Effect of photodynamic therapy mediated by hematoporphyrin derivatives on small cell lung cancer H446 cells and bronchial epithelial BEAS-2B cells.

To investigate the effects of photodynamic therapy (PDT) mediated by hematoporphyrin derivatives (HPD) on the proliferation of small cell lung cancer H446 cells and bronchial epithelial BEAS-2B cells. H446 cells and BEAS-2B cells were cultured in vitro with different concentrations of HPD(0, 5, 10, 12, 15, 20 µg/mL) for 4 h, and then irradiated with 630 nm laser with different energy densities (0, 25, 50, 75, 100 mW/cm2). Cell viability of H446 cells and BEAS-2B cells were detected by CCK8 assay. The cell apoptosis was observed with Annexin V-FTTC/PI double staining and Hoechst 33258. The RT-PCR examination was applied to detect the transcriptional changes of the mRNA of Bax、Bcl-2, and Caspase-9. The results of CCK8 showed that when the HPD was 15 µg/mL and the laser power density reached 50 mW/cm2, the cell viability was significantly decreased compared with the black control group. Hoechst 33258 staining showed that with the increase of HPD concentration, the cell density was reduced, and apoptotic cells increased. Flow cytometry assay revealed that the apoptotic rates of the HPD-PDT group of H446 cells and BEAS-2B cells were significantly different from those of the blank control group. The RT-PCR examination showed that the expression levels of Bax and Caspase-9 mRNA in the HPD-PDT group were up-regulated, while the expression levels of Bcl-2 mRNA were down-regulated significantly. HPD-PDT can inhibit H446 cells and BEAS-2B cells growth. The mechanism may be related to up-regulating the expression levels of Bax and Caspase-9 mRNA and down-regulating the expression levels of Bcl-2 mRNA.

Bisbenzimide compounds inhibit the replication of prototype and pandemic potential poxviruses.

We previously identified the bisbenzimide Hoechst 33342 (H42) as a potent multi-stage inhibitor of the prototypic poxvirus, the vaccinia virus (VACV), and several parapoxviruses. A recent report showed that novel bisbenzimide compounds similar in structure to H42 could prevent human cytomegalovirus replication. Here, we assessed whether these compounds could also serve as poxvirus inhibitors. Using virological assays, we show that these bisbenzimide compounds inhibit VACV spread, plaque formation, and the production of infectious progeny VACV with relatively low cell toxicity. Further analysis of the VACV lifecycle indicated that the effective bisbenzimide compounds had little impact on VACV early gene expression but inhibited VACV late gene expression and truncated the formation of VACV replication sites. Additionally, we found that bisbenzimide compounds, including H42, can inhibit both monkeypox and a VACV mutant resistant to the widely used anti-poxvirus drug TPOXX (Tecovirimat). Therefore, the tested bisbenzimide compounds were inhibitors of both prototypic and pandemic potential poxviruses and could be developed for use in situations where anti-poxvirus drug resistance may occur. Additionally, these data suggest that bisbenzimide compounds may serve as broad-activity antiviral compounds, targeting diverse DNA viruses such as poxviruses and betaherpesviruses.IMPORTANCEThe 2022 mpox (monkeypox) outbreak served as a stark reminder that due to the cessation of smallpox vaccination over 40 years ago, most of the human population remains susceptible to poxvirus infection. With only two antivirals approved for the treatment of smallpox infection in humans, the need for additional anti-poxvirus compounds is evident. Having shown that the bisbenzimide H33342 is a potent inhibitor of poxvirus gene expression and DNA replication, here we extend these findings to include a set of novel bisbenzimide compounds that show anti-viral activity against mpox and a drug-resistant prototype poxvirus mutant. These results suggest that further development of bisbenzimides for the treatment of pandemic potential poxviruses is warranted.

Calycosin inhibits triple-negative breast cancer progression through down-regulation of the novel estrogen receptor-α splice variant ER-α30-mediated PI3K/AKT signaling pathway.

