Photo-induced DNA Cleavage Research Articles

Photo-induced double-strand DNA and site-specific protein cleavage activity of l-histidine (μ-oxo)diiron(iii) complexes of heterocyclic bases

Three oxo-bridged diiron(III) complexes of L-histidine and heterocyclic bases [Fe(2)(micro-O)(L-his)(2)(B)(2)](ClO(4))(2) (1-3), where B is 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq), were prepared and characterized. The bpy complex was structurally characterized by X- ray crystallography. The molecular structure showed a {Fe(2)(micro-O)} core in which iron(III) in a FeN(4)O(2) coordination is bound to tridentate monoanionic L-histidine and bidentate bpy ligands. The FeFe distance is approximately 3.5 A. The Fe-O-Fe unit is essentially linear, giving a bond angle of approximately 172 degrees . The complexes showed irreversible cyclic voltammetric cathodic response near -0.1 V vs. SCE in H(2)O-0.1 M KCl. The binuclear units displayed antiferromagnetic interaction between two high-spin (S = 5/2) iron(III) centers giving a -J value of approximately 110 cm(-1). The complexes showed good DNA binding propensity giving a binding constant value of approximately 10(5) M(-1). Isothermal titration calorimetric data indicated single binding mode to the DNA. The binding was found to be driven by negative free energy change and enthalpy. The dpq complex showed oxidative double-strand DNA cleavage on exposure to UV-A and visible light. The phen complex displayed single-strand photocleavage of DNA. The DNA double-strand breaks were rationalized from theoretical molecular docking calculations. Mechanistic investigations showed formation of hydroxyl radicals as the reactive species through photodecarboxylation of the L-histidine ligand. The complexes exhibited good binding propensity to bovine serum albumin (BSA) protein in Tris-HCl/NaCl buffer medium. The dpq complex showed UV-A light-induced site-specific oxidative BSA cleavage forming fragments of approximately 45 kDa and approximately 20 kDa molecular weights via OH pathway.

Anaerobic Photocleavage of DNA in Red Light by Dicopper(II) Complexes of 3,3′-Dithiodipropionic Acid

Binuclear copper(II) complexes [{(phen)Cu(II)}(2)(mu-dtdp)(2)] (1), [{(dpq)Cu(II)}(2)(mu-dtdp)(2)] (2), [{(phen)Cu(II)}(2)(mu-az)(2)] (3), and [{(dpq)Cu(II)}(2)(mu-az)(2)] (4) and a zinc(II) complex [{(phen)Zn(II)}(2)(mu-dtdp)(2)] (5), having 3,3'-dithiodipropionic acid (H(2)dtdp), azelaic acid (nonanedioic acid), 1,10-phenanthroline (phen), and dipyrido[3,2-d:2',3'-f]quinoxaline (dpq), were prepared and characterized by physicochemical methods. Complex 1 has been structurally characterized by X-ray crystallography. The complexes have each metal center bound to a chelating phenanthroline base and two bridging carboxylate ligands giving a square-planar MN(2)O(2) coordination geometry. The molecular structure of complex 1 shows two sterically constrained disulfide moieties of the dtdp ligands. The complexes show good binding propensity to calf thymus DNA in the major groove. The photoinduced DNA cleavage activity of the complexes has been studied using 365 nm UV light and 647.1 nm and >750 nm red light under both aerobic and anaerobic conditions. The phen complex 1, having dtdp ligand, cleaves supercoiled (SC) DNA to its nicked circular (NC) form. The dpq analogue 2 shows formation of a significant quantity of linear DNA resulting from double-strand breaks (dsb) in air. Mechanistic studies reveal the involvement of HO(*) and (1)O(2) as the reactive species under an aerobic medium. The dsb of DNA is rationalized from the docking studies on 2, showing a close proximity of two photosensitizers, namely, the disulfide moiety of dtdp and the quinoxaline ring of dpq to the complementary strands of DNA. The copper(II) complexes of the dtdp ligand cleave SC DNA to its NC form upon exposure to UV or red light under an argon atmosphere. An enhancement of the DNA cleavage activity under argon has been observed upon increasing the concentration of the DMF solvent in the DMF-Tris buffer medium. Theoretical studies suggest the possibility of sulfide anion radical formation from a copper(II)-bound dtdp ligand in >750 nm red light, which further cleaves the DNA. The copper(II) azelate complexes are inactive under similar reaction conditions. The azelate complex of the dpq ligand cleaves DNA in air following the (1)O(2) pathway. The zinc(II) complex of the dtdp ligand (5) does not show any photoinduced DNA cleavage activity in red light.

