Fluorescence spectral study of interaction of water-soluble metal complexes of Schiff-base and DNA

Water-Soluble Sulfonate Schiff-Base Ligands as Fluorescent Detectors for Metal Ions in Drinking Water and Biological Systems

The ability to detect and selectively identify trace amounts of metal ions is of major importance for drinking water identification and biological studies. Herein, we report a series of water-soluble Schiff-base ligands capable of being fluorescent and colorimetric sensors for metal ions. Upon coordination of the metal ion to the ligand, quenching of fluorescence is observed, typically in a 1:1 ratio. The selectivity of metal ions Cu²⁺, Ni²⁺, Cr₃₊, and Co₂₊ is exhibited via fluorescence quenching accompanied by colorimetric changes, whereas that of Ag⁺ and Co²⁺ is observed through colorimetric changes alone. Additionally, pH sensing studies were performed for the potential use of these ligands in biological applications.

Synthesis, characterization, DNA binding, cleavage activity, cytotoxicity and molecular docking of new nano water-soluble [M(5-CH₂PPh₃-3,4-salpyr)](ClO₄)₂ (M = Ni, Zn) complexes

Some new water soluble complexes [N,N'-bis{5-[(triphenyl phosphonium chloride)-methyl]salicylidine}-3,4-diaminopyridine] M(ii), which are formulated as nano-[Zn(5-CH2PPh3-3,4-salpyr)](ClO4)2 (), [Zn(5-CH2PPh3-3,4-salpyr)](ClO4)2 (), nano-[Ni(5-CH2PPh3-3,4-salpyr)](ClO4)2 (), [Ni(5-CH2PPh3-3,4-salpyr)](ClO4)2 (), and [N,N'-bis{5-[(triphenyl phosphonium chloride)-methyl]salicylidine}-2,3-diaminopyridine]Ni(ii) [Ni(5-CH2PPh3-2,3-salpyr)](ClO4)2 () have been isolated and characterized by elemental analysis, FT-IR, (1)H NMR, (13)C NMR, (31)P NMR, and UV-vis spectroscopy. The morphology and size of the nano complexes were determined using FE-SEM and TEM. In vitro DNA binding studies were investigated by UV-vis absorption spectroscopy, viscosity measurements, CD spectroscopy, cyclic voltammetry, emission spectra and gel electrophoresis, which suggest that the metal complexes act as efficient DNA binders. The absorption spectroscopy of the compounds with DNA reveals that the DNA binding affinity (Kb) has this order: > > > > > Ligand. The metal complexes show DNA binding stronger than the ligand, which is expected due to the nature of the metal. The nano complexes display DNA binding stronger than the other complexes which is related to the effect of size on binding affinity and the Ni(ii) complexes reveal DNA binding stronger than the corresponding Zn(ii) analogues, which is expected due to their z* effect and geometry. The prominent double strand DNA cleavage abilities of compound are observed in the absence of H2O2 with efficiencies of more than 50% even at 70 μM complex concentration. Surprisingly, Zn(ii) complexes (compounds & ) exhibit a higher cytotoxicity (IC50: 7.3 & 10.9 μM at 24 h; IC50: 4.6 & 8.7 μM at 48 h) against human hepatoma (HepG2) and HeLa cell lines than the Ni(ii) complexes (compounds , & ) and 5-fluorouracil as control in spite of their inability to cleave DNA. Finally, DNA binding interactions were performed by docking studies. Density functional theory (DFT) studies were performed using the GAUSSIAN 03 program. The DFT method with B3LYP functional, LANL2DZ basis set for metal centers and 6-311g* for other atoms was used. The synthesized compounds and DNA were simulated by molecular docking to explore more details of the ligands conformation and their orientations in the active site of the receptor.

Enlightening surface plasmon resonance effect of metal nanoparticles for practical spectroscopic application

Pictorial depiction of applications of metal nanoparticles in different fields enlightening surface plasmon resonance effect.