BACKGROUND: Triple-negative breast cancer (TNBC) is a heterogeneous carcinoma characterized by the most aggressive phenotype among all breast cancer subtypes. However, therapeutic options for TNBC patients have limited clinical efficacy due to lack of specific target and efficient targeted therapeutics. AIM: To investigate the biological characteristics of a novel estrogen receptor (ER)-α splice variant ER-α30 in breast cancer cells, and its possible role in the anticancer effects of calycosin, a typical phytoestrogen derived from the herbal plant Astragalus membranaceus, against TNBC. This may also provide a better understanding of the inhibitory activity of calycosin on TNBC progression. METHODS: Breast cancer tissues and para-cancer tissues were collected and analyzed for the expression levels of ER-α30 using immunohistochemistry (IHC), and its expression in two TNBC cell lines (MDA-MB-231 and BT-549) was detected by western blot and qRT-PCR assays. Then the alteration of cell viability, apoptosis, migration, invasion and epithelial-mesenchymal transition (EMT) in response to overexpression or knockdown of ER-α30 was separately determined by CCK-8, Hoechst 33258, wound healing, transwell and western blot assays in two TNBC cell lines. Next, the anticancer effects of calycosin on MDA-MB-231 cells were evaluated through CCK-8, colony formation, flow cytometry, Hoechst 33258 and western blot assays, along with the role of ER-α30 in these effects and the possible downstream targets of ER-α30. In addition, the in vivo experiments were carried out using MDA-MB-231 xenograft model intraperitoneally treated with calycosin. The volume and weight of xenograft tumor were measured to evaluate the in vivo anticancer activities of calycosin, while the corresponding changes of ER-α30 expression in tumor tissues were detected by IHC. RESULTS: It was demonstrated that the novel ER-α splice variant ER-α30 was primarily distributed in the nucleus of TNBC cells. Compared with normal breast tissues, ER-α30 expression was found in significantly higher levels in breast cancer tissues of ER- and progesterone receptor (PR)-negative subtype, so did in TNBC cell lines (MDA-MB-231 and BT-549) when compared to normal breast cell line MCF10A. Moreover, ER-α30 overexpression strikingly enhanced cell viability, migration, invasion and EMT progression and reduced apoptosis in TNBC cells, whereas shRNA-mediated knockdown of ER-α30 revealed the opposite results. Notably, calycosin suppressed the expression of ER-α30 in a dose-dependent manner, accompanied with the inhibition of TNBC growth and metastasis. A similar finding was observed for the xenografts generated from MDA-MB-231 cells. The treatment with calycosin suppressed the tumor growth and decreased ER-α30 expression in tumor tissues. Furthermore, this inhibition by calycosin was more pronounced in ER-α30 knockdown cells. Meanwhile, we found a positive relationship between ER-α30 and the activity of PI3K and AKT, which could also be inactivated by calycosin treatment. CONCLUSION: For the first time, it is demonstrated that the novel estrogen receptor-α splice variant ER-α30 could function as pro-tumorigenic factor in the context of TNBC by participating in cell proliferation, apoptosis, invasion and metastasis, thus it may serve as a potential therapeutic target for TNBC therapy. Calycosin could reduce the activation of ER-α30-mediated PI3K/AKT pathway, thereby inhibited TNBC development and progression, suggesting that calycosin may be a potential therapeutic option for TNBC.

Molecular Mechanism of Ligand Binding to the Minor Groove of DNA.

Although DNA-ligand binding is pervasive in biology, little is known about molecular-level binding mechanisms. Using all-atom, explicit-solvent molecular dynamics simulations in conjunction with weighted ensemble (WE)-enhanced sampling, an ensemble of 2562 binding trajectories of Hoechst 33258 (H33258) to d(CGC AAA TTT GCG) was generated from which the binding mechanism was extracted. In particular, the electrostatic interaction between the positively charged H33258 and the negatively charged DNA backbone drives the formation of initial H33258-DNA contacts. After this initial contact, a hinge-like intermediate state is formed in which one end of H33258 inserts into the minor groove of DNA. Following hinge state formation is a concerted motion whereby the second end of H33258 swings into the minor groove and the spine of hydration along the minor groove causing dehydration. This study illustrates how WE-enhanced simulations of biomolecular ligation processes can offer novel mechanistic insights by generating ensembles of binding events.