Synthesis, crystal structures, DNA binding and cleavage activity of l-glutamine copper(II) complexes of heterocyclic bases

Ternary l-glutamine ( l-gln) copper(II) complexes [Cu( l-gln)(B)(H 2O)](X) (B = 2,2′-bipyridine (bpy), X = 0.5 SO 4 2 - , 1; B = 1,10-phenanthroline (phen), X = ClO 4 - , 2) and [Cu( l-gln)(dpq)(ClO 4)] ( 3) (dpq, dipyridoquinoxaline) are prepared and characterized by physicochemical methods. The DNA binding and cleavage activity of the complexes have been studied. Complexes 1– 3 are structurally characterized by X-ray crystallography. The complexes show distorted square pyramidal (4+1) CuN 3O 2 coordination geometry in which the N,O-donor amino acid and the N,N-donor heterocyclic base bind at the basal plane with a H 2O or perchlorate as the axial ligand. The crystal structures of the complexes exhibit chemically significant hydrogen bonding interactions besides showing coordination polymer formation. The complexes display a d–d electronic band in the range of 610–630 nm in aqueous-dimethylformamide (DMF) solution (9:1 v/v). The quasireversible cyclic voltammetric response observed near −0.1 V versus SCE in DMF–TBAP is assignable to the Cu(II)/Cu(I) couple. The binding affinity of the complexes to calf thymus (CT) DNA follows the order: 3 (dpq) > 2 (phen) ≫ 1 (bpy). Complexes 2 and 3 show DNA cleavage activity in dark in the presence of 3-mercaptopropionic acid (MPA) as a reducing agent via a mechanistic pathway forming hydroxyl radical as the reactive species. The dpq complex 3 shows efficient photo-induced DNA cleavage activity on irradiation with a monochromatic UV light of 365 nm in absence of any external reagent. The cleavage efficiency of the DNA minor groove binding complexes follows the order: 3 > 2 ≫ 1. The dpq complex exhibits photocleavage of DNA on irradiation with visible light of 647.1 nm. Mechanistic data on the photo-induced DNA cleavage reactions reveal the involvement of singlet oxygen ( 1O 2) as the reactive species in a type-II pathway.

Photosensitizer in a Molecular Bowl and its Effect on the DNA-Binding and -Cleavage Activity of 3d-Metal Scorpionates

Ternary 3d -metal complexes [M(Tp (Ph))(B)](ClO 4) ( 1- 8), where M is Co(II), Ni(II), Cu(II) and Zn(II), Tp (Ph) is anionic tris(3-phenylpyrazolyl)borate, and B is N,N-donor heterocyclic base, namely, 1,10-phenanthroline (phen, 1- 4) and dipyrido[3,2- d:2',3'- f]quinoxaline (dpq, 5- 8), were prepared from a reaction of the perchlorate salt of the metal with KTp (Ph) and B in CH 2Cl 2. The complexes were characterized by various physicochemical methods. 4- 6 and 8 were structurally characterized by single-crystal X-ray crystallography. The crystal structures of the complexes show the presence of discrete cationic complexes having a square-pyramidal (4 + 1) coordination geometry in which two nitrogen atoms of the phenanthroline base (B) and two nitrogen atoms of the Tp (Ph) ligand occupy the basal plane and one nitrogen of the Tp (Ph) ligand binds at the axial site. The phenyl groups of the Tp (Ph) form a bowl-shaped structure that essentially encloses the {M(phen/dpq)} moiety. DNA-binding studies were carried out using various spectral techniques and from viscosity measurements. The complexes show moderate binding propensity to calf thymus DNA at the minor groove, giving binding constant values ( K b) of approximately 10 (4) M (-1). The complexes exhibit poor DNA-cleavage activity in the dark in the presence of 3-mercaptopropionic acid (MPA) or hydrogen peroxide (H 2O 2). The photoinduced DNA-cleavage activity of the complexes was investigated using UV-A radiation of 365 nm and visible light of two different wavelengths with a tunable multicolor Ar-Kr mixed gas ion laser source. The dpq complexes show efficient photoinduced DNA-cleavage activity via a metal-assisted photoexcitation process involving the formation of singlet oxygen as the cleavage active species in a type-II pathway. The paramagnetic d (7)-Co(II)-dpq and d (9)-Cu(II)-dpq complexes exhibit efficient DNA-cleavage activity in visible light. The paramagnetic d (8)-Ni(II)-dpq complex displays only minor DNA-cleavage activity in visible light. Diamagnetic d (10)-Zn(II)-dpq complex shows only UV-A light-induced DNA cleavage but no apparent DNA-cleavage activity in visible light. Steric protection of the photoactive quinoxaline moiety of the dpq ligand inside the hydrophobic {M(Tp (Ph))} molecular bowl has a positive effect on the photoinduced DNA-cleavage activity.