Synthesis, characterization and interaction of N,N'-dipyridoxyl (1,4-butanediamine) Co(III) salen complex with DNA and HSA

Co(III) salen complex with N,N'-dipyridoxyl (1,4-butanediamine) Schiff-base ligand as tetradentate ligand was synthesized and characterized by the elemental and spectroscopic analysis. The interaction of this complex with calf thymus DNA (ct DNA) has been investigated in vitro using UV absorption, fluorescence spectroscopy, thermal denaturation and gel electrophoresis techniques. The binding constant has been estimated to be 1×10(4)M(-1) using UV absorption. The addition of ct DNA to Co(III) salen solution resulted in a fluorescence quenching. The binding constant and site size binding have been calculated in connection with other experimental observations show that the interactive model between Co(III) salen and ct DNA is an intercalative one. The interaction between plasmid DNA (pTZ57R DNA) and this complex is confirmed by gel electrophoresis studies. Furthermore, the interaction between HSA and Co(III) salen complex was investigated by UV absorption, fluorescence spectroscopy and molecular modeling. The binding constant for the interaction of this complex with HSA were found to be 3.854×10(4)M(-1) using UV absorption, which was in good agreement with the binding constant obtained from fluorescence method (3.866×10(4)M(-1)). The binding distance between HSA and this complex was estimated to be 2.48nm according to Förster theory of non-radioactive energy transfer. Molecular modeling studies suggested that hydrophobic interaction was the predominant intermolecular forces stabilizing Co(III) complex-HSA system.

Spectroscopic studies on the binding of holmium-1,10-phenanthroline complex with DNA

Fluorescence and absorption spectroscopy, circular dichroism (CD) as well as viscosity experiment have been used to characterize the DNA binding of [Ho(Phen)(2)Cl(3)]·H(2)O, where phen stand for 1,10-phanathroline. This complex exhibits the marked decrease in the emission intensity and some hypochromism in UV-Vis spectrum in the presence of DNA. For characterization of the binding mode between the Ho(III) complex and DNA various procedures such as: absorption and emission titration and EB quenching experiments, viscosity measurements, CD study, iodide quenching assay, salt effect and thermodynamical investigation are used. The intrinsic binding constant of [Ho(Phen)(2)Cl(3)]·H(2)O with DNA is calculated by UV-Vis and florescence spectroscopy. The value of binding constants in 296, 299 and 303 are 1.99 ± 0.07 × 10(4), 1.07 ± 0.09 × 10(4) and 0.84 ± 0.06 × 10(4), respectively. The thermodynamic studies show that the reaction is entropically driven. The above-mentioned physical measurements indicate that the Ho(III) complex binds to fish salmon DNA, presumably via groove binding mode.

Spectroscopic study on the interaction of ct-DNA with manganese Salen complex containing triphenyl phosphonium groups

The DNA binding properties of a bulky and hydrophobic Schiff base complex of manganese(III) [N,N'-bis(5-(triphenyl phosphonium methyl)salicylidene)-1,2-ethylene diamine chloride Mn(III) acetate] was examined by spectroscopic techniques. UV-vis titration data indicate both hypo and hyperchromic effect with addition of DNA to complex. A competitive binding study showed that the enhanced emission intensity of ethidium bromide (EB) in the presence of DNA was quenched by adding Mn Salen complex. This finding indicates that Mn Salen complex displaces EB from its binding site in DNA. Helix melting studies indicate improvement in the helix stability, and an increase in the melting temperature. The analysis of CD spectra represents the structural changes in DNA due to the binding of Mn Salen complex. The binding constant has been calculated using absorbance and fluorescence data. The results also represent that the binding process proceeds by strong electrostatic and hydrophobic interactions.