Unraveling topoisomerase IA gate dynamics in presence of PPEF and its preclinical evaluation against multidrug-resistant pathogens.

Type IA topoisomerases maintain DNA topology by cleaving ssDNA and relaxing negative supercoils. The inhibition of its activity in bacteria prevents the relaxation of negative supercoils, which in turn impedes DNA metabolic processes leading to cell death. Using this hypothesis, two bisbenzimidazoles, PPEF and BPVF are synthesized, selectively inhibiting bacterial TopoIA and TopoIII. PPEF stabilizes the topoisomerase and topoisomerase-ssDNA complex, acts as an interfacial inhibitor. PPEF display high efficacy against ~455 multi-drug resistant gram positive and negative bacteria. To understand molecular mechanism of inhibition of TopoIA and PPEF, accelerated MD simulation is carried out, and results suggested that PPEF binds, stabilizes the closed conformation of TopoIA with -6Kcal/mol binding energy and destabilizes the binding of ssDNA. The TopoIA gate dynamics model can be used as a tool to screen TopoIA inhibitors as therapeutic candidates. PPEF and BPVF cause cellular filamentation and DNA fragmentation leading to bacterial cell death. PPEF and BPVF show potent efficacy against systemic and neutropenic mouse models harboring E. coli, VRSA, and MRSA infection without cellular toxicity.

Fragment-Based Design of Small Molecules to Study DNA Minor Groove Recognition.

DNA-protein interactions are ubiquitous in cellular processes. Impeding unwanted nucleic acid interactions and protein recognition have therapeutic implications. Therefore, new chemical scaffolds and studies related to their structural basis of nucleic acid recognition are essential for developing high-affinity DNA binders. In this study, we have employed a fragment-based strategy to design and synthesize benzimidazole-guanidinium hybrid compounds and study the individual fragment's role in imparting selectivity and specificity in DNA recognition. The fragments were extensively studied using thermal denaturation, circular dichroism, UV-vis absorption spectroscopy, and molecular docking techniques. The results indicate an interdependent role of the benzimidazole core, polar ends, and the DNA composition in imparting sequence-selective binding of the benzimidazole-guanidinium hybrid compounds in the DNA minor groove. Circular dichroism and molecular docking studies indicated minor groove binding analogous to classical minor groove binders such as DAPI and Hoechst 33258. Thermal denaturation studies indicated that the best binder (compound 8) gave similar thermal stabilization to B-DNA as given by DAPI.

Saikosaponin A Inhibits Growth of Human Bladder Carcinoma T24 and 5637 Cells Both in Vitro and in Vivo.

Saikosaponin A (SSA)-a natural compound extracted from Radix bupleuri-possesses antitumor properties in several types of carcinomas. However, the role of SSA on bladder cancer and the mechanisms remain unclear. In this study, we have described the effect of SSA on human bladder cancer cell lines T24 and 5637 in the context of the regulation of mitochondrial pathways of apoptosis. In vitro, the Cell Counting Kit-8 (CCK-8) assay and cell wound healing assays were used to determine the proliferative effect of SSA treatment. Flow cytometry and Western blotting were performed to evaluate the apoptosis and related mechanisms. To further confirm that apoptosis is mediated through Caspase activation, Hoechst 33258 fluorescence staining assay was done after cells were treated with SSA and caspase inhibitor-Z-VAD-FMK. In vivo, an orthotopic xenograft mice model was adopted to evaluate the effect of SSA. The tumors were analyzed by hematoxylin-eosin (H&E) staining, immunohistochemical analysis, and Western blotting. In vitro, the results with CCK-8 assay showed obvious SSA-induced suppression in cell growth in a dose- and time-dependent manner. Flow cytometry analysis, Hoechst 33258 fluorescence staining assay and the assessment of the changes in the B-cell lymphoma 2 (Bcl-2) family protein expression level revealed that SSA could significantly induce cell apoptosis, which was associated with apoptosis via the mitochondrial pathways. In vivo, the results revealed a reduction in cell proliferation. In conclusion, our data suggest that SSA inhibits the growth of bladder cancer cells by activating the mitochondrial apoptosis pathway and inducing cell apoptosis.

