Contributions of a long side chain to the binding affinity of an anthracene derivative to DNA

Systematic studies on the DNA binding of a new anthracene derivative, carrying a 1,8-octyldiamine side chain, were carried out. Calorimetric, spectroscopic, and helix melting studies show that the side chain, consisting of eight methylene groups, enhances the binding constant by a factor of approximately 35 when compared to the binding of a probe lacking the long side chain. Furthermore, the enthalpy of binding of the long-chain derivative to calf thymus DNA (Delta H = 4.1 +/- 0.1 kcal/mol) is far greater than the sum of the enthalpy changes associated with the binding of the probe lacking the long side chain, and the enthalpy for the binding of 1,8-octyldiamine.2HCl. Strong synergistic effects, therefore, are seen with the long-chain derivative. Spectroscopic data indicate bathochromism, strong hypochromism, and quenching of anthryl fluorescence when the above ligand binds to calf thymus DNA. Fluorescence energy transfer studies and circular dichroism data strongly suggest intercalation of the anthryl ring system. The binding stabilizes the double helix, and the helix melting temperature is increased from 78 degrees C to >90 degrees C. The binding to DNA is reversible, depended on the ionic strength, and the major binding mode was suppressed at high ionic strengths and a new mode begins to dominate binding. Substitution of the anthracene ring with 1,8-octyldiamine chain provided a simple method to enhance the binding constant by nearly a factor of 35.     

Calorimetric and thermal analysis studies on the binding of phenothiazinium dye thionine with DNA polynucleotides

Binding of the phenothaizinium dye thionine with four sequence specific deoxyribopolynucleotides, poly(dG-dC).poly(dG-dC), poly(dG).poly(dC), poly(dA-dT).poly(dA-dT), and poly(dA).poly(dT) has been investigated by means of thermal helix melting, isothermal titration calorimetry, and differential scanning calorimetry experiments. The binding affinity values evaluated from isothermal titration calorimetry suggests that thionine exhibits the highest binding affinity to poly(dG-dC).poly(dG-dC). The binding to poly(dG-dC).poly(dG-dC), poly(dA-dT).poly(dA-dT), and poly(dG).poly(dC) is exothermic and favoured by negative enthalpy changes while binding to poly(dA).poly(dT) is endothermic and anomalous. The values of heat capacity changes of the interaction are negative and in the range (?0.4 to ?0.5) kJ · K^?1 · mol^?1. The binding is characterized by strong stabilization of the polynucleotides against thermal strand separation. The binding affinity values derived from thermal melting data are in excellent agreement with those obtained from isothermal titration calorimetry data. Insights into the energetic aspects and guanine–cytosine selectivity of the DNA interaction of thionine have been obtained from these studies.     

Thermodynamic profiles of the DNA binding of benzophenanthridines sanguinarine and ethidium: A comparative study with sequence specific polynucleotides

Energetics of the binding of two known classical DNA intercalating molecules, ethidium and sanguinarine with four sequence specific polynucleotides, poly(dG-dC).poly(dG-dC), poly(dG).poly(dC), poly(dA-dT).poly(dA-dT), and poly(dA).poly(dT) have been compared under identical conditions. The binding of both the molecules was characterized by strong stabilization of the polynucleotides against thermal strand separation in optical melting as well as differential scanning calorimetry studies. Isothermal titration calorimetry results revealed that the binding of both sanguinarine and ethidium to poly(dG-dC).poly(dG-dC), poly(dA-dT).poly(dA-dT), and poly(dG).poly(dC) was exothermic and favoured by negative enthalpy changes. On the other hand, the binding of both molecules to poly(dA).poly(dT) was endothermic and entropy driven. The binding affinity values obtained from isothermal titration calorimetry data was in close proximity to that derived from thermal melting data. The heat capacity changes obtained from temperature dependence of the enthalpy change gave negative values in the range (?0.4 to 1.25) kJ · mol^?1 · K^?1 for the binding of ethidium and sanguinarine to these polynucleotides. The variations in the values indicate important differences in the formation of the complexes. New insights into the energetics and specificity aspects of interaction of these molecules to DNA have emerged from these studies.     