New modulated design and synthesis of chiral CuII/SnIV bimetallic potential anticancer drug entity: in vitro DNA binding and pBR322 DNA cleavage activity

A new chiral ligand scaffold L derived from (R)-2-amino-2-phenyl ethanol and diethyl oxalate was isolated and thoroughly characterized by various spectroscopic methods. The ligand L was allowed to react with CuCl(2)·2H(2)O and NiCl(2)·6H(2)O to achieve monometallic complexes 1 and 2, respectively. Subsequently modulation of 1 and 2 was carried out in the presence of SnCl(4)·5H(2)O to obtain heterobimetallic potential drug candidates 3 and 4 possessing (Cu(II)/Sn(IV) and Ni(II)/Sn(IV)) metallic cores, respectively and characterized by elemental analysis and spectroscopic data including (1)H, (13)C and (119)Sn NMR in case of 3 and 4. In vitro DNA binding studies revealed that complex 3 avidly binds to DNA as quantified by K(b) and K(sv) values. Complex 3 exhibits a remarkable DNA cleavage activity (concentration dependent) with pBR322 DNA and the cleavage activity of 3 was significantly enhanced in the presence of activators and follows the order H(2)O(2)>Asc>MPA>GSH. Complex 3 cleave pBR322 DNA via hydrolytic pathway and accessible to major groove of DNA.

Structural, vibrational and hyperpolarizability calculation of (E)-2-(2-hydroxybenzylideneamino)-3-methylbutanoic acid

The (E)-2-(2-hydroxybenzylideneamino)-3-methylbutanoic acid (E)-2HBAMBA was synthesized. The FT-IR, FT-Raman and UV-vis spectra have been recorded and characterized. Theoretical wavenumbers along with IR and Raman intensities were calculated using B3LYP/6-31G(d,p) level and total energy distribution (TED) of the various normal mode of vibrations were also studied. The conformational analysis was performed for a stable conformer by selecting the dihedral angles and the optimized bond parameters were calculated for the stable structure. Effect of intramolecular interactions is calculated by changing the orientation of hydroxyl hydrogen. To know the charge transfer while changing the hydroxyl group hydrogen orientation, the NBO analysis was performed. Using the same level of calculation, the electronic charge transfers were calculated and compared.

Synthesis and enantiopreferential DNA-binding profile of late 3d transition metal R- and S-enantiomeric complexes derived from N,N-bis-(1-benzyl-2-ethoxyethane): Validation of R-enantiomer of copper(II) complex as a human topoisomerase II inhibitor

To evaluate the biological preference of chiral drug candidates for molecular target DNA, new potential metal-based chemotherapeutic agents 1-3 (a and b) of late 3d transition metals Ni(II), Cu(II), and Zn(II), respectively, derived from (R)- and (S)-2-amino-2-phenylethanol with CH(2) CH(2)  linker were synthesized and thoroughly characterized. Interaction studies of 1-3 (a and b) with calf thymus DNA in Tris buffer were studied by electronic absorption titrations, luminescence titrations, cyclic voltammetry, and circular dichroism. The results reveal that the extent of DNA binding of R-enantiomer of copper 1a was highest in comparison to rest of the complexes via electrostatic interaction mode. The nuclease activity of 1(a and b) with supercoiled pBR322 DNA was further examined by gel electrophoresis, which reveals that complex 1a exhibits a remarkable DNA cleavage activity (concentration dependent) with pBR322DNA, and the cleavage activity of both enantiomers of complex 1 was significantly enhanced in the presence of activators. The activating efficiency follows the order Asc > H(2) O(2) > MPA for 1a, and reverse order was observed for 1b, because of the differences in enantioselectivity and conformation. Further, it was observed that cleavage reaction involves singlet oxygen species and superoxide radicals via oxidative cleavage mechanism. In addition, complex 1a exhibits significant inhibitory effects on the topoisomerase II (topo II) activity at a very low concentration ∼24 μM, which suggest that complex 1a is indeed catalytic inhibitor or (poison) of human topo II.

