Supramolecular Architecture in an Oxovanadium (V)-Schiff Base

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Supramolecular Architecture in an Oxovanadium(V)-Schiff Base Complex: Synthesis, Ab initio Structure Determination from X-ray Powder Diffraction, DNA Binding and Cleavage Activity Swastik Mondal,† Monika Mukherjee,*,† Koushik Dhara,‡ Soumen Ghosh,# Jagnyeswar Ratha,§ Pradyot Banerjee,‡ and Alok K. Mukherjee#

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 9 1716-1721

Department of Solid State Physics and Department of Inorganic Chemistry, Indian Association for the CultiVation of Science, JadaVpur, Kolkata-700032, India, Department of Physics, JadaVpur UniVersity, Kolkata-700032, India, and Cellular Biochemistry DiVision, Indian Institute of Chemical Biology, JadaVpur, Kolkata-700 032, India ReceiVed October 26, 2006; ReVised Manuscript ReceiVed June 8, 2007

ABSTRACT: The synthesis, spectroscopic characterization, X-ray powder structure determination, and thermal behavior of a binuclear bis(µ-oxo)-bridged vanadium(V) complex, [(VO2L)2], L ) N,N′-dimethylenediamine(o-hydroxyl acetophenon), along with its DNA binding ability and photoinduced DNA cleavage activity, have been described. The compound crystallizes in a monoclinic system with a ) 7.679(3) Å, b ) 12.020(5) Å, c ) 13.882(6) Å, β ) 90.799(4)°, space group P21/c, and Z ) 2. The crystal structure has been solved from laboratory X-ray powder diffraction data using the direct space approach and refined by the Rietveld method. The dimeric complex consists of two edge-sharing vanadium octahedra with each metal center coordinated to one oxo-, one phenolate-, and two bridging-oxygen ligands, and two nitrogen donor atoms. The molecular structure reveals a two-dimensional grid of R44(24) rings in the (011) plane, which on combination with one-dimensional polymeric chains along the [100] direction, forms a novel three-dimensional supramolecular framework. Thermogravimetric analysis of the complex indicates multiple overlapping decomposition steps in the temperature range 180-800 °C with the end product being V2O5. The complex binds to double-stranded DNA giving a Kapp value of 1.56 × 107 M-1 and displays DNA cleavage activity on UV (300 nm) irradiation via a mechanistic pathway involving formation of singlet oxygen as the reactive species. Introduction Vanadium(V) complexes that are capable of binding and cleaving DNA under physiological conditions are of current interest for their potential utility as diagnostic agents in medicinal applications, as probes for conformational studies of nucleic acid, and for genomic research.1-3 When these systems are photoactivated, the chemical reactivity can be spatially localized and initiated on demand. The insulin-enhancing, insulin-mimetic properties of oxovanadium(V) complexes, their model character in relation to the active site of vanadoenzymes, and their use as catalysts in biological and industrial processes have been reported.4-7 Furthermore, these compounds exhibit antitumor activity by inhibiting growth of malignant cell lines by induction of cell-cycle arrest and/or cytotoxic effects.8,9 Several anionic and neutral oxo-, dioxo-vanadium(V) complexes containing tridentate Schiff base related ligands have been structurally characterized using single-crystal X-ray analyses. Depending on the number, types of donor atoms, and the nature of substitution in these oligodentate ligands, a variety of mononuclear10,11 and polynuclear12,13 oxo-, dioxo-vanadium(V) compounds and complex clusters14,15 have been reported. The metal centers with a N2O3 chromophore in the oxovanadium(V) monomers normally exhibit a square-pyramidal or trigonal bipyramidal environment.16,17 The two five-coordinate vanadium(V) centers often dimerize into octahedral bis(µ-oxo)bridged complexes via (L)VdO---VO(L) intermolecular interac* To whom correspondence should be addressed. E-mail: sspmm@ iacs.res.in. † Department of Solid State Physics, Indian Association for the Cultivation of Science. ‡ Department of Inorganic Chemistry, Indian Association for the Cultivation of Science. # Jadavpur University. § Indian Institute of Chemical Biology.

