded' geometry for the [CuL1]2+ complex. Among the tetrathioether complexes, the CuII-CuI redox potential increases with increase in the number of six-membered chelate rings, implying an increase in preference for the copper(I) over the copper(II) state. For copper(II) complexes of bis(benzimidazolyl) thioether ligands with all-five-membered chelate rings the potential increases with increase in the number of thioether donors