Circular dichroism spectroscopic studies reveal pH dependent binding of curcumin in the minor groove of natural and synthetic nucleic acids

For the first time, an interaction between the non-toxic, cancer chemopreventive agent curcumin and both natural and synthetic DNA duplexes has been demonstrated by using circular dichroism (CD) and absorption spectroscopy techniques. Upon addition of curcumin to calf thymus DNA, poly(dG-dC).poly(dG-dC) and poly(dA-dT).poly(dA-dT) solutions, an intense positive induced CD band centered around 460-470 nm was observed depending on the actual pH and Na+ ion concentration of the medium; no CD signal was obtained, however, with single stranded poly(dC). Interaction of curcumin with calf thymus DNA was observed already at pH 6.5 in contrast with poly(dG-dC).poly(dG-dC) which induces no extrinsic Cotton effect above a pH value of 5. The protonated, Hoogsteen base-paired structure of poly(dG-dC).poly(dG-dC) is necessary for curcumin binding while the alternating AT-rich polymer formed complexes with curcumin only at certain Na+ concentrations. Evaluation of the spectral data and molecular modeling calculations suggested that curcumin, this dietary polyphenolic compound binds in the minor groove of the double helix. The mechanism of the induced CD activity, the effects of the pH and Na+ ions on the ligand binding and conformation of the double helix are discussed in detail. As well as being an essentially new phenolic minor groove binder agent curcumin is also a promising molecular probe to study biologically important, pH and cation induced conformational polymorphisms of nucleic acids.     

Hypochromism of polynucleotides in the nearest neighbour approximation

A method of calculating the hypochromism of polynucleotides in the nearest neighbour approximation is given by perturbation theory. The calculated hypochromism values of the first UV-absorption band of polynucleotides poly(dA)·poly(dT) and poly(dG)·poly(dC) are in good agreement with experiment. In this approximation the origin of the hypochromic effect is studied for the double-stranded polynucleotides; the dependence of the hypochromism upon the length of the polynucleotide is given.     

Can Copper(II) Mediate Hoogsteen Base‐Pairing in a Left‐Handed DNA Duplex? A Pulse EPR Study

Pulse EPR spectroscopy is used to investigate possible structural features of the copper(II) ion coordinated to poly(dG‐dC)?poly(dG‐dC) in a frozen aqueous solution, and the structural changes of the polynucleotide induced by the presence of the metal ion. Two different copper species were identified and their geometry explained by a molecular model. According to this model, one species is exclusively coordinated to a single guanine with the N7 nitrogen atom forming a coordinative bond with the copper. In the other species, a guanine and a cytosine form a ternary complex together with the copper ion. A copper crosslink between the N7 of guanine and N3 of cytosine is proposed as the most probable coordination site. Moreover, no evidence was found for an interaction of either copper species with a phosphate group or equatorial water molecules. In addition, circular dichroism (CD) spectroscopy showed that the DNA of the Cu^II‐poly(dG‐dC)?poly(dG‐dC) adducts resembles the left‐handed Z‐form. These results suggest that metal‐mediated Hoogsteen base pairing, as previously proposed for a right‐handed DNA duplex, can also occur in a double‐stranded left‐handed DNA.     

The mode of binding ACMA-DNA relies on the base-pair nature

A thermodynamic and kinetic study on the mode of binding of 9-amino-6-chloro-2-methoxi-acridine (ACMA) to poly(dA-dT)·poly(dA-dT) and poly(dG-dC)·poly(dG-dC) has been undertaken at pH = 7.0 and I = 0.1 M. The spectrophotometric, kinetic (T-jump), circular dichroism, viscometric and calorimetric information gathered point to formation of a fully intercalated ACMA complex with poly(dA-dT)·poly(dA-dT) and another one only partially intercalated (7%) with poly(dG-dC)·poly(dG-dC). The ACMA affinity with the A-T bases was higher than with the G-C bases. The two polynucleotide sequences give rise to external complexes when the ACMA concentration is raised, namely, the electrostatic complex poly(dA-dT)·poly(dA-dT)-ACMA and the major groove binding complex poly(dG-dC)·poly(dG-dC)-ACMA. A considerable quenching effect of the ACMA fluorescence is observed with poly(dA-dT)·poly(dA-dT), ascribable to face-to-face location in the intercalated A-T-ACMA base-pairs. The even stronger effect observed in the presence of poly(dG-dC)·poly(dG-dC) is related to the guanine residue from on- and off-slot ACMA positions.     