tions with highly asymmetric metal-(µ-O) distances.18-20 In the context of supramolecular architecture, these oxo-, dioxovanadium(V) compounds incorporating appropriate hydrogen bonding donors are also of particular importance and can lead to a variety of one-, two-, and three-dimensional (1D, 2D, and 3D) framework structures.6 Although single-crystal X-ray diffraction is undoubtedly the most powerful and widely used technique for elucidating the structures of organic and metal-organic compounds and inorganic complexes, an intrinsic limitation of this technique is the requirement to prepare single crystals of sufficient size, quality, and stability, which are not always met for all compounds using the chosen crystal growth conditions and within a reasonable time-scale. In such circumstances, X-ray powder diffraction can be an alternative route for structural analysis. Two classes of strategies, traditional (Patterson and Direct methods) and direct-space approaches, are generally used for solving crystal structures from X-ray powder data. In the direct-space approaches, which overcome some of the limitations encountered by the traditional methods, trial structures are generated in direct space, independently of the experimental powder diffraction data, and the suitability of each trial structure is assessed by comparing the powder diffraction pattern calculated for the trial structure and the experimental powder diffraction profile.21,22 Recent developments in the methodologies as implemented in the software packages FOX,23 DASH,24 EAGER25 have shown considerable promise for structure solution from powder diffraction data.26 Crystal structures of several molecular compounds and metal-organic complexes using X-ray powder diffraction data have been determined following the reciprocal or direct-space methodologies or a combination of both.27-29 In the present paper, we describe the synthesis, spectroscopic characterization, and crystal structure determination of a novel

10.1021/cg060753i CCC: $37.00 © 2007 American Chemical Society Published on Web 07/21/2007

Oxovanadium(V)-Schiff Base Complex

binuclear bis(µ-oxo)-bridged vanadium(V) complex, [{VO2(C12H17N2O)}2] (1), from X-ray powder diffraction data as well as its thermal behavior, DNA binding efficiency, and DNA cleavage activity. Experimental Section All reagents and chemicals were of AR grade and used as purchased. Calf thymus (CT) DNA, agarose (molecular biology grade), and ethidium bromide (EB) were obtained from Sigma. Supercoiled (SC) pUC19 DNA (cesium chloride purified) was purchased from Bangalore Genie (India). Tris(hydroxymethyl)-aminomethane-HCl(Tris-HCl) buffer was prepared using deionized and sonicated triple distilled water. Solvents used for spectroscopic studies were purified and dried by standard procedures before use.30 The elemental analysis was carried out using a Perkin-Elmer 2400 Series-II CHN analyzer. The IR, UVvis, and luminescence spectra were recorded on Simazu FTIR-8300, UV-2401 PC, UV-2100 and Perkin-Elmer LS 55 luminescence spectrometers, respectively. Thermogravimetric analysis was performed using a Perkin-Elmer Diamond TG/DTA thermobalance (model N5350030) under a flow of nitrogen (50 mL/min) from 40 to 800 °C at 10 °C min-1. Synthesis. O-Hydroxy acetophenone (0.34 g, 2.5 mmol) in tetrahydrofuran (THF) (10 mL) was added dropwise to a solution of N,Ndimethyl ethylene diamine (0.22 g, 2.5 mmol) in THF (20 mL) and refluxed for 1 h. To the resulting yellow solution, VO(acac)2 (0.663 g, 2.5 mmol), (acac ) acetonitrile acetone) was added and the solutuion was refluxed for 12 h; a stream of dry air was bubbled through the solution. On keeping the solution at 5 °C overnight, red microcrystalline powder separated out (yield 65%). Elemental analysis: Calculated for [VO2(C12H17N2O)]2: C 50.01, H 5.95, N 9.72%; found: C 48.97, H 6.04, N 9.47%. X-ray Powder Data Collection. The sample was ground to a fine homogeneous powder using an agate pestle and mortar and mounted in a top-loaded sample holder. X-ray powder diffraction data were collected in a Bruker D8 Advance powder diffractometer using monochromatic CuKR1 radiation (λ ) 1.5406 Å) selected with an incident beam germanium monochromator. The diffraction pattern at room temperature (20 °C) was recorded in step-scan mode with a step size (2θ) 0.02° and counting time 25 s-1 over an angular range 8-88° (2θ) using the Bragg-Brentano geometry. Partial recollection of data was undertaken to confirm sample integrity, and it showed no significant evidence of degradation in the X-ray beam. Indexing and Space Group Determination. The first 25 peaks of the powder pattern in the 2θ range 8-30° were fitted using the program NTREOR,31 and refined 2θ positions were input into the auto-indexing module of NTREOR. The solution with the highest figure of merit [M(25) ) 24.0, F(25) ) 60.0(0.008, 55)]32,33 indexed all peaks in a monoclinic cell having a ) 7.729(2) Å, b ) 12.090(3) Å, c ) 13.968(3) Å, and β ) 90.79(2)°. The results of indexing with the programs TREOR34 and DICVOL9135 also indicated an almost similar monoclinic solution. The unambiguous determination of space group using powder diffraction data is not straightforward, and usually more than one space group may be compatible with the diffraction profile. Additional problems may arise for space groups containing only screw axes since the number of reflections extracted from the powder pattern obeying the extinction rule of a screw axis is often quite small. The probabilistic approach36 as incorporated in EXPO200437 provides a quantitative basis for determining the possible extinction symbols from the experimental powder data. The full pattern decomposition was performed with EXPO2004 according to the Le Bail algorithm38 using a split type pseudo-Voigt peak profile function39 and space group P2/m, the Laue symmetry corresponding to the monoclinic system. Analysis of the powder pattern by the FINDSPACE module of EXPO2004 revealed a possible extinction symbol as P21/c, which was used for subsequent structure analysis. Incidentally, this happens to be the most common space group among the metal-organic compounds belonging to the monoclinic system, and a search of Cambridge Structural Database (Version 1.8, 2006) reveals that out of 1822 hits for vanadium compounds in the monoclinic system, 1238 (67.95%) crystallize in space group P21/c. Structure Solution and Refinement. The calculated density with two [(VO2L)2], L ) N,N′-dimethylenediamine(o-hydroxyl acetophenon)

