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Catalytic Aspects of a Copper(II) Complex: Biological Oxidase to Oxygenase Activity

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Catalytic Aspects of a Copper(II) Complex: Biological Oxidase to Oxygenase Activity J. Chem. Sci. Vol. 129, No. 10, October 2017, pp. 1627–1637. © Indian Academy of Sciences DOI 10.1007/s12039-017-1379-y REGULAR ARTICLE Catalytic aspects of a copper(II) complex: biological oxidase to oxygenase activity BISWAJIT CHOWDHURYa, MILAN MAJIc and BHASKAR BISWASa,b,∗ aDepartment of Chemistry, Raghunathpur College, Purulia, West Bengal 723 133, India bDepartment of Chemistry, Surendranath College, 24/2 M G Road, Kolkata, West Bengal 700 009, India cDepartment of Chemistry, National Institute of Technology, Mahatma Gandhi Avenue, Durgapur, West Bengal 713 209, India E-mail: [email protected] MS received 19 February 2017; revised 30 August 2017; accepted 7 September 2017; published online 3 October 2017 ) , = Abstract. A coper(II) complex, [Cu(dpa)2(OAc)](ClO4 (1) [dpa =2 2 -dipyridylamine; OAc acetate], has been synthesized and crystallographically characterized. X-ray structure analysis revealed that this mononuclear Cu(II) complex crystallizes as a rare class of hexa coordination geometry named bicapped square pyramidal −1 geometry with P21/c space group. This copper complex displays excellent catalytic efficiency, kcat/KM(h ) = 6.17 × 105 towards the oxidative coupling of 2-aminophenol (2-AP) to aminophenoxazin-3-one. Further, upon stoichiometric addition of copper(II) complex to 3,5-DTBC in presence of molecular oxygen in ethanol medium, the copper complex affords predominantly extradiol cleavage products along with a small amount of −3 −1 benzoquinone and a trace amount of intradiol cleavage products at a rate, kobs = 1.09 × 10 min , which provide substantial evidence for the oxygen activation mechanism. This paper presents a novel addition of a copper(II) complex having the potential to mimic the active site of phenoxazinone synthase and catechol dioxygenase enzymes with significant catalytic efficiency. Keywords. Copper(II); crystal structure; phenoxazinone synthase activity; catechol dioxygenase; bio-mimetic chemistry. 1. Introduction these copper proteins is phenoxazinone synthase which is a copper-containing oxidase that catalyzes the cou- Over the past few decades, biochemical oxidation pling of 2-aminophenols to form the 2-aminophenoxaz- reactions have significantly stimulated the coordination inone chromophore. This reaction constitutes the final chemists to design various metal complexes as struc- step in the biosynthesis of the potent antineoplastic agent tural and functional mimic of the active site of different actinomycin. 26–29 On the other hand, catechol dioxy- metalloproteins and metalloenzymes. 1–3 Bio-inspired genases of intradiol class utilizes a non-heme iron(III) catalysis mediated by different copper proteins and cofactor in catalyzing the cleavage of the carbon– enzymes include primarily dioxygen transport (hemo- carbon bond between the two catechol oxygens; and cyanin, Hc), 4 aromatic ring oxidations (tyrosinase, Tyr, 5 the extradiol dioxygenases utilize a nonheme iron(II) catechol oxidase, 6 and quercetin-2,3-dioxygenase, 7–9 cofactor in catalyzing the cleavage of the carbon–carbon the biogenesis of neurotransmitters and peptide hor- bond adjacent to the catechol oxygens. 30,31 In the last mones (dopamine-β-monooxygenase, D β M, 10 and pep- decade, Youngme et al., and Choudhury et al., produced tidylglycine α-amidating monooxygenase, PHM, 11,12 same Cu(II)-dipyridyl complex using different reac- hydrogen peroxide generation (galactose and glyoxal tion methodology. 32 Here we introduce a copper(II)- oxidases, 13–18 iron homeostasis (ceruloplasmin3 and ) dipyridylamine complex [Cu(dpa)2(OAc)](ClO4 (1) Fet3p, 19–21 and methane oxidation (particulate methane [dpa = 2, 2-dipyridylamine; OAc = acetate], with monoxygenase, pMMO). 22–25 A prominent member of potential ability to mimic the functional sites of phenox- −1 3 azinone synthase enzyme with kcat (h ) = 1.83 × 10 *For correspondence and catechol dioxygenase enzyme with the Electronic supplementary material: The online version of this article (doi:10.1007/s12039-017-1379-y) contains supplementary material, which is available to authorized users. 