硫堇与寡核苷酸结合序列选择性的光谱研究

用UV-Vis吸收光谱、荧光光谱、圆二色谱以及核磁共振光谱等手段研究了硫堇(TH)与两个不同序列寡核苷酸的作用。TH与寡核苷酸作用后的吸收光谱和荧光光谱产生了明显的减色红移和荧光猝灭效应。分别计算了TH与[oligo d(GC)]2和[oligo d(AT)]2作用的荧光猝灭常数和结合常数,结果表明TH与GC序列的结合能力比与AT序列更强。通过TH与[oligo d(GC)]2作用后双螺旋链构象变化以及TH质子的1HNMR谱峰明显变宽,进一步说明TH与寡核苷酸结合的序列选择性。     

EFFICIENCY OF PHOTOAFFINITY LABELING DNA HOMOPOLYMERS AND COPOLYMERS WITH ETHIDIUM MONOAZIDE *

Abstract— Photoaffinity labeling of synthetic DN As with ethidium monoazide was studied to determine if the efficiency of adduct formation was related to DNA sequence. Equilibrium drug binding to DNA homopolymers and copolymers was quanitified by phase partition techniques. The amount of drug bound to a deoxypolymer at equilibrium was then compared to the fraction of ethidium analog covalently-linked following photoactivation at the same drug/DNA input ratio. There were significant sequence-related differences in the ability of the photoaffinity probe to label DNA covalently. The efficiency of covalent-adduct formation decreased in the order poly(dG-dC)^. poly(dG-dC)> poly-(dG). poly(dC)poly(dA-dT). poly(dA-dT)poly(dA). poly(dT). Ethidium monoazide was about 2-fold more efficient in labeling deoxyhomopolymers and deoxycopolymers composed of G-C pairs than the A-T base counterparts. In low ionic buffers (0.015 M Na⁺), the efficiency of photoactivation decreased with increasing ethidium monoazide concentrations. However. the base sequence effect was observed over a 40-fold range of drug concentrations. Therefore, the amount of ethidium monoazide bound to a DNA site after irradiation does not appear to represent the true affinity of the drug for that site.     

Contributions of hydroxyethyl groups to the DNA binding affinities of anthracene probes

Contributions of hydroxyethyl functions to the DNA binding affinities of substituted anthracenes are evaluated by calorimetry and spectroscopy. Isothermal titration calorimetry indicated that binding of the ligands to calf thymus DNA (5 mM Tris buffer, 50 mM NaCl, pH 7.2, 25 degrees C) is exothermic. The binding constants increased from 1.5 x 10(4) to 1.7 x 10(6) M(-1) as a function of increase in the number of hydroxyethyl functions (0-4). DNA binding was accompanied by red-shifted absorption (approximately 630 cm(-1)), strong hypochromism (>65%), positive induced-circular dichroism bands, and negative linear dichroism signals. DNA binding, in general, increased the helix stabilities to a significant extent (DeltaT(m) approximately 7 degrees C, DeltaDeltaH approximately 3 kcal/mol, DeltaDeltaS approximately 6-20 cal/K.mol). The binding constants showed a strong correlation with the number of hydroxyethyl groups present on the anthracene ring system. Analysis of the binding data using the hydrophobicity parameter (Log P) showed a poor correlation between the binding affinity and hydrophobicity. This observation was also supported by a comparison of the affinities of probes carrying N-ethyl (Kb = 0.8 x 10(5) M(-1)) versus N-hydroxyethyl side chains (Kb = 5.5 x 10(5) M(-1)). These are the very first examples of a strong quantitative correlation between the DNA binding affinity of a probe and the number of hydroxyethyl groups present on the probe. These quantitative findings are useful in the rational design of new ligands for high-affinity binding to DNA.     

Synthesis and nucleic acid-binding properties of water-soluble porphyrins appending platinum(II) complexes.

We synthesized two water-soluble porphyrins appending platinum(II) complexes [alpha,beta-(4a) and alpha,alpha-(4b) 5,15-bis(2-trans-[PtCl(NH3)2]N-2-aminoethylaminocarbonylphenyl) 2,3,7,8,12,13,17,18-octamethylporphyrin] and studied their reactions with a variety of nucleic acids [disodium adenosine-5'-monophosphate (AMP), disodium guanosine-5'-monophosphate (GMP), disodium thymidine-5'-monophosphate (TMP), disodium cytidine-5'-monophosphate (CMP), synthetic polymer poly(dG)-poly(dC), poly(dA)-poly(dT)] by 1H-NMR, UV-vis and FAB-MS spectroscopies. Based on the denaturation experiments of synthetic nucleic acid polymers, we conclude that the presence of the porphyrins (5.6 microM) does not cause significant changes in the melting temperature of poly(dA)-poly(dT) (28 microM) (deltaT=1 degrees C) and shows reannealing. On the other hand, gradual melting of poly(dG)-poly(dC) (28 microM) occurs at a low temperature (deltaT= -27 degrees C) in the presence of the porphyrins (5.6 microM), and the solutions do not show reannealing phenomena. The results of UV-vis and 1H-NMR experiments revealed that the porphyrins bind to guanine bases and that the porphyrins bind to GMP more strongly than to the other nucleotides. The binding modes between the porphyrins and synthetic nucleic acids are affected more by the coordination of the nucleobase [poly(dG)-poly(dC)] to the Pt(II) in the porphyrins than by Coulomb and hydrophobic interactions.     