Crystal Growth & Design, Vol. 7, No. 9, 2007 1717 moieties per unit cell agreed well with that reported for other bis(oxobridged) dinuclear vanadium complexes.40,41 The structure solution was carried out in space group P21/c with half of the formula unit as the crystallographic asymmetric unit. Attempts to solve the structure by Patterson or direct methods were not successful. In either case, however, the V atom could be located, but positions of C, N, and O atoms leading to a sensible structural motif could not be determined from successive difference Fourier or E-maps. The structure was therefore solved by global optimization of a structural model in direct space using the program FOX.23 This attempts to minimize the difference between the observed and the calculated powder intensities by a simulated-annealing approach (in parallel tempering mode) that moves the constituent fragments defining the structure within the unit cell, varying their positions, orientations, and, when appropriate, their conformation. Lattice and profile parameters, zero-point and interpolated background calculated from previous powder-pattern decomposition based on Le Bail algorithm, were introduced into the program FOX. The initial molecular geometry adopted from the standard data incorporated in the MOPAC 5.0 program package,42 which included the AM1 Hamiltonian,43 was fully optimized by an energy gradient method. The parallel tempering algorithm of the program FOX was used for 2θ range 8-60° with the bond length and angles constrained within 0.10 Å and 5.0°, respectively. After 2 × 107 trials with approximately 3 h of computer time on a Pentium IV (512 MB RAM) PC, a solution with Rwp ) 0.21 and GOF ) 4.39 was obtained. The atomic coordinates obtained from the simulated annealing procedure of FOX were taken as the starting model of Rietveld refinement with the program GSAS.44 The refinement was carried out using a 2θ angular range 8.00-88.00° with soft constraints on bond distances and angles; the planar phenyl ring was treated as a rigid body.45 The internal coordinates of the rigid body were built by the sum of vectors having an overall scalar multiplier ti (translation length), which allowed for variation of their length. The origin was set at the center of the ring, and the initial length of the translation vector was set to 1.392 Å. The rotation matrices Rx, Ry, Rz for transforming the Cartesian reference system to a rigid body reference system were determined using the GEOMETRY module of the GSAS program package. Regarding the coordination geometry around vanadium atom, no constraint was, however, applied. Two isotropic thermal parameters, 0.02 Å2 for V atom and 0.03 Å2 for the remaining non-hydrogen atoms, were refined. The background was described by the shifted Chebyshev function of the first kind with 36 points regularly distributed over the entire 2θ range. The peak profiles were fitted with pseudo-Voigt functions using the Thompson-Cox-Hasting formalism.46 These functions take into account the experimental resolution and peak broadening due to size and strain effects. Hydrogen atoms were placed in calculated positions. In the final stage of refinement, preferred orientation correction was applied using two different approaches, the generalized spherical harmonics47 and the March-Dollase48 models. The agreement parameters with the spherical harmonics description of preferred orientation (order 12) (Rp ) 0.0511, Rwp ) 0.0678) compared to those with the MarchDollase model (Rp ) 0.0632, Rwp ) 0.0825) indicate the superiority of the former approach over the latter one. Final Rietveld refinement of 145 parameters [36 coordinates, 6 translation vector lengths, 2 isotropic thermal parameters, 4 lattice parameters, 36 background points, 12 profile parameters, 48 orientation distribution function coefficients, and 1 scale factor] resulted in excellent agreement between the observed and the calculated patterns (Figure 1). The molecular view of the compound with an atom numbering scheme, drawn with DIAMOND,49 is shown in Figure 2. DNA-Binding and Cleavage Experiments. The concentration of CT-DNA was determined from its absorption intensity at 260 nm with a known molar absorption coefficient value of 6600 dm3 mol-1 cm-1.50 The binding of title vanadium complex to CT-DNA was studied by fluorescence spectral methods using the emission intensity of EB. The apparent binding constant (Kapp) for the vanadium complex was estimated from the equation: KEB[EB] ) Kapp[complex] using the KEB value as 1 × 107 M-1.51 Absorption titration experiments were carried out by monitoring the absorption spectra of the metal complex (30 µM) in Tris-HCL buffer (50 mM, pH 7.2) containing NaCl (50 mM) with varying concentrations of CT-DNA. Due correction was made for the absorbance of DNA itself. The DNA cleavage activity of the complex was studied by agarose gel electrophoresis. Supercoiled pUC19 DNA (2.5 µL, 0.5 µg) in Tris-