1627 1628 Biswajit Chowdhury et al. − decomposition rate, 1.09 × 10−3 min 1, respectively. CHN microanalyser. Electrospray ionization (ESI) mass spec- Coordinative unsaturation of the Cu(II) centre in ethanol trum was recorded on a Q-TOF MicroTM Mass Spectrometer. medium facilitates the formation of substrate-enzyme The electrochemical studies were carried out using Cyclic voltammograms were recorded in CH3CN solutions contain- adduct and account in favour of such oxidase to oxyge- ◦ nase activity. ing 0.1 M TBAP at 25 C using a three-electrode configuration (Pt working electrode, Pt counter electrode, Ag/AgCl refer- ence) and a PC-controlled PAR model 273A electrochemistry system. All the experimental solutions were degassed for 30 2. Experimental min with high-purity argon gas before any cyclic voltammetry of a sample was done. The Electron Paramagnetic Resonance 2.1 Materials (EPR) spectrum was recorded on a Bruker EMX-X band spec- trometer. High purity 2, 2 -dipyridylamine (Aldrich, UK), copper(II) perchlorate hexahydrate (Fluka, Germany), sodium acetate (E. Merck, India), 2-aminophenol (E. Merck, India), 3,5- 2.4 Crystal structure determination and refinement di-tert-butylcatechol (Sigma Aldrich Corporation, St. Louis, MO, USA) were obtained from commercial sources and Single crystal X-ray diffraction data of the copper(II) complex used as purchased. All other chemicals and solvents were were collected using a Rigaku XtaLABmini diffractometer of analytical grade and were used as received without further equipped with Mercury CCD detector. The data were col- α λ = purification. lected with graphite monochromated Mo-K radiation ( 0.71073 Å) at 293 K using ω scans. The data were reduced Caution! Perchlorate salts of metal ions are potentially explo- using Crystal Clear suite 2.0 33 and the space group determi- sive, especially in the presence of organic ligands. Only a nation was done using Olex2. The structure was resolved by small amount of material should be prepared and it should be direct method and refined by full-matrix least-squares proce- handled with care. dures using the SHELXL-2014/7 34 software package through the OLEX2 suite. 35 The crystallographic bond distance and 2.2 Synthesis of [Cu(dpa)2(OAc)](ClO4)(1) bond angle are given in Table 1 and Table S1 (in Supplemen- tary Information). The copper(II) complex was synthesized by addition of aque- ous solution of dipyridylamine (0.3420 g, 2 mmol) into a 2.5 Catalytic oxidation of 2-aminophenol ) · solution of Cu(ClO4 2 6H2O (0.3650 g, 1 mmol)] in the same solvent (20 mL) keeping the solution on a magnetic In order to examine the penoxazinone synthase activity, stirrer with stirring. Then solid sodium acetate (0.0820 g, 1 1 × 10−3M solution of 1 in EtOH was treated with 10 equiv. mmol) was added in solid into the blue solution and stirring continued to 30 min more. The blue solutions were turned into green and the supernatant liquids were kept in air for Table 1. Crystallographic refinement parameters [ ]( ) slow evaporation. After 7–10 days the fine microcrystalline of Cu(dpa)2(OAc) ClO4 (1). compound was separated out and washed with hexane and Parameters Cu(II) compound dried in vacuo over silica gel indicator. The spectroscopic measurements and elemental analyses confirm the struc- Empirical formula C22H21N6O6ClCu tural formation of the complex. Yield =∼0.28g, (∼ 77% Formula weight 564.44 based on metal salt). Anal. calc. (%) C22H22N6ClO6Cu: C, Temperature (K) 293 46.73; H, 3.92; N, 14.86; Found(%): C, 46.69; H, 3.88; N, Crystal system Monoclinic − 14.89. Selected IR bands (KBr pellet, cm 1): 3321 (m), Space group P21/c 1637(s), 1604(s),1423(m), 1378(s), 1095(s). UV-Vis (λ,nm; a (Å) 13.885(12) 10−4M, 1 cm cell length, abs, ethanol): 298(1.86), 725– b (Å) 7.899(7) 745(0.00180) (broad band); ESI-MS (MeCN): m/z, 406.10 c (Å) 22.202(18) ([ ] +) ( 3) Cu(dpa)2 -H ; (Calc. 406.09). Volume Å 2582.7(2) Z4 ρ ( −3) 2.3 Physical measurements gcm 1.541 μ (mm−1) 1.058 F (000) 1156 Infrared spectrum (KBr) was recorded with a FTIR-8400S ◦ ◦ ◦ − θ ranges ( ) 3.1 to 27.50 SHIMADZU spectrophotometer in the range 400–3600 cm 1. R 0.038 1H NMR spectrum in DMSO-d6 was obtained on a Bruker int ◦ R (reflections) 15670 Avance 300 MHz spectrometer at 25 C and was recorded at wR2 (reflections) 5578 299.948 MHz. Ground-state absorption measurements were Final R indices 0.0634, 0.1929 − made with a Jasco model V-730 UV-Vis spectrophotometer. Largest peak and hole (eA◦ 3) 0.77, −0.44 Elemental analyses were performed on a Perkin Elmer 2400 Catalytic aspects of a copper(II) complex 1629 of 2-aminophenol (2-AP) under aerobic conditions at room MHz, CDCl3): δ = 3, 5-di-tert-butyl-2-pyrone: 6.09 (m, 2H); temperature. Absorbance vs. wavelength (wavelength scans) 4,6- di-tert-butyl-2-pyrone: 7.11 (d,1H), 7.24 (d, 1H); 3,5- of the solution was recorded at a regular time interval of 15 min di-tert-butylbenzoquinone: 6.19 (d, 1H), 6.74 (d, 1H); 3,5-di- for aminophenol oxidation in the wavelength range 300–800 tert-butyl-5-(carboxymethyl)-2-furanone: 6.84 (s, 1H). nm. Kinetic experiments were performed spectrophotometri- cally 26–28 with Cu(II) complex and 2-AP in EtOH at 25◦C for aminophenol oxidation activity. 0.04 mL of the complex solution, with a constant concentration of 1 × 10−3 M, was 3. Results and Discussion added to 2 mL of 2-AP of a particular concentration (varying its concentration from 1 × 10−3 Mto1× 10−2 M) to achieve 3.1 Synthesis and formulation of the Cu(II)-dpa the ultimate concentration of the complex as 1×10−3 M. The complex (1) conversion of 2-aminophenol to 2-aminophenoxazine-3-one was monitored with time at 433 nm (time scan) 28 in EtOH.
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