Mixed ligand Ni(II) complexes: DNA binding, oxidative and photo-cleavage studies

Mixed ligand complexes of the type [Ni(phen)2(bnp)](PF6)2 · 2H2O (1) and [Ni(bpy)2(bnp)](PF6)2 · 2H2O (2) (where phen, 1,10,Phenanathroline; bpy, 2,2 bipyridine; bnp, benzo[1.8]naphthyridinone-2-ol) have been synthesized and characterized by employing analytical and spectral methods. The DNA binding properties of the complexes with calf thymus-DNA were studied by using absorption spectra, viscosity measurements as well as thermal denaturation experiments. The absorption spectra indicated that the Ni(II) complexes intercalate tightly between the base pairs of the DNA with intrinsic DNA binding constants of 1.9 × 104 M−1 for (1) and 2.5 × 104 M−1 for (2) in 5 mM Tris-HCl/50 mM NaCl buffer at pH 7.2, respectively. The large enhancement in the relative viscosity of DNA on binding to the bnp supports the proposed DNA binding modes. The oxidative cleavage and photo-cleavage activity of complexes (1) and (2) were studied. The photo-induced DNA cleavage activities of the complexes in the presence and absence of d...

Photo-induced DNA cleavage by (heterocyclo)carbonyl oxime esters of anthraquinone

Various (heterocyclo)carbonyl mono-oxime esters of anthraquinone were synthesized, which exhibited an ability for DNA cleavage upon UV irradiation. Their structure–activity relationship was established, in which the most potent compound was anthraquinone O-9-(1,3-benzothiazole-2-carbonyl)oxime ( 4j). It can produce radical species and nick DNA at the concentration as low as 1.0 μM.

Intermolecular vs. intramolecular photoinduced electron transfer from nucleotides in DNA to acridinium ion derivatives in relation with DNA cleavage

Photoirradiation of various 10-methylacridinium ions (AcrR +, R = H, i Pr, and Ph) intercalated in DNA results in ultrafast intramolecular electron transfer, followed by rapid back electron transfer between AcrR + and nucleotides in DNA. The electron-transfer dynamics in DNA were monitored by femtosecond time-resolved transient absorption spectroscopy. Both acridinyl radical and nucleotide radical cations, formed in the photoinduced electron transfer in DNA, were successfully detected in an aqueous solution. These transient absorption spectra were assigned by the comparison with those of DNA nucleotide radical cations, which were obtained by the intermolecular electron-transfer oxidation of nucleotides with the electron-transfer state of 9-mesityl-10-methylacridinium ion (Acr –Mes +) produced upon photoexcitation of Acr +–Mes. Photoinduced cleavage of DNA with various acridinium ions (AcrR +, R = H, i Pr, Ph, and Mes) has also been examined by agarose gel electrophoresis, which indicates that the rapid intramolecular back electron transfer between acridinyl radical and nucleotide radical cation in DNA suppresses the DNA cleavage as compared with the intermolecular electron-transfer oxidation of nucleotides with Acr –Mes +.

New Insights into the Visible-Light-Induced DNA Cleavage Activity of Dipyridoquinoxaline Complexes of Bivalent 3d-Metal Ions