1718 Crystal Growth & Design, Vol. 7, No. 9, 2007

Figure 1. Final observed (crosses), calculated (red line), background (green line), and difference (blue line) profiles for [(VO2)2(C12H17N2O)2].

Figure 2. The molecular view of [(VO2)2(C12H17N2O)2] with atom numbering scheme. Symmetry code: (i) 1 - x, 2 - y, -z. HCL buffer (50 mM, pH 7.2) containing NaCl (50 mM) was treated with the complex in the presence or absence of additives. For photoinduced DNA cleavage studies, the reactions were carried out under illuminated conditions using UV source of 300 nm (12 W). After photoexposure, the sample was incubated for 1 h at 37 °C followed by the addition of the loading buffer containing 25% bromophenol blue, 0.25% xylene cyanol, 30% glycerol (3 µL), and the solution was finally loaded on 1% agarose gel containing 1.0 µg mL-1 ethidium bromide. Electrophoresis was carried out in a dark chamber for 2 h at 40 V in Tris-acetate-EDTA (TAE) buffer. Bands were visualized by UV light and photographed. The extent of cleavage of SC DNA and formation of nicked circular (NC) DNA were estimated from the intensities of the bands using the UVITEC Gel Documentation System. Corrections for the low level of NC form present in original (SC) DNA sample and the low affinity of EB binding to SC compared to NC form of DNA were applied.52 In the control experiments, different reagents, such as sodium azide (100 µM), dimethyl sulfoxide (DMSO) (4 µM), and L-histidine (100 µM), were added to (SC) DNA prior to addition of the complex.