Molecular spectroscopic and molecular simulation studies on the interaction of oral contraceptive drug Ormeloxifene with CT-DNA.

The interaction between oral contraceptive drug Ormeloxifene (ORM) and calf thymus DNA (CT-DNA) was studied using UV-Vis, fluorescence, circular dichroism (CD) and 1H NMR spectral techniques under physiological buffer (pH 7.4). Competitive binding assays with ethidium bromide (EB) and Hoechst 33258, viscosity measurements, KI quenching studies, molecular docking and metadynamics simulation studies were also substantiated the spectroscopic results. ORM is found to binds in the minor groove of CT-DNA as evidenced by: (1) non-displacement of EB from EB/CT-DNA complex; (2) appreciable displacement of Hoechst 33258 from its CT-DNA complex; (3) slight alteration in the CD signal; (4) small shifts (Δδ < 0.033 ppm) without broadening in 1H NMR signals and (5) the nearly equal extent of quenching of fluorescence of ORM by KI in the absence and presence of CT-DNA. Negative values of both enthalpy and entropy changes pointed out that the interaction between ORM and CT-DNA is governed mainly by H-bonding and van der Waals forces. Negative free energy change suggested a spontaneous interaction between ORM and CT-DNA. The free energy landscape of the binding process was computed using metadynamics simulation. The simulation study results disclosed that ORM binds to the minor groove of DNA through H-bonding and π-π stacking interactions. The results of molecular docking and simulation studies corroborate the available experimental data.

Study of complexes of Hoechst 33258 with poly(rA)-poly(rU) depending on various ionic strengths in the water-saline solution.

Comparative study of the complexes of groove-binding ligand Hoechst 33258 (H33258) with synthetic homopolynucleotides poly(rA)-poly(rU) and poly(dA)-poly(dT) has been carried out at various concentration ratios of r = ligand/nucleic acids (NA) and different ionic strengths of the water-saline solution 0.02, 0.04 and 0.1 M, using the method of UV-melting. It was revealed that the melting curves of the complexes of poly(dA)-poly(dT) with H33258 at the low concentrations of ligand are biphasic, which actually does not depend on the solution ionic strength. In the case of the complexes of poly(rA)-poly(rU)-H33258, the melting curves become quasi-biphasic only at the ionic strength 0.02 M and relatively high concentrations of the ligand. Differential melting curves (DMC) of the mentioned polynucleotides and their complexes with H33258 were obtained as well. DMC of poly(rA)-poly(rU) were found to be significantly wide at the ionic strengths of the solution 0.02 and 0.04 M and to show an intrinsic heterogeneity of double-stranded structure of this polynucleotide.Communicated by Ramaswamy H. Sarma.

Identification and characterization of bisbenzimide compounds that inhibit human cytomegalovirus replication.