Dipyridoquinoxaline (dpq) complexes of bivalent 3d-metal ions, viz., [FeII(dpq)3](PF6)2 (1), [CoII(dpq)3](ClO4)2 (2), [NiII(dpq)3](ClO4)2 (3), [CuII(dpq)2(H2O)](ClO4)2 (4), [ZnII(dpq)3](ClO4)2 (5), and [ZnII(dpq)2(DMF)2](ClO4)2 (5a) (DMF = N,N-dimethylformamide), are prepared and their photoinduced DNA cleavage activity studied. Structural characterization for the complexes 1 and 5a is done by single-crystal X-ray crystallography. All the complexes show efficient binding propensity to calf thymus DNA with a binding constant (K) value of approximately 10(5) M(-1). Complexes 1, 2, and 4 show metal-based cyclic voltammetric responses at 1.2, 0.4, and 0.09 V (vs SCE) in DMF 0.1 M [Bun4N](ClO4) assignable to the respective FeIII/FeII, CoIII/CoII, and CuII/CuI couples. The NiII and ZnII complexes do not show any metal-based redox process. The dpq-based reductions are observed in the potential range of -1.0 to -1.7 V vs SCE. DNA melting and viscosity data indicate the groove-binding nature of the complexes. Control experiments using distamycin-A suggest a minor groove-binding propensity of the complexes. The complexes exhibit photoinduced cleavage of supercoiled pUC19 DNA in UV light of 365 nm. The diamagnetic d6-FeII and d10-ZnII complexes are cleavage-inactive on irradiation with visible light. The paramagnetic d7-CoII and d9-CuII complexes exhibit efficient DNA cleavage activity on photoirradiation at their respective d-d band. The paramagnetic d8-NiII complex displays only minor DNA cleavage activity on irradiation at its d-d band. The DNA cleavage reactions at visible light under aerobic conditions involve the formation of hydroxyl radical. The CoII complex shows photocleavage of DNA under an argon atmosphere. Theoretical calculations on the complexes suggest a photoredox pathway in preference to a type-2 process forming singlet oxygen for the visible-light-induced DNA cleavage activity of the 3d-metal complexes. The theoretical data also predict that the photoredox pathway is favorable for the 3d7-CoII and 3d9-CuII complexes to exhibit DNA cleavage activity, while the analogous 3d6-FeII and 3d8-NiII complexes are energetically unfavorable for the exhibition of such activity under visible light. The CoII and CuII complexes are better suited for designing and developing new metal-based PDT agents than their cleavage-inactive FeII, NiII, and ZnII analogues.

DNA Cleavage by New Oxovanadium(IV) Complexes of N-Salicylidene α-Amino Acids and Phenanthroline Bases in the Photodynamic Therapy Window

Oxovanadium(IV) complexes [VO(salmet)(B)] (1-3) and [VO(saltrp)(B)] (4-6), where salmet and saltrp are N-salicylidene-l-methionate and N-salicylidene-l-tryptophanate, respectively, and B is a N,N-donor heterocyclic base (viz. 1,10-phenanthroline (phen, 1, 4), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq, 2, 5), and dipyrido[3,2-a:2',3'-c]phenazine (dppz, 3, 6)) are prepared and characterized and their DNA binding and photoinduced DNA cleavage activity studied. Complexes 1, 2, and 4 are structurally characterized by single-crystal X-ray crystallography. The molecular structure shows the presence of a vanadyl group in the VO3N3 coordination geometry. The dianionic alpha-amino acid Schiff base acts as a tridentate O,N,O-donor ligand in a meridional binding mode. The N,N-donor heterocyclic base displays a chelating mode of bonding with a N-donor site trans to the oxo group. The complexes show a d-d band in the range of 680-710 nm in DMF with a shoulder near 840 nm. They exhibit an irreversible oxidative cyclic voltammetric response near 0.8 V assignable to the V(V)/V(IV) couple and a quasi-reversible V(IV)/V(III) redox couple near -1.1 V vs SCE in DMF-0.1 M TBAP. The complexes show good binding propensity to calf thymus DNA giving binding constant values in the range from 5.2 x 10(4) to 7.2 x 10(5) M(-1). The binding site size, thermal melting, and viscosity data suggest DNA surface and/or groove binding nature of the complexes. The complexes show poor "chemical nuclease" activity in the dark in the presence of 3-mercaptopropionic acid or hydrogen peroxide. The dpq and dppz complexes show efficient DNA cleavage activity on irradiation with UV-A light of 365 nm via a mechanistic pathway involving formation of singlet oxygen as the reactive species. They also show significant DNA cleavage activity on photoexcitation in red light (>750 nm) by (1)O2 species. Observation of red-light-induced cleavage of DNA is unprecedented in the vanadium chemistry. The DNA cleavage activity is metal promoted as the ligands or vanadyl sulfate alone are cleavage inactive on photoirradiation at these wavelengths.

Ferrocene-conjugated copper(II) dipyridophenazine complex as a multifunctional model nuclease showing DNA cleavage in red light

The copper(II) complex [Cu(L)(dppz)](ClO 4) 2 ( 1) having a tripodal ligand ferrocenylmethylbis(2-pyridylmethylamine) (L) with a pendant ferrocenyl unit and a planar NN-donor dipyrido-[3,2-a:2′,3′-c]-phenazine (dppz) base is prepared and its DNA binding and cleavage properties studied. The complex is redox active showing cyclic voltammetric responses at 0.52 and –0.01 V vs. SCE due to Fe(III)/Fe(II) and Cu(II)/Cu(I) couples, respectively. The complex that binds to the major groove of DNA shows dual chemical nuclease activity involving both the metal centres. The complex displays efficient photo-induced DNA cleavage activity in visible laser light of 458 and 568 nm wavelengths forming cleavage active hydroxyl radicals. Significant DNA cleavage is also observed in red light of 647 nm within the photodynamic therapy (PDT) window.