Electrical conductance of DNA molecules with varied density of itinerant pi electrons

The electrical transport of DNA is closely related to the density of itinerant pi electrons because of the strong electron-lattice interaction. The resistivities of two typical DNA molecules [poly(dG)-poly(dC) and lambda-DNA] with varied densities of itinerant pi electrons are calculated. It is found that the dependence of the resistivity on the density of itinerant pi electrons is symmetrical about the half-filling state of itinerant pi electrons in poly(dG)-poly(dC). At the half-filling state, the Peierls phase transition takes place and poly(dG)-poly(dC) has a large resistivity. When the density of itinerant pi electrons departs far from the half-filling state, the resistivity of poly(dG)-poly(dC) becomes small. For lambda-DNA, there is no Peierls phase transition due to the aperiodicity of its base pair arrangement. The resistivity of poly(dG)-poly(dC) decreases with increasing length of the molecular chain, but the resistivity of lambda-DNA increases with increasing length. The conducting mechanisms for poly(dG)-poly(dC) and a few lambda-DNA molecules with varied densities of itinerant pi electrons are analyzed.     

Chirality of silver nanoparticles synthesized on DNA

Silver nanocrystals grown on a poly(dG)-poly(dC) double stranded DNA scaffold displayed circular dichroism at their surface plasmon excitation band. This chiral plasmon signature was not observed in a control experiment where silver nanocrystals of similar size were produced without the DNA template and adsorbed to the DNA. It is concluded that the DNA templated Ag nanocrystals had a preferred structural handedness.     

Interpreting DNA vibrational circular dichroism spectra using a coupling model from two-dimensional infrared spectroscopy

Two-dimensional infrared spectroscopy was recently used to measure the vibrational couplings between carbonyl bonds located on DNA nucleobases (Krummel, A. T.; Mukherjee, P.; Zanni, M. T. J. Phys. Chem. B 2003, 107, 9165 and Krummel, A. T.; Zanni, M. T. J. Phys. Chem. B 2006, 110, 13991). Here, we extend the coupling model derived from these 2D IR experiments to simulate the vibrational absorption and vibrational circular dichroism (VCD) spectra of three double-stranded DNA oligomers: poly(dG)-poly(dC), poly(dG-dC), and dGGCC. Using this model, we determine that the VCD spectrum of A-form poly(dG)-poly(dC) is dominated by interactions between stacked bases, whereas the coupling between base pairs and stacked bases carries equal importance in the VCD spectrum of B-form poly(dG-dC). We also simulate the absorption and VCD spectra of dGGCC, which is a combination of A- and B-form configurations. These simulations give insight into the structural interpretation of VCD and absorption spectroscopies that have long been used to monitor DNA secondary structure and kinetics.     

Base-specific and enantioselective studies for the DNA binding of iron(II) mixed-ligand complexes containing 1,10-phenanthroline and dipyrido[3,2-a:2',3'-c]phenazine

Base specificity and enantioselectivity for the DNA binding of [Fe(phen)2(dppz)]2+ (phen=1,10-phenanthroline and dppz=dipyrido[3,2-a:2',3'-c]phenazine) have been studied by determining the equilibrium binding constant (Kb) of the iron(II) complex to calf thymus DNA (ct-DNA), poly[(dA-dT)2], poly[(dG-dC)2] and poly[(dI-dC)2] using spectrophotometric titration and by monitoring the CD spectral profile of the iron(II) complex in the presence and absence of different types of DNA using circular dichroism (CD) spectroscopy, respectively. It has been shown that [Fe(phen)2(dppz)]2+ prefers to intercalate into the A-T and I-C sequences of poly[(dA-dT)2] and poly[(dI-dC)2] rather than into the G-C sequences of poly[(dG-dC)2] or into the base pairs of ct-DNA. In contrast to previous reports, it is a surprising observation that the enantioselectivity of the DNA binding for [Fe(phen)2(dppz)]2+ is base-dependent in nature. The Delta-enantiomer of [Fe(phen)2(dppz)]2+ is preferentially intercalated into the base pairs of poly[(dG-dC)2] or ct-DNA as indicated by its CD spectral profiles. On the other hand, the Lambda-enantiomer of [Fe(phen)2(dppz)]2+ is favorably intercalated into poly[(dA-dT)2] or poly[(dI-dC)2] as suggested by the opposite CD spectral profile. This preferential binding of Lambda-[Fe(phen)2(dppz)]2+)for the A-T sequence may be attributed to the fact that the binding site for the A-T sequence is relatively facile and thus the steric effect caused by the ancillary (non-intercalated) phen ligands is alleviated. The degree of enantioselectivity represented by inversion constants (Kinv) decreases as the salt concentration in the solution increases, indicating that electrostatic interaction is also operating in the ct-DNA-binding events of the iron (II) complex.     