Chiral Salarg and its metal complex: unique extrinsic fluorophores

We present a fluorescence spectroscopic study of a chiral amino acid Schiff base ligand Salarg and its metal complex Mn-Salarg. This study reveals that the Salarg ligand and its metal complex have the properties of a typical extrinsic fluorophore. A detailed comparison has been made with the fluorescence spectroscopic properties of various other extrinsic and intrinsic fluorophores. It is suggested that Salarg and Mn-Salarg, which are economic, efficient, easy to synthesize and environment-friendly extrinsic fluorophores, might be used to label DNA and lipids respectively.

Synthesis and DNA binding studies of Mg(II) complex of Schiff base derived from vanillin and L-tryptophan

The Mg(II) complex of Schiff base (K[HL]) derived from vanillin and L-tryptophan could bind with herring sperm DNA. The binding behaviors between them in physiological pH environment (pH 7.40) have been studied by spectroscopy, cyclic voltammetry and viscosity methods. Binding ratios of n(Mg(II)): n(K[HL]) = 1:1 and n(Mg(II)L): n(DNA) = 5:1 were confirmed. The obtained thermodynamic parameters suggest that the interaction between Mg(II)L and DNA is driven mainly by entropy. Combined with fluorimeteric studies, cyclic voltammetry, CD spectroscopy and viscosity methods, the results indicate the interaction modes between Mg(II)L and DNA are mainly with intercalation and involving some electrostatic interaction.

Synthesis, spectroscopic characterization and antiproliferative evaluation in vitro of novel Schiff bases related to benzimidazoles

A series of novel benzimidazole substituted Schiff bases were synthesized by reaction of aromatic aldehydes with corresponding 2-aminobenzimidazoles. Their structure has been studied by 1H and 13C NMR, IR and UV/Vis spectroscopy. Majority of prepared Schiff bases were tested on their antiproliferative activity in vitro and exerted non-specific antiproliferative activity on the tested cell lines at the highest tested concentration. Compounds 18 and 19 exerted the strongest non-specific antiproliferative effect on all cell lines and a concentration-dependent effect on HeLa and MCF-7 cell lines at micromolar concentrations but simultaneously being highly cytotoxic on human fibroblasts as well.

Synthesis, experimental and theoretical characterization of N,N'-dipyridoxyl (1,4-butanediamine) Schiff-base ligand and its Cu(II) complex

A new N,N'-dipyridoxyl(1,4-butanediamine) [=H(2)BS] Schiff-base ligand and its Cu(II) salen complex, [Cu(BS)(H(2)O)(CH(3)OH)], were synthesized and characterized by IR, UV-vis, (1)H NMR, mass spectrometry and elemental analysis. Also, full optimization of the geometries, (1)H NMR chemical shifts (for the H(2)BS) and vibrational frequencies were calculated by using density functional theory (DFT) method. Structure of the H(2)BS ligand is not planar, i.e. two pyridine rings are not in the same plane. In the structure of the Cu complex, the Schiff-base ligand acts as a dianionic tetradentate ligand in N, N, O(-), O(-) manner. The coordinating atoms of BS(2-) occupy equatorial positions of the octahedral complex, where the H(2)O and CH(3)OH ligands locate at axial positions. The calculated results are in good agreement with the experimental data, confirming the suitability of the proposed and optimized structures for the H(2)BS ligand and its Cu complex.