Supramolecular Architecture in an Oxovanadium(V)-Schiff Base Complex: Synthesis, Ab initio Structure Determination from X-ray Powder Diffraction, DNA Binding and Cleavage Activity

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) A, b = 12.020(5) A, c = 13.882(6) A, 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 (24) rings in the (011) plane, which on combination with one-dimensional polymeric chains along the [100] direction, forms a novel three-dimensi...

DNA Cleavage on Photoexposure at the d−d Band in Ternary Copper(II) Complexes Using Red-Light Laser

Ternary copper(II) complexes [Cu(L1)B](ClO4) (1, 2) and [Cu(L2)B](ClO4) (3, 4), where HL1 and HL2 are tridentate NSO- and ONO-donor Schiff bases and B is a heterocyclic base, viz. dipyrido[3,2-d:2',3'-f]quinoxaline (dpq, 1 and 3) or dipyrido[3,2-a:2',3'-c]phenazine (dppz, 2 and 4), were prepared and their DNA binding and photoinduced DNA cleavage activity studied. Complex 1, structurally characterized by single-crystal X-ray crystallography, shows an axially elongated square-pyramidal (4 + 1) coordination geometry in which the monoanionic L1 binds at the equatorial plane. The NN-donor dpq ligand exhibits an axial-equatorial binding mode. The complexes display good binding propensity to calf thymus DNA, giving a relative order 2 (NSO-dppz) > 4 (ONO-dppz) > 1 (NSO-dpq) > 3 (ONO-dpq). They cleave supercoiled pUC19 DNA to its nicked circular form when treated with 3-mercaptopropionic acid (MPA) by formation of hydroxyl radicals as the cleavage active species under dark reaction conditions. The photoinduced DNA cleavage activity of the complexes was investigated using UV radiation of 365 nm and red light of 633, 647.1, and 676.4 nm (CW He-Ne and Ar-Kr mixed gas ion laser sources) in the absence of MPA. Complexes 1 and 2, having photoactive NSO-donor Schiff base and dpq/dppz ligands, show dual photosensitizing effects involving both the photoactive ligands in the ternary structure with significantly better cleavage properties when compared to those of 3 and 4, having only photoactive dpq/dppz ligands. Involvement of singlet oxygen in the light-induced DNA cleavage reactions is proposed. A significant enhancement of the red-light-induced DNA cleavage activity is observed for the dpq and dppz complexes containing the sulfur ligand when compared to their earlier reported phen (1,10-phenanthroline) analogue. Enhancement of the cleavage activity on photoexposure at the d-d band indicates the occurrence of metal-assisted photosensitization processes involving the LMCT and d-d band in the ternary structure.

Photocleavage of DNA by copper(II) complexes

The chemistry of ternary and binary copper(II) complexes showing efficient visible light-induced DNA cleavage activity is summarized in this article. The role of the metal in photo-induced DNA cleavage reactions is explored by designing complex molecules having a variety of ligands. Ternary copper(II) complexes with amino acid like L-methionone or L-lysine and phenanthroline base are efficient photocleavers of DNA. Complexes of formulation [Cu(Ln)(phen)](C1O4) with NSO-donor Schiff base (HLn) and NN-donor heterocyclic base 1,10-phenanthroline (phen) show significant cleavage of super-coiled (SC) DNA on exposure to red light at ≈ 700 nm. Thed-d and CT electronic bands of the copper(II) complexes play important roles in DNA cleavage reactions. The mechanistic pathways are found to be dependent on the types of ligands present in the copper(II) complexes and the photo-excitation energy. While UV exposure generally proceeds via a type-II process forming singlet oxygen as the reactive species, red-light exposure leads to DNA cleavage following different mechanistic pathways, viz. type-I, type-II and photo-redox pathways. Ternary copper(II) complexes with phen as DNA binder and Schiff base with a thiomethyl group as photosensitizer, cleave SC DNA to its nicked circular (NC) form in a type-II process in red-light. The binary complex [Cu(dpq)2(H20)](C104)2 (dpq, dipyridoquinoxaline) cleaves DNA by photo-redox pathway at 694 nm. The binuclear complex [Cu II 2 (RSSR)2], where H2RSSR is a Schiff base derived from 2-(thioethyl)salicylaldimine, cleaves SC DNA at 632.8 nm (CW He-Ne laser) and 694 nm (ruby laser) involving sulphide (type-I process) and hydroxyl radicals (photo-redox pathway) as the reactive species.

