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  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09B—ORGANIC DYES OR CLOSELY-RELATED COMPOUNDS FOR PRODUCING DYES, e.g. PIGMENTS; MORDANTS; LAKES
  • C09B23/00—Methine or polymethine dyes, e.g. cyanine dyes
  • C09B23/02—Methine or polymethine dyes, e.g. cyanine dyes the polymethine chain containing an odd number of >CH- or >C[alkyl]- groups
  • C09B23/04—Methine or polymethine dyes, e.g. cyanine dyes the polymethine chain containing an odd number of >CH- or >C[alkyl]- groups one >CH- group, e.g. cyanines, isocyanines, pseudocyanines
• • C—CHEMISTRY; METALLURGY
  • C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
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  • C12Q1/00—Measuring or testing processes involving enzymes, nucleic acids or microorganisms; Compositions therefor; Processes of preparing such compositions
  • C12Q1/68—Measuring or testing processes involving enzymes, nucleic acids or microorganisms; Compositions therefor; Processes of preparing such compositions involving nucleic acids
  • C12Q1/6844—Nucleic acid amplification reactions
  • C12Q1/686—Polymerase chain reaction [PCR]

Definitions

• the present invention relates to new cyanine dyes particularly suited for use in DNA sequencing in particular minor groove [poly(dA-dT)] 2 binders.
• Fluorogenic compounds can provide tremendous sensitivity due to large quantum emission yield upon excitation.
• a limitation is that there are not many fluorophores that give a high increase in fluorescence upon hybridisation or reaction with targets.
• Asymmetric cyanine dyes have achieved much interest due to their excellent nucleic acid staining properties. Upon binding to nucleic acids such dyes usually exhibit a large enhancement in fluorescence intensity 1 and are widely used as fluorescent markers for DNA in various contexts. 2"4
• the interaction between double stranded DNA and the asymmetric cyanine dyes TO and YO ( Figure 1) have been investigated spectroscopically in several studies and were found to bind by intercalation 5"7 in a nonspecific fashion. 8 They also bind strongly to single stranded DNA with a large accompanying increase in fluorescence intensity. 9 This makes the dyes less useful in studies where only a signal from double stranded DNA is desirable.
• fluorescent ligands binding in the minor groove instead of by intercalation that bind selectively to double and not to single stranded DNA, e.g. DAPI 10 and Hoechst- derivatives. 11
• these ligands have a DNA sequence selectivity, preferably for A/T-rich segments. 12
• the intercalating dyes exert a smaller perturbation of the DNA-duplex upon binding. This is valuable in studies where its critical that the DNA is not stretched out, for example in certain fluorescence microscopy studies.
• BEBO asymmetric cyanine dye
• Fluorescein and BODIPY are the most common fluorescent reporter groups for covalent labeling of proteins whereas DAPI, Hoechst and Cyanine dyes are the most common fluorophores for detection of nucleic acid.
• DAPI abbreviations: abbreviations: abbreviations: abbreviations: abbreviations: abbreviations: abbreviations: abbreviations: abbreviations: abbreviations: abbreviations: abbreviations: abbreviations: abbreviations: abbreviations: abbreviations: abbreviations: abbreviations: abbreviations: abbreviations: abbreviations: abbreviations
• a cyanine dye that bind in a more organised way may have high fluorescence increase upon hybridisation and thus, be a more sensitive fluorophores.
• the minor groove is a convenient site for attack since it is normally unoccupied by cellular compounds such as proteins. It is also a perfect complement to concave cationic dyes due to the negative electrostatic potential and the convex floor of the minor groove.
• Certain minor groove binders stabilise DNA duplexes and can work as regulators of DNA-protein function. As a consequence, the development of sequence-specific minor groove binders may generate new compounds with anticancer and/or antiviral properties and thus, serve as an alternative and complementary approach to the antisense oligonucleotide strategy.
• the minor groove binders stabilising effect upon DNA duplexes can be used in probes, consisting of a minor-groove ligand- nucleic acid conjugate, to increase the melting temperatures (Tm) of probe-DNA duplexes.
• Tm melting temperatures
• An increase of the Tm of probes will allow a more flexible assay design since the oligo in the probe can be shorter and still have an optimal Tm.
• Sequence selective minor groove binders also has mismatch discrimination.
• Nucleic acid probes with minor groove binders as reporter group should have an increased difference between the Tm of match and single-base mismatch nucleic acids than the corresponding probe with an intercalator as reporter group. Thereby increasing the discriminatory power of hybridisation assays.
• a useful feature of minor groove binders are a preference for double stranded DNA compared to single stranded DNA whereas intercalators usually has no preference for single or double stranded DNA. This feature results in that minor groove binder probes will have lower background fluorescence than probes with an intercalator and as a consequence, a greater signal-to-noise ratio upon hybridisation.
• dyes specific for duplex-DNA can be used for quantification of DNA in mixtures contaminated by RNA or single stranded DNA.
• cyanine dyes can have up to a 18.000-fold increase in fluorescence upon hybridisation which is almost 1000 times higher than the minor groove binders that are used today. Also the absorption and emission are easily tuned by varying the conjugated system in cyanine dyes. Thus, a cyanine dye substituted so that binding in the minor groove is govern but with the extraordinary fluorescence properties of the known cyanine dyes retained seems to be a highly interesting target compound.
• a cyanine dye binding in the groove of DNA selected from the group of
• A_ and A 2 are each independently O, S, or N, and R is H or a carbohydrate that may contain a hetero atom, and m is 0 to 5, and n is 0 to 5.