Design and synthesis of heterobimetallic topoisomerase I and II inhibitor complexes: in vitro DNA binding, interaction with 5'-GMP and 5'-TMP and cleavage studies

New potential cancer chemotherapeutic complexes Cu-Sn(2)/Zn-Sn(2) 3 and 4 were designed and prepared as topoisomerases inhibitors; their in vitro DNA binding studies were carried out which reveal strong electrostatic binding via phosphate backbone of DNA helix, in addition to other binding modes viz. coordinate covalent and partial intercalation. To throw insight to molecular binding event at the target site, UV-vis titrations of 3 and 4 with mononucleotides of interest, viz, 5'-GMP and 5'-TMP were carried out, (in case of 4) by (1)H and (31)P NMR. Cleavage studies employing gel electrophoresis demonstrate both the complexes 3 and 4 are efficient cleavage agents and are specific groove binders (complex 3 binds to both major and minor groove while complex 4 is specifically minor groove binder only). In addition, the complexes show high inhibition activity against topoisomerase I and II. However, complex 4 exhibits significant inhibitory effects on the Topo I activity at a very low concentration approximately 2.5 microM.

DNA interaction studies of new nano metal based anticancer agent: validation by spectroscopic methods

A new nano dimensional heterobimetallic Cu-Sn containing complex as a potential drug candidate was designed, synthesized and characterized by analytical and spectral methods. The electronic absorption and electron paramagnetic resonance parameters of the complex revealed that the Cu(II) ion exhibits a square pyramidal geometry with the two pyrazole nitrogen atoms, the amine nitrogen atom and the carboxylate oxygen of the phenyl glycine chloride ligand located at the equatorial sites and the coordinated chloride ion occupying an apical position. (119)Sn NMR spectral data showed a hexa-coordinated environment around the Sn(IV) metal ion. TEM, AFM and XRD measurements illustrate that the complex could induce the condensation of CT-DNA to a particulate nanostructure. The interaction of the Cu-Sn complex with CT-DNA was investigated by UV-vis absorption and emission spectroscopy, as well as cyclic voltammetric measurements. The results indicated that the complex interacts with DNA through an electrostatic mode of binding with an intrinsic binding constant K(b) = 8.42 x 10(4) M( - 1). The Cu-Sn complex exhibits effective cleavage of pBR322 plasmid DNA by an oxidative cleavage mechanism, monitored at different concentrations both in the absence and in the presence of reducing agents.

Synthesis and characterization of glucose-bis(pyrazole)-Cu(II)/Ni(II) complexes and their in vitro DNA binding studies

The D-glucose-bis pyrazolyl complexes of Cu(II) 1 and Ni(II) 2 were synthesized and characterized by elemental analysis, molar conductance measurements and spectroscopic methods. The solution structures of the complex have been assessed to square pyramidal using electronic absorption and electronic paramagnetic resonance (EPR) spectroscopy. The interaction of 1 and 2 with calf thymus DNA (CT DNA) has been carried out by absorption, emission, viscometric and electrochemical methods. The intrinsic binding constant K(b) was determined as 13.4x10(5) M(-1), 4.5x10(5) M(-1) for 1 and 2, respectively suggestive of strong binding of complexes with DNA. Furthermore, higher value of K(b) for 1 implies that this complex interacts more strongly with CT DNA in comparison to 2. The quenching constant "K" of 1 and 2 obtained from emission spectral methods was 1.33, 0.55, respectively. Complex 1 hydrolytically cleaved pBR322 supercoiled DNA in absence of an activating agent. The enhanced cleavage of pBR322 DNA was observed in presence of ascorbic acid as a reducing agent, 1 also displays efficient photonuclease activity through double strand DNA breaks when irradiated at 365 nm through mechanistic pathway involving hydroxyl radicals. In addition to the above binding studies, an in vitro binding study of complex 1 with protein human serum albumin (HSA), tyrptophan and mixtures of HSA, L-tryptophan with CT DNA was carried out. The in vitro "binding study" also supports that 1 shows higher binding affinity towards CT DNA.