The shortcomings of current anti-human cytomegalovirus (HCMV) drugs has stimulated a search for anti-HCMV compounds with novel targets. We screened collections of bioactive compounds and identified a range of compounds with the potential to inhibit HCMV replication. Of these compounds, we selected bisbenzimide compound RO-90-7501 for further study. We generated analogues of RO-90-7501 and found that one compound, MRT00210423, had increased anti-HCMV activity compared to RO-90-7501. Using a combination of compound analogues, microscopy and biochemical assays we found RO-90-7501 and MRT00210423 interacted with DNA. In single molecule microscopy experiments we found RO-90-7501, but not MRT00210423, was able to compact DNA, suggesting that compaction of DNA was non-obligatory for anti-HCMV effects. Using bioinformatics analysis, we found that there were many putative bisbenzimide binding sites in the HCMV DNA genome. However, using western blotting, quantitative PCR and electron microscopy, we found that at a concentration able to inhibit HCMV replication our compounds had little or no effect on production of certain HCMV proteins or DNA synthesis, but did have a notable inhibitory effect on HCMV capsid production. We reasoned that these effects may have involved binding of our compounds to the HCMV genome and/or host cell chromatin. Therefore, our data expand our understanding of compounds with anti-HCMV activity and suggest targeting of DNA with bisbenzimide compounds may be a useful anti-HCMV strategy.

In silico investigation of DNA minor groove binding bibenzimidazoles in the context of UVA phototherapy.

The versatility of DNA minor groove binding bibenzimidazoles extends to applications in cancer therapy, beyond their typical use as DNA stains. In the context of UVA phototherapy, a series of halogenated analogues designated ortho-, meta-, and para-iodoHoechst have been investigated. Phototoxicity involves dehalogenation of the ligands following exposure to UVA light, resulting in the formation of a carbon-centred radical. While the cytotoxic mechanisms have been well established, the nature and severity of DNA damage induced by the ortho-, meta-, and para-iodoHoechst isomers requires clarification. Our aims were to measure and compare the binding constants of iodoHoechst analogues, and to determine the proximity of the carbon-centred radicals formed following photodehalogenation to the C1', C4', and C5' DNA carbons. We performed molecular docking studies, as well as classical molecular dynamics simulations to investigate the interactions of Hoechst ligands with DNA including a well-defined B-DNA dodecamer containing the high affinity AATT minor groove binding site. Docking highlighted the binding of Hoechst analogues to AATT regions in oligonucleotides, nucleosomes, and origami DNA helical bundles. Further, MD simulations demonstrated the stability of Hoechst ligands in the AATT-containing minor groove over microsecond trajectories. Our findings reiterate that the efficiency of dehalogenation per se, rather than the proximity of the carbon-centred radicals to the DNA backbone, is responsible for the extreme phototoxicity of the ortho- isomer compared to the meta- and para-iodoHoechst isomers. More generally, our analyses are in line with the potential utility of ortho-iodoHoechst in DNA-targeted phototherapy, particularly if combined with a cell-specific delivery system.

Biological evaluation of antiproliferative and anti-invasive properties of an androstadiene derivative on human cervical cancer cell lines.

Gynaecological cancers are leading cause of death: breast cancer is the most frequently diagnosed type of malignancies, and cervical neoplasms rank fourth for both incidence and mortality among women worldwide. In one of our previous studies, favourable antiproliferative and antimetastatic properties of a newly synthesized androstane derivative, 17APAD have been demonstrated on breast cancer cell lines with different expression patterns of hormone receptors. The aim of the current study was to investigate the antitumoral potential of this molecule in cervical cancer cell lines, including SiHa cells positive for human papilloma virus (HPV) type 16 and HPV-negative C33A cells. 17APAD exerted pronounced growth-inhibition (with IC50 values ranging from 0.76 to 1.72 µM with considerable cancer selectivity), while cisplatin used as a reference agent yielded higher IC50 values (ranging from 3.69 to 12.43) and less selectivity, as evidenced by MTT assay. The proapoptotic effect and morphological changes induced by 17APAD were detected by Hoechst 33258-propidium iodide or Annexin V-Alexa488-propidium iodide fluorescent double staining methods, supplemented with a caspase-3 activity assay to identify the mechanism behind the programmed cell death induced by 17APAD. Additionally, significant and concentration-dependent elevation of the ratio of cells in the G2/M phase, on the expense of G0/G1 phase, was observed after 48 h of exposure to 17APAD. Besides its potent antiproliferative properties against both cervical cancer cell lines, 17APAD elicited a remarkable inhibition of cell migration and invasion as detected in wound-healing and Boyden chamber assays, respectively. The mechanisms of action underlying the effects of 17APAD on cell proliferation and motility were independent of androgenic activity, as demonstrated by the Yeast Androgen Screen method. Our results provide new evidence for the proapoptotic and anti-invasive properties of 17APAD, suggesting that it is worth of further research, as a promising prototype for designing novel anticancer agents.