A novel porphyrin–polypyridyl ruthenium(II) hybrid. Synthesis, characterization and photoinduced DNA cleavage activity

A novel porphyrin–polypyridyl ruthenium(II) hybrid compound, [C4-TPP-(ip)Ru(phen)2](ClO4)2 (TPP = 5,10, 15,20-tetraphenylporphyrin, ip = imidazo[4,5-f][1,10]phenanthroline and phen = 1,10-Phenanthroline), in which a polypyridyl ruthenium(II) moiety is linked to a porphyrin moiety by a butyl chain, has been synthesized by the reaction of (C4ip)TPP with cis-Ru(phen)2Cl2 in glacial acetic acid under an Ar atmosphere and characterized by elemental analysis, i.r., 1H-n.m.r., u.v.–vis., ESI-MS and electrochemistry. Its photoinduced DNA cleavage activity was investigated preliminarily. It has been found that the hybrid compound promoted photocleavage of plasmid pBR 322 DNA upon irradiation. Singlet oxygen (1O2) is suggested to be the reactive species responsible for the cleavage.

Synthesis, crystal structure, DNA binding and photo-induced DNA cleavage activity of ( S-methyl- l-cysteine)copper(II) complexes of heterocyclic bases

Ternary S-methyl- l-cysteine (SMe- l-cys) copper(II) complexes [Cu(SMe- l-cys)(B)(H 2O)](X) ( 1– 4), where the heterocyclic base B is 2,2′-bipyridine (bpy, 1), 1,10-phenanthroline (phen, 2), dipyridoquinoxaline (dpq, 3) and dipyridophenazine (dppz, 4), and X is ClO 4 - ( 1– 3) or NO 3 - ( 4), are prepared and their DNA binding and cleavage properties studied. Complexes 2 and 4 are structurally characterized by X-ray crystallography. Both the crystal structures show distorted square-pyramidal (4 + 1) CuN 3O 2 coordination geometry of the complexes in which the N,O-donor S-methyl- l-cysteine and N,N-donor heterocyclic base bind at the basal plane with a water molecule as the axial ligand. In addition, the dppz structure shows the presence of a 1D-chain formed due to covalent linkage of the carboxylate oxygen atom belonging to another molecule at the elongated axial site. The crystal structures show chemically significant non-covalent interactions like hydrogen bonding involving the axial aqua ligand and π–π interactions between dppz ligands. The complexes display a d–d band in the range of 605–654 nm in aqueous dimethylformamide (DMF) solution (9:1 v/v). The redox active complexes show quasireversible cyclic voltammetric response near 0.1 V in DMF assignable to the Cu(II)/Cu(I) couple. The complexes show good binding affinity to calf thymus (CT) DNA giving the order: 4 (dppz) > 3 (dpq) > 2 (phen) ≫ 1 (bpy). The intrinsic binding constants, obtained from UV–visible spectroscopic studies, are 1.3 × 10 4 and 2.15 × 10 4 M −1 for 3 and 4, respectively. Control DNA cleavage experiments using pUC19 supercoiled (SC) DNA and minor groove binder distamycin suggest major groove binding propensity for the dppz complex, while the phen and dpq complexes bind at the minor groove of DNA. Complexes 2– 4 show DNA cleavage activity in dark in the presence of a reducing agent 3-mercaptopropionic acid (MPA) via a mechanistic pathway involving formation of hydroxyl radical as the reactive species. The complexes also show efficient photo-induced DNA cleavage activity on irradiation with a monochromatic UV light of 365 nm in absence of any external reagent. The cleavage efficiency follows the order: 3 > 4 > 2. The complexes exhibit significant DNA cleavage activity on irradiation with visible light of 633 nm. Control experiments show inhibition of cleavage in presence of singlet oxygen quenchers like sodium azide, histidine and enhancement of cleavage in D 2O, suggesting formation of singlet oxygen as a reactive species in a type-II process. The photosensitizing effect of the thiomethyl group of the amino acid is evidenced from the observation of significant DNA photocleavage activity of the phen complex 2 as the phen ligand itself is not a photosensitizer.

A Theoretical Elucidation on the Solvent‐dependent Photosensitive Behaviors of C60

In this paper, the solvent-dependent photosensitive behaviors of fullerene (C(60)) were investigated in polar and nonpolar solvents by time-dependent density functional theory (TD-DFT) calculation. Based on the calculated physicochemical parameters on triplet state, it is revealed that excited-state C(60) only generates (1)O(2) via energy transfer in benzene, but can give birth to O(2)(.-) and (1)O(2) in water via energy transfer and electron transfer, respectively. Considering the fact that electron transfer is more favorable compared with energy transfer in polar biological systems, especially with the presence of electron donors, the O(2)(.-)-generating process will get predominant in physiological systems. These results account well for the experimental observations that O(2)(.-) and (.)OH are primarily responsible for the photoinduced DNA cleavage by C(60) under physiological conditions, whereas (1)O(2) plays a critical role in nonpolar solvents.