Results and Discussion UV-vis and IR Spectra. The UV-vis spectrum of the complex displays two absorption bands ca. 232 and 277 nm, assignable to intra ligands π f π* and n f π* transitions. The red color of the compound arises from absorption tailing in from the UV region (λmax ) 518 nm), originating in ligand to metal charge transfer from the phenolate oxygen to an empty d orbital of the vanadium ion. In the IR spectrum, strong bands in the range 1270-1480 cm-1 can be attributed to the organic ligands. The intense absorption bands at 927 and 941 cm-1 are due to ν(CdO) of the cis- VO2+ structural unit, while the features at 550-760 cm-1 are ascribed to ν(V-O-V).53 Furthermore, the bands at 467 and 403 cm-1 can be assigned to the stretching modes of the metal to ligand bonds, ν(V-O) and ν(V-N),

Mondal et al.

respectively, as they lie in similar ranges for other vanadium(V) compounds with O, N donor ligands.54 These observations indicated oxo-bridged octahedral vanadium(V) centers, which was in agreement with X-ray structure analysis. Description of Structure. The crystallographic asymmetric unit of the centrosymmetric bis(µ-oxo)-bridged vanadium(V) dimer (Figure 2) consists of one vanadium atom (V1), two oxo groups (O2, O3), and one tridentate Schiff base ligand binding to the metal center via imine (N1), amine (N2) nitrogen, and the phenolate oxygen (O1) atoms. The vanadium(V) ions in the dimer exhibit highly distorted octahedral geometry with N2, O1, O2, and O2i (symmetry code: 1 - x, 2 - y, -z) atoms defining the equatorial plane, and N1, O3 atoms at the apical positions; the metal atom is displaced toward the axial oxoatom (O3) by 0.30(1) Å from the equatorial plane. Depending on the orientation of VdO groups with respect to the plane through two vanadium centers and two bridging oxo-groups, the {V2O4}2+ core can have five different configurations (synand anti-orthogonal, syn- and anti-coplanar, twist) in complexes constructed from two cis-VO2+ (pervanadyl) units.55,56 The title complex may be classified as anti-orthogonal with the two V-(µ-O) distances in the {V2O4} unit markedly asymmetric [V1-O2 1.68(2) Å, V1-O2i (1 - x, 2 - y, -z) 2.31(2) Å]. The coordination polyhedra can be best described as two edgeshared vanadium octahedra that are significantly distorted. This distortion arises mainly from the large variation of V-O bond distances and an O1-V1-O2i (1 - x, 2 - y, -z) angle of 157.0(12)°. The terminal V1-O3 (oxo) [1.61(2) Å], V1-O2(µ-bridging) [1.68(2) Å], and V1-O1(phenoxy) [1.90(2) Å] bond distances agree well with the corresponding values reported for related systems.11,44,57 To rationalize the changes in the bonding structure of VO2L′, [L′ ) N,N-dimethylenediamine (salicylaldehyde)],58 upon dimerization, the structural data of the present complex are compared with that of VO2L′, where the two Vd O(oxo) distances are 1.610(2) and 1.622(2) Å, respectively. The asymmetry between V1-O2 and V1-O3 bond lengths in the title compound, [(VO2)2(C12H17N2O)2], is due to the formation of weak intermolecular V‚‚‚O bond bridging the two halves of the binuclear complex. Similar bond asymmetry in V-O distances on dimerization of monomeric dioxovanadium(V) complex has been reported in the literature.18 The trans influence of the oxo ligand in one apical position of the vanadium octahedron manifests itself by elongation of the bond distance for the apically bonded imine nitrogen atom [V1-N1 2.18(2) Å] when compared to the distance for the amine nitrogen atom [V1-N2 2.11(2) Å] in the equatorial position. The intramolecular V‚‚‚V separation in the complex is 3.030(14) Å and falls within the range of known V‚‚‚V distances in double-bridged vanadium polynuclear systems.59,60 The assignment of the oxidation state for the vanadium atom in the compound is confirmed by the bond valance sums calculation,61 which gives 5.08 valance units for vanadium. The crystal packing of the complex exhibits intermolecular C-H‚‚‚O and C-H···π(arene) hydrogen bonds. Adjacent dimers are linked via pairs of C-H···π(arene) interactions with a C9‚‚‚Cg′ (C1-C6, symmetry: x - 1, y, z) distance of 3.65 Å and a C9-H9A‚‚‚Cg′ angle of 139° to form an infinite 1D chain (Figure 3) running parallel to the [100] direction. The successive dimeric units along the [100] direction are separated by 7.7 Å. The C8-H8A‚‚‚O3 hydrogen bonds [C8‚‚‚O3 (x, 3/2 - y, 1/2 + z) 3.60 Å, C8-H8A‚‚‚O3 160°] connect the dimeric molecules related by the c-glide and translation into a 2D grid in the (011) plane (Figure 4). The molecular assembly can also