Electron attachment induced proton transfer in a DNA nucleoside pair: 2'-deoxyguanosine-2'-deoxycytidine

To elucidate electron attachment induced damage in the DNA double helix, electron attachment to the 2'-deoxyribonucleoside pair dG:dC has been studied with the reliably calibrated B3LYP/DZP++ theoretical approach. The exploration of the potential energy surface of the neutral and anionic dG:dC pairs predicts a positive electron affinity for dG:dC [0.83 eV for adiabatic electron affinity (EAad) and 0.16 eV for vertical electron affinity (VEA)]. The substantial increases in the electron affinity of dG:dC (by 0.50 eV for EAad and 0.23 eV for VEA) compared to those of the dC nucleoside suggest that electron attachment to DNA double helices should be energetically favored with respect to the single strands. Most importantly, electron attachment to the dC moiety in the dG:dC pair is found to be able to trigger the proton transfer in the dG:dC- pair, surprisingly resulting in the lower energy distonic anionic complex d(G-H)-:d(C+H).. The negative charge for the latter system is located on the base of dC in the dG:dC- pair, while it is transferred to d(G-H) in d(G-H)-:d(C+H)., accompanied by the proton transfer from N1(dG) to N3(dC). The low energy barrier (2.4 kcal/mol) for proton transfer from dG to dC- suggests that the distonic d(G-H)-:d(C+H). pair should be one of the important intermediates in the process of electron attachment to DNA double helices. The formation of the neutral nucleoside radical d(C+H). is predicted to be the direct result of electron attachment to the DNA double helices. Since the neutral radical d(C+H). nucleotide is the key element in the formation of this DNA lesion, electron attachment might be one of the important factors that trigger the formation of abasic sites in DNA double helices.     

Long-range assemblies on poly(dG-dC)2 and poly(dA-dT)2: phosphonium cationic porphyrins and the importance of the charge

The present paper describes synthesis and spectroscopic properties of novel cationic meso-tetraphenylporphyrins bearing two (trans) (P2) or three (P3) triphenylphosphonium substituents. The porphyrin aggregation in aqueous solutions is discussed in detail. Porphyrin binding to and self-organization onto long-range assemblies on poly(dA-dT)2 or poly(dG-dC)2 were probed by combination of absorption, fluorescence, circular dichroism (CD), transient and resonance light-scattering (RLS) techniques. The higher hydrophobicity of P2 is manifested by more extensive self-organization. Induced CD and intensive RLS indicate binding to the chiral environment on the nucleic acids exterior and exciton coupling between adjacent porphyrin moieties. The CD spectra of P2 on poly(dG-dC), and poly(dA-dT)2 suggest that the binding geometry is essentially independent of the base sequence. The fluorescence lifetime of about 4 ns was attributed to the long-range assembly. In the case of P3 the distinctly different CD spectra induced by GC or AT base-pair regions reveal that the number of the substituents determines how closely the porphyrin can approach the specific electronic environment on the nucleic acid exterior. The fluorescence lifetime of the P3 assembly is about 2 ns.     