Steric Protection of a Photosensitizer in a N,N-Bis[2-(2-pyridyl)ethyl]-2-phenylethylamine-copper(II) Bowl that Enhances Red Light-Induced DNA Cleavage Activity

Ternary copper(II) complexes [Cu(py2phe)B](ClO4)2 (1-3), where py2phe is a tripodal ligand N,N-bis[2-(2-pyridyl)ethyl]-2-phenylethylamine and B is a heterocyclic base (viz., 1,10-phenanthroline (phen, 1), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq, 2), or dipyrido[3,2-a:2',3'-c]phenazine (dppz, 3)), are prepared and their DNA-binding and photoinduced DNA-cleavage activities are studied. Complex 1 has been structurally characterized by single crystal X-ray crystallography. The molecular structure shows an axially elongated square-pyramidal (4 + 1) coordination geometry in which the phen ligand binds at the basal plane. The tripodal ligand py2phe displays an axial-equatorial binding mode with the amine nitrogen bonded at the axial site. A chemically significant CH-pi interaction involving the CH moiety of the phenyl group of the tripodal ligand and the aromatic ring of phen is observed. The complexes display good binding propensity to calf thymus DNA giving a relative order of 3 (dppz) > 2 (dpq) > 1 (phen). The DNA binding constants (K(b)) for 1-3, determined from absorption spectral studies, are 6.2 x 10(3), 1.0 x 10(4), and 5.7 x 10(4) M(-1), respectively. The complexes show chemical nuclease activity in the presence of 3-mercaptopropionic acid as a reducing agent forming hydroxyl radicals as the cleavage active species. The photoinduced DNA-cleavage activity of the complexes has been studied using UV radiation of 365 nm and red light of 632.8 and 694 nm. The phen complex in absence of any photosensitizing moiety does not show any DNA cleavage upon photoirradiation. The dpq and dppz ligands with their photoactive quinoxaline and phenazine moieties display significant photoinduced DNA-cleavage activity. The dppz complex is more active than its dpq analogue because of the better steric protection of the DNA-bound photosensitizing dppz ligand from the solvent molecules. Control experiments reveal the formation of singlet oxygen in the light-induced DNA-cleavage reactions. The observed efficient photoinduced DNA-cleavage activity of 2 and 3 is akin to the "light switch" effect known for the tris-chelates of ruthenium(II).

Effect of copper-sulphur bond on the DNA photo-cleavage activity of 2-(methylthio)ethylpyridine-2-carbaldimine copper(II) complexes

The binding and photo-induced DNA cleavage activity of a binary complex [CuL2](ClO4)2 (1) and the in situ generated ternary complexes [CuLB](ClO4)2 from 1 (B: 1,10-phenanthroline, phen, 2; dipyrido[3,2-d: 2′,3′-f]quinoxaline, dpq, 3) are studied, where L is a N2S-donor tridentate Schiff base 2- (methylthio)ethylpyridine-2-carbaldimine. Complex 1, structurally characterized by X-ray diffraction study, has six-coordinate meridional geometry showing CuN4S2 coordination. The Cu-N bond lengths are in the range of 1·968(3) to 2·158(4) Å. The Cu-S bond lengths of 2·599(2) and 2·705(2) Å are significantly long indicating weak covalent interaction between copper and sulphur atoms. The thiomethyl groups are cis to each other giving S-Cu-S angle of 75·82(5)°. The Cu-N(pyridyl) bond distances are longer than the Cu-N(imine) bonds. The complexes are redox active and display a quasi-reversible cyclic voltammetric response assignable to the Cu(II)/Cu(I) couple near 0·0 V vs SCE in DMF-Tris buffer (1: 4 v/v) using 0·1 M KCl as supporting electrolyte. Electronic spectra of the complexes show a d-d band in the range 630 to 700 nm in DMF along with higher energy charge transfer bands. While complex 1 is a poor binder to DNA, the ternary complexes show good DNA binding propensity. The photo-nuclease activity of 1–3 is studied using UV and visible wavelengths. The DNA cleavage activity at 365 nm follows the order: 3 > 2 > 1. The cleavage reaction involves the formation of singlet oxygen as the reactive species in a type-II process.