Combining steady state and temperature jump IR spectroscopy to investigate the allosteric effects of ligand binding to dsDNA.

Changes in the structural dynamics of double stranded (ds)DNA upon ligand binding have been linked to the mechanism of allostery without conformational change, but direct experimental evidence remains elusive. To address this, a combination of steady state infrared (IR) absorption spectroscopy and ultrafast temperature jump IR absorption measurements has been used to quantify the extent of fast (∼100 ns) fluctuations in (ds)DNA·Hoechst 33258 complexes at a range of temperatures. Exploiting the direct link between vibrational band intensities and base stacking shows that the absolute magnitude of the change in absorbance caused by fast structural fluctuations following the temperature jump is only weakly dependent on the starting temperature of the sample. The observed fast dynamics are some two orders of magnitude faster than strand separation and associated with all points along the 10-base pair duplex d(GCATATATCC). Binding the Hoechst 33258 ligand causes a small but consistent reduction in the extent of these fast fluctuations of base pairs located outside of the ligand binding region. These observations point to a ligand-induced reduction in the flexibility of the dsDNA near the binding site, consistent with an estimated allosteric propagation length of 15 Å, about 5 base pairs, which agrees well with both molecular simulation and coarse-grained statistical mechanics models of allostery leading to cooperative ligand binding.

Quantitative spectrofluorometric assay detecting nuclear condensation and fragmentation in intact cells.

At present, nuclear condensation and fragmentation have been estimated also using Hoechst probes in fluorescence microscopy and flow cytometry. However, none of the methods used the Hoechst probes for quantitative spectrofluorometric assessment. Therefore, the aim of the present study was to develop a spectrofluorometric assay for detection of nuclear condensation and fragmentation in the intact cells. We used human hepatoma HepG2 and renal HK-2 cells cultured in 96-well plates treated with potent apoptotic inducers (i.e. cisplatin, staurosporine, camptothecin) for 6-48 h. Afterwards, the cells were incubated with Hoechst 33258 (2 µg/mL) and the increase of fluorescence after binding of the dye to DNA was measured. The developed spectrofluorometric assay was capable to detect nuclear changes caused by all tested apoptotic inducers. Then, we compared the outcomes of the spectrofluorometric assay with other methods detecting cell impairment and apoptosis (i.e. WST-1 and glutathione tests, TUNEL, DNA ladder, caspase activity, PARP-1 and JNKs expressions). We found that our developed spectrofluorometric assay provided results of the same sensitivity as the TUNEL assay but with the advantages of being fast processing, low-cost and a high throughput. Because nuclear condensation and fragmentation can be typical markers of cell death, especially in apoptosis, we suppose that the spectrofluorometric assay could become a routinely used method for characterizing cell death processes.

DNA electrochemical biosensor for detection of Alicyclobacillus acidoterrestris utilizing Hoechst 33258 as indicator.

Alicyclobacillus acidoterrestris is an acidophilic and thermophilic bacterium present in the soil, often associated with the spoilage of acidic juices, such as orange juice. Their spores resist pasteurization and, when reactivated, modify the organoleptic properties of the juice, making it unsuitable for consumption, due mainly to production of guaiacol. Biosensors are detection devices that respond quickly and are easy to handle, with great potential for use in the juice production chain. In this context, this work reports an electrochemical genosensor for detection of A. acidoterrestris, based on a graphite electrode modified with electrochemically reduced graphene oxide, a polymer derived from 3-hydroxybenzoic acid and a specific DNA probe sequence complementary with the genomic DNA of A. acidoterrestris. Detection of the target was performed by monitoring the oxidation peak of the Hoechst 33258, a common DNA stainer. The genosensor detection limit was 12 ng mL-1 and it kept 77% of response after ten weeks, and a test showed that orange juice does not interfere with bacteria lysate detection. This biosensor is the first platform for electrochemical detection of the genomic DNA of A. acidoterrestris in the literature, and the first to use Hoechst 33258 as indicator with whole genomic DNA molecules.

