Drug-DNA sequence-dependent interactions analysed by electric linear dichroism

Binding of biologically important molecules to DNA, probed using electro-fluorescence polarization spectroscopy

When small fluorescent tag molecules are bound with a single geometry to larger carrier molecules the fluorescence excited using polarized light may itself be partially polarized. If this is the case then alignment of the carrier molecules changes the fluorescence polarization. Electro-fluorescence polarization spectrometry (EFPS) makes use of the fluorescence polarization changes which occur when the carrier molecules are aligned by an electric field. EFPS allows the determination of the binding geometry of the small tag molecules to the larger carrier molecules. In this study, EFPS has been used to probe the interactions between DNA and a series of chromosomal stains and anti-neoplastic (anti-cancer) agents. Results are presented for calf thymus DNA, treated with the dyes acridine orange, ethidium bromide, proflavine, Hoechst 33258, Hoechst 33342, 4',6-diamidino-2-phenylindole and the anti-tumour agents, doxorubicin, daunomycin, actinomycin C and actinomycin V.

Use of electric linear dichroism and competition experiments with intercalating drugs to investigate the mode of binding of Hoechst 33258, berenil and DAPI to GC sequences

The drugs Hoechst 33258, berenil and DAPI bind preferentially to the minor groove of AT sequences in DNA. Despite a strong selectivity for AT sites, they can interact with GC sequences by a mechanism which remains so far controversial. The 2-amino group of guanosine represents a steric hindrance to the entry of the drugs in the minor groove of GC sequences. Intercalation and major groove binding to GC sites of GC-rich DNA and polynucleotides have been proposed for these drugs. To investigate further the mode of binding of Hoechst 33258, berenil and DAPI to GC sequences, we studied by electric linear dichroism the mutual interference in the DNA binding reaction between these compounds and a classical intercalator, proflavine, or a DNA-threading intercalating drug, the amsacrine-4-carboxamide derivative SN16713. The results of the competition experiments show that the two acridine intercalators markedly affect the binding of Hoechst 33258, berenil and DAPI to GC polynucleotides but not to DNA containing AT/GC mixed sequences such as calf thymus DNA. Proflavine and SN16713 exert dissimilar effects on the binding of Hoechst 33258, berenil and DAPI to GC sites. The structural changes in DNA induced upon intercalation of the acridine drugs into GC sites are not identically perceived by the test compounds. The electric linear dichroism data support the hypothesis that Hoechst 33258, berenil and DAPI interact with GC sites via a non-classical intercalation process.

Characterization of the binding of YO to [poly(dA-dT)]2 and [poly(dG-dC)]2, and of the fluorescent properties of YO and YOYO complexed with the polynucleotides and double-stranded DNA

The interaction between the fluorescent dye YO (oxazole yellow) and the alternating polynucleotides [poly(dA-dT)]2 [the duplex of alternating poly(dA-dT)] and [poly(dG-dC)]2 [the duplex of alternating poly(dG-dC)] has been studied with optical spectroscopic techniques including absorbance, flow linear dichroism, CD, and fluorescence measurements. The principal features of the spectra are very similar for the two polynucleotide solutions, showing that YO binds quite similarly to AT and GC base pairs. From a strongly negative reduced linear dichroism (LDr) in the dye absorption band, an induced negative CD, and transfer of energy from the bases to bound YO, we conclude that at low mixing ratios YO is intercalated in both [poly(dA-dT)]2 and [poly(dG-dC)]2. At higher mixing ratios an external binding mode starts to contribute, evidenced from the appearance of an exciton CD. The conclusion that YO binds in a similar way to AT and GC base pairs should be valid also for the dimer YOYO since its YO units have been found to bind to double-stranded (dsDNA) in the same way as the YO monomer. The fluorescence properties of YO and YOYO complexed with DNA or the polynucleotides have been characterized by studying the dependence of fluorescence intensity on temperature, mixing ratio, and ionic strength. The fluorescence intensity and fluorescence lifetime of YO-DNA decrease strongly with increasing mixing ratio, whereas the fluorescence intensity of YOYO-DNA shows a weaker dependence, indicating that the quantum yield depends on the distance between the YO chromophores on the DNA chain. Further, the fluorescence intensity of YO depends on the base sequence; the quantum yield and fluorescence lifetime for YO complexed with [poly(dG-dC)]2 are about twice as large as for YO complexed with [poly(dA-dT)]2. Measurements of excitation spectra at different mixing ratios and different emission wavelengths indicate that the fluorescence of the externally bound chromophores is negligible compared to the intercalated ones.