Synthesis, crystal structure and photo-induced DNA cleavage activity of ternary copper(II) complexes of NSO-donor Schiff bases and NN-donor heterocyclic ligands

New ternary copper(II) complexes [CuL n B](ClO 4) ( 1– 3), where HL n is the NSO donor Schiff base derived from the condensation of 2-mercaptoethylamine hydrochloride with salicylaldehyde (HL 1) or 2-hydroxy-3-methoxybenzaldehyde (HL 2) and B is NN-donor heterocyclic base like 2,2′-bipyridine (bpy, 1), 1,10-phenanthroline (phen, 2) or 2,9-dimethyl-1,10-phenanthroline (dmp, 3), are prepared, structurally characterized by X-ray crystallography and their DNA cleavage activity studied. The complexes show distorted square-pyramidal (4 + 1) CuN 3OS coordination geometry in which the NSO-donor Schiff base is bonded at the basal plane and the NN-donor heterocyclic base displays axial-equatorial mode of bonding [Cu–S distance: ∼2.4 Å]. The one-electron paramagnetic ( μ eff = ∼1.9 μ B) complexes display axial EPR spectra in DMF glass at 77 K giving g ∥ = ∼2.2 (A ∥ = 162 G) and g ⊥ = ∼2.0, indicating { d x 2 - y 2 } 1 ground state. The complexes exhibit visible spectral d–d band in MeCN near 650 nm and two charge transfer bands near 400 nm. Complexes 1 and 2 display quasireversible cyclic voltammetric response in DMF–Tris buffer (1:4 v/v, pH 7.2) for the Cu(II)/Cu(I) couple at ca. −0.1 V vs. SCE. Complex 3 exhibits an irreversible reduction process forming [Cu I(dmp) 2] +. Binding of 1– 3 to calf thymus DNA shows the relative order: 2 (phen) ⩾ 3 (dmp) > 1 (bpy). Complex 2 efficiently cleaves supercoiled pUC19 DNA in the presence of mercaptopropionic acid (MPA) forming hydroxyl radical or on irradiation with light of 312, 532 and 632.8 nm wavelength in a type-II process. Complexes 1 and 3 are cleavage inactive.

Effect of charge transfer bands on the photo-induced DNA cleavage activity of [1-(2-thiazolylazo)-2-naphtholato]copper(II) complexes

Ternary copper(II) complex [Cu(TAN)(O 2CMe)] ( 1), where H–TAN is 1-(2-thiazolylazo)-2-naphthol, is prepared and structurally characterized by X-ray crystallography. The complex has a distorted square pyramidal (4 + 1) CuN 2O 3 coordination geometry with the acetate showing chelating axial–equatorial binding mode and TAN as a tridentate ligand bonded to the metal in the basal plane. Complex 1 is one-electron paramagnetic and displays ligand-to-metal charge transfer bands at 575 and 398 nm in dimethylformamide. The reactions of 1 with bases (B) like 1,10-phenanthroline (phen) and kanamycin-A (kan-A) afford ternary complexes of formulation [Cu(TAN)B] + (B = phen, 2; kan-A, 3) under in situ reaction conditions. Complexes 2 and 3, prepared to explore their DNA binding and photo-induced DNA cleavage activity, display good binding propensity to calf thymus (CT) DNA giving a relative order: 2– 3 > 1. The apparent binding constant ( K app) for 1 is determined as 9.8 × 10 5 M −1 from fluorescence quenching experiments using ethidium bromide. The quenching constants ( K) values of 1- 3, obtained from the Stern–Volmer plots, are 0.28, 0.52 and 0.49, respectively. All the complexes show photo-induced DNA cleavage activity when irradiated with a monochromatic UV light of 365 nm wavelength. A 200 μM complex 1 cleaves ∼75% supercoiled (SC) DNA on 2 h exposure time at 365 nm. A 50 μM solution of 1 in presence of 100 μM phen and kanamycin-A cleaves ∼99% and ∼60% SC DNA to its nicked circular form, respectively, for an exposure of 30 min. The complexes also exhibit significant cleavage of SC DNA on irradiation with visible light of wavelengths 532, 575 and 632.8 nm. Control experiments reveal the minor groove binding nature of the complexes. The cleavage reactions involve the formation reactive hydroxyl species as significant inhibition in the presence of dimethyl sulfoxide (DMSO) and catalase is observed. There is no apparent inhibition in cleavage in the presence of singlet oxygen quenchers like sodium azide. The cleavage activity has been found to be higher at the CT band position of 575 nm in comparison to those at 532 and 632.8 nm. The results indicate the involvement of the CT band in the photo-excitation process.