Sequence-dependent interactions of cationic naphthalimides and polynucleotides

The binding interactions of three naphthalimide derivatives with heteropoly nucleic acids have been evaluated using fluorescence, absorption and circular dichroism spectroscopies. Mono- and bifunctionalized naphthalimides exhibit sequence-dependent variations in their affinity toward DNA. The heteropoly nucleic acids, [Poly(dA-dT)]2 and [Poly(dG-dC)]2, as well as calf thymus (CT) DNA, were used to understand the factors that govern binding strength and selectivity. Sequence selectivity was addressed by determining the binding constants as a function of polynucleotide composition according to the noncooperative McGhee-von Hippel binding model. Binding affinities toward [poly(dA-dT)](2) were the largest for spermine-substituted naphthalimides (Kb = 2-6 x 10(6) M(-1)). The association constants for complex formation between the cationic naphthalimides and [poly(dG-dC)]2 or CT DNA (58% A-T content) were 2-500 times smaller, depending on the naphthalimide-polynucleotide pair. The binding modes were also assessed using a combination of induced circular dichroism and salt effects to determine whether the naphthalimides associate with DNA through intercalative, electrostatic or groove-binding. The results show that the monofunctionalized spermine and pyridinium-substituted naphthalimides associate with DNA through electrostatic interactions. In contrast, intercalative interactions are predominant in the complex formed between the bifunctionalized spermine compound and all of the polynucleotides.     

三维有序金掺杂纳米TiO_2修饰电极用于抗肿瘤药物与DNA相互作用的研究

利用模板法在氧化铟锡(ITO)电极表面制备了三维有序多孔结构的金掺杂纳米Ti O2薄膜修饰电极(3DOM GTD/ITO),并在此修饰电极上成功固定小牛胸腺DNA(ct DNA),从而构建了一种新型的DNA生物传感器(DNA/3DOM GTD/ITO),并通过透射电镜(TEM)、扫描电镜(SEM)对修饰电极的表面形貌进行表征。采用电化学交流阻抗(EIS)法研究了ct DNA在3DOM GTD/ITO修饰电极表面的固定情况,结果表明,ct DNA已被成功地固定在3DOM GTD/ITO修饰电极表面。采用循环伏安法、微分脉冲伏安法等电化学方法研究了抗肿瘤药物槲皮素(Qu)在3DOM GTD/ITO修饰电极表面的电化学性质及与ct DNA的相互作用。结果表明,Qu在3DOM GTD/ITO修饰电极表面有1对准可逆的氧化还原峰,其氧化还原反应为2电子和2质子的转移过程。Qu可与固定在修饰电极上的ct DNA发生较强的结合作用,其结合常数(K)为3.61×106L/mol。循环伏安实验、紫外-可见吸收光谱、分子荧光光谱、圆二色性光谱均表明Qu与ct DNA之间的相互作用模式为嵌插作用。Qu与ct DNA的碱基结合具有序列选择性,对Qu与聚(d G-d C)及聚(d A-d T)的结合常数进行计算,得到结合常数比K(d G-d C)/K(d A-d T)=3.5,表明Qu与ct DNA发生嵌插作用时更倾向于结合在GC富集区域。     

DNA binding and cytotoxicity of ruthenium(II) and rhenium(I) complexes of 2-amino-4-phenylamino-6-(2-pyridyl)-1,3,5-triazine

[Ru(tBu2bpy)2(2-appt)](PF6)2 [1.(PF6)2, tBu2bpy = 4,4'-di-tert-butyl-2,2'-bipyridine, 2-appt = 2-amino-4-phenylamino-6-(2-pyridyl)-1,3,5-triazine] and [Re(CO)3(2-appt)Cl] (2) were prepared and characterized by X-ray crystal analysis. The binding of 1.(PF6)2 and 2 to calf thymus DNA (ct DNA) led to increases in the DNA melting temperature (Delta Tm = +12 degrees C), modest hypochromism (29% and 5% of the absorption bands at lambda max = 450 and 376 nm, respectively), and insignificant shifts in the absorption maxima. The binding constants of 1.(PF6)2 and 2 with ct DNA, as determined by absorption titration, are (8.9 +/- 0.5) x 104 and (3.6 +/- 0.1) x 104 dm3 mol-1, respectively. UV-vis absorption titration, DNA melting studies, and competition dialysis using synthetic oligonucleotides [poly(dA-dT)2 and poly(dG-dC)2] revealed that 1.(PF6)2 and 2 exhibit a binding preference for AT sequences. A modeling study on the interaction between 1 or 2 and B-DNA revealed that the minor groove is the most favored binding site and an extensive hydrogen-bonding network is formed. As determined by MTT assays, 1.(PF6)2 and 2 exhibited moderate cytotoxicities toward several human cancer cell lines (KB-3-1, HepG2, and HeLa), as well as a multi-drug-resistant cancer cell line (KB-V-1). According to confocal microscopic and flow cytometric studies, 1.(PF6)2 and 2 induced apoptosis (50-60%) in cancer cells with <5% necrosis detected.