Intercalative interactions of ethidium dyes with triplex structures

The binding of phenanthridine dyes to triplex poly(dT)*poly(dA).poly(dT) and its precursor duplex poly(dA).poly(dT) is characterized using linear dichroism and circular dichroism spectroscopy, and thermal denaturation. The two monomeric dyes ethidium bromide and propidium iodide are shown to behave similarly to each other in intercalating into and stabilizing both the duplex and the triplex structures. However, contrary to expectations, the extra cationic side-chain of propidium iodide provides no significant extra stabilization of triplex compared with ethidium bromide, although propidium does stabilize the duplex more than ethidium. The monomeric dyes appear to have somewhat different binding geometries with the duplex and triplex polymers. The dimeric dye ethidium homodimer is found to bis-intercalate in the triplex as well as the duplex but, in contrast to the monomers, no variation in geometry between duplex and triplex is observed. However, although dimer stabilizes the duplex, it has no effect on the thermal stability of the triplex. This lack of binding preferentiality of the dimer for triplex compared with the monomeric dyes indicates greater constraints on the accommodation of a bis-intercalator in the triplex structure than in the duplex.

Stimulation of site-specific topoisomerase II-mediated DNA cleavage by an N-methylpyrrolecarboxamide-anilinoacridine conjugate: relation to DNA binding

The DNA binding properties and effects on topoisomerase II of MePyGA, an anilinoacridine derivative bearing an N-methylpyrrolecarboxamide unit at position 1', have been compared with those of its precursor glycylanilinoacridine and the structurally related antileukaemic drug amsacrine. Electric linear dichroism spectroscopy reveals that MePyGA intercalates its acridine chromophore between DNA base pairs with a preference for GC-rich sequences, whereas both its structural analogue lacking the N-methylpyrrole unit and amsacrine intercalate into DNA without any strong sequence preference. The effects of the test drug on the catalytic activities of topoisomerase II were studied in vitro using purified calf thymus enzyme and 32P-labeled DNA. MePyGA stabilizes the topoisomerase II-DNA covalent complex and stimulates the cutting of DNA at a subset of preexisting topoisomerase II cleavage sites. The removal of the N-methylpyrrole unit abolishes both the GC-preferential binding to DNA and the topoisomerase II-mediated DNA cleavage. MePyGA and amsacrine stimulate the cleavage of DNA by topoisomerase II at different places: cleavage stimulated by amsacrine is consistent with the expected adenine requirement at position +1 whereas the predominant sites of DNA cleavage stimulated by MePyGA contain a cytosine at position +/- 1. This is the first instance where an anilinoacridine derivative differing only by the nature of the substituent at position 1' has been found to affect the catalytic activity of topoisomerase II differently. The spectroscopic and biochemical data lead to the conclusion that two functional domains can be identified in MePyGA: its anilino group can be regarded as a skeletal core to which are connected (i) the tricyclic acridine moiety which represents the DNA-binding domain and (ii) the N-methylpyrrole moiety which constitutes the topoisomerase II-targeted domain. The structure of the substituent at position 1' of the anilinoacridine chromophore evidently determines the location of the sites of DNA cleavage by topoisomerase II. These findings provide guidance for the synthesis and development of new topoisomerase II-targeted antitumor anilinoacridine derivatives.

Mode of DNA binding of bis-benzimidazoles and related structures studied by electric linear dichroism

The binding mode of a series of bis-benzimidazole analogues of Hoechst 33258 to a variety of DNAs and polynucleotides has been investigated by electric linear dichroism. Two groups of compounds were examined: (i) benzoxazole and pyridoimidazole derivatives and (ii) pyridoimidazole analogs substituted with an N-alkoxyalkyl group either directed towards the minor groove or directed away from the minor groove. The ELD data indicate that the mode of binding of these drugs varies significantly with the sequence of the target DNA sequence. The DNA binding properties of these drugs are related to their topoisomerase inhibitory properties.