Synthesis of new heterometallic macromolecules: their DNA binding, cleavage activity and in vitro model electrochemotherapy study

The homodinuclear C(16)H(30)N(8)O(5)Sn(2)Cl(4) (1), heterotetranuclear C(16)H(38)N(8)O(9)Sn(2)Cu(2)Cl(8) (2) and C(16)H(38)N(8)O(9)Sn(2)Mn(2)Cl(8) (3) macrocyclic complexes were synthesized and characterized by elemental analysis, spectroscopic techniques and molar conductance measurements. The interaction studies of 1-3 with calf thymus DNA (CT-DNA) were carried out by UV-vis titration, fluorescence, cyclic voltammetry and viscosity measurements. These results were further authenticated by carrying out interaction studies of 1-3 with plasmid pBR322 DNA employing gel electrophoresis. To overcome the dose resistance, auto toxicity of the drugs, a model study based on electrochemotherapy (ECT) was carried out and the results were compared in the presence and in the absence of the applied electrical potential.

Distance dependent interhelical couplings of organic rods incorporated in DNA 4-helix bundles

The synthesis of a conjugated linear organic module containing terminal salicylaldehyde groups and a central activated ester, designed for conjugation to amino-modified oligonucleotides, is presented. The organic module has a phenylene-ethynylene backbone and is highly fluorescent. It is conjugated to oligonucleotide sequences and incorporated into specific locations in a well-defined DNA 4-helix bundle (4-HB). The DNA-nanostructure offers precise location control of the organic modules which allows for selective interhelical coupling reactions. In this study, metal-salen formation as well as dihydrazone formation are used to covalently interlink the organic modules. Both coupling reactions are highly dependent on the distances between the organic modules in the 4-HB. Neighboring modules dimerize easier, whereas more distanced modules are less prone to react, even when the linkers are extended. The dimeric products are characterized by denaturing polyacrylamide gel electrophoresis (PAGE), high performance liquid chromatography (HPLC), and matrix assisted laser desorption/absorption ionization time-of-flight (MALDI TOF) mass spectrometry.

Synthesis of new piperazine derived Cu(II)/Zn(II) metal complexes, their DNA binding studies, electrochemistry and anti-microbial activity: validation for specific recognition of Zn(II) complex to DNA helix by interaction with thymine base

New 3,4:9,10-dibenzo-2,11-dihydroxy-1,12-bispiperazine-5,8-dioxododecane complexes [C(24)H(36)N(4)O(6)Cu] (1), [C(24)H(32)N(4)O(4)Zn] (2) have been synthesized and characterized by elemental analysis, IR, NMR, Mass, EPR, UV-vis spectroscopy and molar conductance measurements. The complexes are non-ionic in nature and possess octahedral geometry around Cu(2+), Zn(2+) central metal ions. The binding studies of 1 and 2 with calf thymus DNA (CT-DNA) were investigated by UV-vis, fluorescence, cyclic voltammetery and viscosity measurements. The calculated binding constant K(b) for 1 and 2 obtained from UV-vis absorption studies was 7.6x10(3)M(-1), 80.8x10(4)M(-1), respectively. The intrinsic binding constants were also estimated to be 7.0x10(4)M(-1) and 7.53x10(5)M(-1) for 1 and 2, respectively by using emission titrations. These experimental results suggest that complexes are groove binders and interact to CT-DNA with different affinities. Both the complexes in presence and absence of CT-DNA show quasireversible wave corresponding to Cu(II)/Cu(I) and Zn(II)/Zn(I) redox couple. The changes in E(1/2), DeltaE, I(pa)/I(pc) ascertain the interaction of 1 and 2 with CT-DNA. Further, decrease in viscosity of CT-DNA with increasing concentration of complexes was observed. In vitro, antimicrobial activity against fungi A. brassicicola, A. niger and bacteria E. coli, P. aeruginosa of complexes were carried out, which indicate that complex 2 is more active against both fungal and bacterial strains as shown by % inhibition data.