Spectroscopic study of interaction of various GC-content DNA with Hoechst 33258 depending on Na+ concentration.

Communicated by Ramaswamy H. Sarma.

Fluorescence anisotropy studies on the Hoechst 33258-DNA interaction: the solvent effect.

Fluorescence anisotropy method was applied to characterize the interactions of DNA minor groove binder Hoechst 33258 with different solvents without and in the presence of DNA. It is important to study the interaction of small molecules with DNA for the purpose of better understanding the mechanism of their action, as well as to design novel and more effective compounds. Spectroscopic study of the ligand in different binary mixed solvents containing DMSO, alcohols and buffer was carried out. Studies were performed without and in the presence of DNA. Fluorescence anisotropy studies reveal the characteristics of Hoechst 33258 in different mixed solvents. The results show the strong dependence of the anisotropy of Hoechst 33258 on solvent content, viscosity and intermolecular interactions. Communicated by Ramaswamy H. Sarma.

DNA minor-groove binder Hoechst 33258 destabilizes base-pairing adjacent to its binding site.

Understanding the dynamic interactions of ligands to DNA is important in DNA-based nanotechnologies. By structurally tracking the dissociation of Hoechst 33258-bound DNA (d(CGCAAATTTGCG)2) complex (H-DNA) with T-jump 2D-IR spectroscopy, the ligand is found to strongly disturb the stability of the three C:G base pairs adjacent to A:T the binding site, with the broken base pairs being more than triple at 100 ns. The strong stabilization effect of the ligand on DNA duplex makes this observation quite striking, which dramatically increases the melting temperature and dissociation time. MD simulations demonstrate an important role of hydration water and counter cations in maintaining the separation of terminal base pairs. The hydrogen bonds between the ligand and thymine carbonyls are crucial in stabilizing H-DNA, whose breaking signal appearing prior to the complete dissociation. Thermodynamic analysis informs us that H-DNA association is a concerted process, where H cooperates with DNA single strands in forming H-DNA.

DNA sequence-specific ligands. XVIII. Synthesis, physico-chemical properties; genetic, virological, and biochemical studies of fluorescent dimeric bisbenzimidazoles DBPA(n).

A set of AT-specific fluorescent dimeric bisbenzimidazoles DBPA(n) with linkers of different lengths bound to DNA in the minor groove were synthesized and their genetic, virological, and biochemical studies were performed. The DBPA(n) were shown to be effective inhibitors of the histon-like protein H-NS, a regulator of the DNA transcription factor, as well as of the Aliivibrio logei Quorum Sensing regulatory system in E. coli cells. Their antiviral activity was tested in model cell lines infected with herpes simplex virus type I. Also, it was found that DBPA(n) could inhibit catalytic activities of HIV-1 integrase at low micromolar concentrations. All of the dimeric bisbenzimidazoles DBPA(n) manifested fluorescent properties, were well soluble in water, nontoxic up to concentrations of 200 µM, and could penetrate into nuclei followed by binding to DNA.

The Sister Chromatid Exchange (SCE) Assay.

A fully optimized staining method for detecting sister chromatid exchanges in cultured cells is presented. The method gives reproducibly robust quantitative results. Sister chromatid exchange is a classic toxicology assay for genotoxicity and for detecting alterations to the biochemistry underlying cellular homologous recombination. Growth of cells in the presence of 5'-bromo-deoxyuridine for two rounds of DNA replication followed by collecting metaphase spreads on glass slides, treatment with the UV-sensitive dye Hoechst 33258, long-wave UV light exposure, and Giemsa staining gives a permanent record of the exchanges.