DNA recognition by intercalators and hybrid molecules

Experiments are described which probe the role of the 2-amino group of guanine as a critical determinant of the recognition of nucleotide sequences in DNA by specific ligands. Homologous samples of tyrT DNA substituted with inosine or 2,6-diaminopurine residues in place of guanosine or adenine respectively yield characteristically modified footprinting patterns when challenged with sequence-selective antibiotics such as echinomycin, actinomycin or netropsin. The capacity of small molecules to recognise particular DNA sequences is exploited in the 'combilexin' strategy to target small molecules to defined sites in DNA. A composite molecule containing a distamycin moiety linked to an intercalating ellipticine derivative has been synthesised and shown to bind tightly to DNA but without much sequence-selectivity. Refinement of this molecule based on predictions from molecular modelling has led to the synthesis of a second generation derivative bearing an additional positive charge: this new hybrid molecule is strongly selective for binding to AT-rich tracts in DNA.

Analysing DNA complexes by circular and linear dichroism

The application of linear and circular dichroism (LD and CD) in nucleic acid research is illustrated by recent results aimed at answering specific structural problems in the interaction of DNA with molecules of biological importance. We first consider the circumstances under which ligands, such as DAPI (4',6-diamidino-2-phenylindole), change their preferred binding mode in the minor groove to major groove binding or intercalation. As an extension of this problem we refer to the switch between groove binding and intercalation of structurally similar ligands such as ellipticines and trigonal ruthenium complexes. We also explore the use of LD and CD in the determination of the structure of the complex formed between the polynucleotide poly(dA) and the novel 'peptide nucleic acid', consisting of nucleic acid bases joined by a polyamide homomorphous with the deoxyribose-phosphate backbone of DNA. Finally, the structure and interaction of the recombination enzyme RecA with DNA is discussed, in particular the influence of the presence of intercalators, groove binders or covalent DNA adducts.

Interaction of two peptide-acridine conjugates containing the SPKK peptide motif with DNA and chromatin

The interaction between DNA and two peptide-acridine conjugates containing one (1) or two (2) moieties of the Ser-Pro-Lys-Lys (SPKK) minor groove-binding peptide motif has been studied by a combination of hydrodynamic, biochemical and spectroscopic methods including diffusion-enhanced luminescence energy transfer (DELET) measurements with a Tb(III) lanthanide chelate as donor. Viscometric titrations do not reveal any significant difference between the two hybrid molecules which both unwind (by about 15 degrees) and extend the DNA similarly. DELET measurements show that the acridinyl chromophore of compounds 1 and 2 is much more accessible than that of a simple monointercalating drug such as acridine orange or ethidium. The accessibility factor increases proportionally with the peptide length, reflecting the extent of perturbation imposed upon the intercalating chromophore by the binding to DNA of the peptide moiety of the hybrids. Experiments with the osmium tetroxide-bispyridine reagent indicate that the two hybrid compounds both affect the local conformation of DNA rendering certain thymine residues conspicuously accessible to the probe. The drug-induced sites of hyperreactivity towards OsO4 in DNA are very similar with the exception of a short run of three T residues which is attacked more strongly in the presence of tetrapeptide-acridine conjugate 1 than with the octapeptide-acridine conjugate 2. These results are fully in agreement with previous footprinting studies and support the view that a minimum of two SPKK motifs is required to mimic the AT-specific minor groove binding antibiotic netropsin. On the basis of the DNA-binding properties of these two peptide-acridine hybrids, we present DNA-binding models in which the acridinyl moiety of compound 1 protrudes slightly outside the double helix but remains more or less parallel to the plane of the base-pairs. In contrast, with compound 2, where the octapeptide SPKKSPKK is bound to the minor groove, we postulate that the chromophore lies only partially overlapped with the base pairs in the intercalation site and, in addition, the heterocyclic chromophore is significantly tilted with respect to the double helix axis. Electric linear dichroism and DELET measurements with chromatin reveal that the presence of histone proteins affects the intercalative binding of compound 2 while it has practically no effect on the binding of compound 1.