Oxovanadium(V)-Schiff Base Complex

Crystal Growth & Design, Vol. 7, No. 9, 2007 1719

Figure 3. Formation of 1D polymeric chain along the [100] direction.

Figure 6. The effect of addition of complex 1; (9) on the emission intensity of 50 µM CT DNA-bound ethidium bromide in Tris-HCl/ NaCl buffer (50 mM, pH 7.2) at 25 °C; (0) on the emission intensity of the ethidium bromide in absence of CT DNA but at different concentrations of 1.

Figure 4. 2D grid in the (011) plane, viewed along the a-axis.

Figure 7. Emission spectral traces of complex 1 (30 µM) showing enhancement of intensity at 435 nm (excitation wavelength: 345 nm) with time in the presence of CT DNA (437 µM) in water.

Figure 5. TG curve for [(VO2)2(C12H17N2O)2].

be visualized in terms of a simpler hexanuclear R44(24) ring having dimension of 9.0 × 13.9 Å. The combination of the 2D grid in the (011) plane and a 1D chain along the [100] direction results into a novel 3D supramolecular architecture. Interestingly, only a few dinuclear vanadium complexes with a multidimensional framework have been structurally characterized using single-crystal X-ray analysis, where a 2D or 3D molecular assembly is established via hydrogen bonds involving either the solvent atoms or the cation.18,57 Thermal Study. The thermogravimetric (TG) curve of the complex (Figure 5) shows that the structure is stable up to 180 °C. Further increase of the temperature causes weight loss between 181 and 800 °C in three overlapping steps, corresponding to the release of the organic ligands minus one oxygen. The

end product of the thermal decomposition is V2O5 (observed mass loss 67.4%, calculated mass loss 68.2%). DNA Binding and Photoinduced Cleavage. The propensity of binding of the complex to CT-DNA has been analyzed by the fluorescence spectral technique using emission intensity of EB. Although EB does not show any emission in the buffer medium due to fluorescence quenching by the solvent molecules, it shows emission in the presence of CT-DNA due to intercalative binding to DNA.62 Addition of the vanadium complex having a planar π conjugated system results in competitive binding to DNA causing reduction of the emission intensity due to displacement of EB from the DNA bound state to the free state and a change of vanadium complex from the free state to the DNA bound state. The reduction of emission intensity of EB at different complex concentrations (0-90 µM) is shown in Figure 6. The apparent binding constant (Kapp) has been estimated as 1.56 × 107 M-1 where KEB is 1.0 × 107 M-1 and the concentration of EB and CT-DNA is taken as 50 µM. The emission spectral behavior of vanadium complex in the presence of CT-DNA has been studied. The complex is emissive at 435 nm (345 nm excitation) in water (blue line in Figure 7). The emission intensity increase with time reaching a constant value after ca. 30 min (Figure 7) indicates significant intercalative binding of the complex to CT-DNA. An intercalative binding

1720 Crystal Growth & Design, Vol. 7, No. 9, 2007

Figure 8. Absorption spectral changes on addition of CT DNA to complex 1 in Tris-HCl/NaCl buffer (50 mM, pH 7.2) (shown by arrow). The inset shows a plot of the absorbance at 271 nm vs the amount of DNA added.