Synthesis, experimental and theoretical characterization of tetra dentate N,N'-dipyridoxyl (1,3-propylenediamine) salen ligand and its Co(III) complex

The new tetra dentate dianionic H2PS (N,N'-dipyridoxyl (1,3-propylenediamine)) Schiff-base ligand and its octahedral Co(III) salen complex [Co(PS)(H2O)(CH3OH)]+CH3COO(-) were synthesized, where coordinating atoms of H2PS (N,N,O(-),O(-)) occupied equatorial positions with H2O and CH3OH as axial ligands. The nature of the H2PS and its complex were determined by elemental and spectrochemical (IR, UV-vis, 1H NMR and Mass) analysis. Also, the fully optimized geometries and vibrational frequencies of them together with the 1H NMR chemical shifts of H2PS have been calculated using density functional theory (B3LYP) method. Obtained structural parameters are in good agreement with the experimental data reported for similar compounds. The calculated and experimental results confirmed the suggested structures for the ligand and complex.

New homodi-and heterotrinuclear metal complexes of Schiff base compartmental ligand: interaction studies of copper complexes with calf thymus DNA

The new homodinuclear complexes 1–4 of the type [LMII 2Cl2], heterotrinuclear complexes 5 and 6 of the type [LMII 2SnIVCl6] where M = CuII, MnII, CoII, NiII and CuII and NiII, respectively have been synthesized and characterized by elemental analysis and various spectroscopic techniques. The homodinuclear complexes possess two different environments (N2 and N2O2donor sets) for holding the metal ions. The metal ion in N2 set exhibits square planar geometry with two chloride ions in the inner sphere but rhombic structure is found in tetradentate N2O2 Schiff base cavity while in heterotrinuclear complexes SnIV atom is in the octahedral environment. The interaction of complexes 1 and 5 with calf thymus DNA was carried out by absorption spectroscopy and cyclic voltammetry. The intrinsic binding constants (K b) of complex 1 and 5 were determined as 3.2 × 103 M−1 and 9.6 × 103 M−1, respectively suggesting that complex 5 binds more strongly to CT-DNA than complex 1. Fluorescence studies along with viscosity measurements have also been checked to authenticate the binding of metal complexes with DNA.

Effects of hydrogen peroxide upon nicotinamide nucleotide metabolism in Escherichia coli: changes in enzyme levels and nicotinamide nucleotide pools and studies of the oxidation of NAD(P)H by Fe(III)

DNA is damaged in vivo by the Fenton reaction mediated by Fe2+ and cellular reductants such as NADH, which reduce Fe3+ to Fe2+ and allow the recycling of iron. To study the response of Escherichia coli to such cycling, the activities of several enzymes involved in nicotinamide nucleotide metabolism were measured following an H2O2 challenge. NADPH-dependent peroxidase, NADH/NADP+ transhydrogenase, and glucose-6-phosphate dehydrogenase were most strongly induced, increasing 2.5-3-fold. In addition, the cellular ratios of NADPH to NADH increased 6- or 92-fold 15 min after exposure to 0.5 or 5 mm H2O2, respectively. In vitro, NADH was oxidized by Fe3+ up to 16-fold faster than NADPH, despite their identical reduction potentials. To understand this rate difference, the interactions of Fe3+ and Ga3+ with NAD(P)H were examined by 1H, 13C, and 31P NMR spectroscopy. Association with NADH occurred primarily with adenine at N7 and the amino group, but for NADPH, strong metal interactions also occurred at the 2'-phosphate group. Interaction of M3+ (Fe3+ or Ga3+) with the adenine ring would bring it into close proximity to the redox-active nicotinamide ring in the folded form of NAD(P)H, but interaction of M3+ with the 2'-phosphate group would avoid this close contact. In addition, as determined by absorbance spectroscopy, the energy of the charge-transfer species was significantly higher for the Fe3+.NADPH complex than for the Fe3+.NADH complex. We therefore suggest that upon exposure to H2O2 the NADH pool is depleted, and NADPH, which is less reactive with Fe3+, functions as the major nicotinamide nucleotide reductant.