Mondal et al.

vis and FTIR spectroscopy. The crystal structure of the dimeric complex has been determined from X-ray powder data and reveals two edge-sharing vanadium octahedra with N2O4 chromophores. The C-H···π(arene) interactions link the molecules into infinite polymeric chains along the [100] direction. The molecular assembly generated by C-H‚‚‚O hydrogen bonds exhibits hexanuclear R44(24) rings in the (011) plane, which combine with the chains along the [100] direction to form a 3D supramolecular architecture. The vanadium(V) complex binds to DNA and shows photoinduced DNA cleavage activity on irradiation with UV light (300 nm) via a pathway involving formation of singlet oxygen. It is proposed that the dinuclear vanadium complex disintegrates into two mononuclear species with an essentially planar salicylidene amino group so as to fit between the two consecutive base pairs of DNA. To the best of our knowledge, the present work is the first report in which the molecular structure of a multidimensional oxovanadium(V) complex with DNA binding and cleavage activity has been established from laboratory X-ray powder diffraction data. Acknowledgment. Financial assistance from the University Grants Commission, New Delhi, and the Department of Science and Technology, Government of India, through DRS (SAP-I) and FIST programs to Department of Physics, Jadavpur University, for purchasing the X-ray powder diffractometer is gratefully acknowledged. S.G. thanks Government of West Bengal, India, for a research fellowship. K.D. thanks CSIR New Delhi, for a research fellowship.

Figure 9. Photocleavage of SC pUC19 DNA (0.5 µg) by 1. Lane 1, DNA control (λ ) 300 nm); lane 2, DNA + 1 (in dark); lane 3, DNA + 1 (10 min, under argon); lane 4, DNA + 1 (5 min); lane 5, DNA + 1 (10 min); lane 6, DNA + 1 + NaN3 (100 µM) (10 min); lane 7, DNA + 1 + DMSO (4 µL) (10 min); lane 8, DNA + 1 + L-histidine (100 µM) (10 min) [lanes 2-8: 1 (35 µM, 300 nm)].

of a complex to DNA generally results in hypochromism along with a blue shift (hypochromic shift) of the electronic spectral band. The extent of the hypochromism gives a measure of the strength of the intercalative binding/interaction.63,64 The observed hypochromic shift ∼ 8 nm in the absorption spectra of the complex with increasing concentration of CT-DNA (Figure 8) is consistent with intercalative stacking. The electrospray mass spectra of the complex in acetonitrile solution shows a peak at 289 that can be assigned to the mononuclear (VO2L) species. The photoinduced DNA (SC pUC19) cleavage experiments (Figure 9) in UV (300 nm, 12 W) using 35 µM concentration of the complex show ∼ 96% of DNA (500 ng) cleavage for an exposure of 10 min; the extent of cleavage at 300 nm is ∼62% on 5 min photoexposure. The mechanistic aspects of the reaction have been probed using external reagents. Control experiments showed that addition of singlet oxygen quencher sodium azide65 or L-histidine66 inhibits cleavage activity, whereas the hydroxyl radical scavenger DMSO has no inhibitory effect on 300 nm photoexposure. The complex is cleavage inactive under argon atmosphere (lane 7 in Figure 9). The results are indicative of the formation of singlet oxygen (1O2) as the reactive species in the process, 3O

2

SC DNA

[VO2(L)](I) + hν 98 I* f 1O2 98 cleaved DNA67 Conclusion A novel binuclear bis(µ-oxo)-bridged vanadium(V)-Schiff base compound has been synthesized and characterized by UV-

Supporting Information Available: Tables containing relevant crystal data, atomic coordinates from final Rietveld refinement, selected bond distances and angles, and molecular packing diagram of [(VO2)2(C12H17N2O)2], FTIR spectrum, and electrospray mass spectrum of the complex. This material is available free of charge via the Internet at http://pubs.acs.org.

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CG060753I