• the cyanine dye has R being methyl, or ethyl, and m being 1 and n being 0. In one embodiment the cyanine dye has R being methyl, or ethyl, and m being 1 and n being 0 and Ai and A 2 being S.
• the cyanine dye has R being methyl, or ethyl, and m being 1 and n being 0 and Ai and A 2 being O.
• cyanine dye has R being methyl, or ethyl, and m being 1 and n being 0 and Ai being S and A 2 being O.
• the cyanine dye has the pyridine/quinoline ring in 2-position.
• One aspect of the present invention provides a probe for nucleic acid hybridization comprising a cyanine dye according to the above.
• a further aspect of the present invention provides a method for carrying out a real-time PCR-reaction of a DNA template, wherein a fluorescent dye increasing its fluorescent reaction when it is looked in a minor groove position in a double stranded DNA is used, whereby the dye comprises at least 2 aromatic ring systems both comprising at least one nitrogen atom, which rings are linked by a alkine goup having up to four carbon atoms to form a conjugated bond, and the dye further comprises at least a third aromatic system linked thereto via a bond having a significant double string character, such as a single bond or a ethin bond, to provide a stiff conjugated system.
• the dye is an asymmetric cyanine dye.
• one of the cyanine residues contains S and/or O.
• the dye compound is crescent shaped.
• the dye is a derivative according to the general formulas given above.
• asymmetric cyanine dyes are prepared by condensation of two quaternary heterocyclic salts with a thiomethyl group acting as leaving group on one of the salts.
• the use of an alternative condensation method developed by Deligeorgiev et al 20 furnished a synthetic route to BEBO of only four steps starting from the commercially available 4-substituted aniline 1 (Scheme 1).
• Thiocyanation of the aniline 1 with potassium thiocyanate and bromine in DMF afforded the 2-aminobenzothiazole 2 in a 40 % yield.
• Figure 2 shows the flow linear dichroism (LD) spectra of BEBO and BO with different DNA.
• LD is defined as the difference in absorption of light polarized parallel and perpendicular to the macroscopic axis of orientation.
• the LD-spectra of oriented DNA- ligand complexes may be analysed in terms of angles that the electronic transition moments of the ligands make with the DNA-helix axis to provide information about binding geometries.
• the orientation of the DNA complexes was achieved using a flow Couette cell with outer rotating cylinder.
• Figure 4a shows the titration of poly-AT into BEBO with binding ratios R, defined as the total number of dye molecules per base, varying from 0.025 to 0.1.
• the CD signal for BO in presence of ctDNA was only weakly negative (data not shown) and this further illustrates the different binding mode of BEBO compared to BO.
• BEBO in particular its strict minor groove binding to poly-AT, give promise for the development of a new class of asymmetric cyanine dyes with a strong preference for minor groove binding and a large increase in fluorescence upon binding. Synthesis and studies of analogous dyes are underway and will be reported in due time.
• the dye 1 was prepared in four steps starting from the commercially available aniline 1. Thiocyanation of the 4-substituted aniline 1 with potassium thiocyanate and bromine in DMF afforded the 2-aminobenzothiazole 2 in a 40 % yield. Methylation and deprotonation of compound 2 proceeded in a total 70 % yield to produce the 2-imino-3- methylbenzothiazoline 3.
• the dye 5 was prepared in 20 % by melting compound 3 together with the pyridinium salt 4 at 160 °C under vacuum.
• 2-(Tii-n-butylstannyl)-benzothiuole (1) and 2-(Tri-n-butylstannyl)-benzoxuole (2) was prepared by treating benzothiazole and benzoxazole, respectively with n-BuLi at -78 °C, followed by addition of tii-n-butyltin chloride.
• 2-Methylthio-benzothiazole is simply brominated in acetic acid with FeCI 3 as catalyst.
• triphenylarsine as the palladium-ligand has been reported to show up to a 1100-fold increase in reaction rates, compared to triphenylphosphine.2" '1
• triphenylarsine was less effective than triphenylphosphine in the experiments performed. This may, ironically enough, depend on triphenylarsine's superiority as ligand, which makes Pd(O) more liable to oxidize and the catalyst far more air-sensitive than the one with triphenylphosphirle.
• a small contamination of air-oxygen in the reaction vessel nught substantially decrease the catalytic effect of tripherrylarsine-coordinated palladium, whereas the catalyst with phosphine-ligand is less affected.
• the metylated salts (12) and (14) were formed in 70% and 56% yields respectively. These salts were allowed to react with 1-methyl-quinolinium tosylate in dichloromethane to produce the desired dyes. The yields in the last step were 27% and 30% respectively.
• Fluorescence spectra were recorded using a SPEX fluorolog ⁇ 2 spectrofluorimeter.
• the LD and CD spectra were recorded on a JASCO-720 spectropolarimeter.
• the orientation of the DNA complexes was achieved using a flow Couette cell with outer rotating cylinder. All spectroscopic measurements were performed at 25 °C in 25 mM sodium phosphate buffer (pH 7.0).
• Aqueous solutions of BEBO and BO were typically obtained from 2 mM stock solutions in DMSO.
• [Poly (dA- dT)] 2 and [poly (dG-dC)] 2 ) were purchased as solutions in buffer from Pharmacia.
• Calf thymus DNA was purchased from Fluka. Commercial reagents were purchased from Sigma-Aldrich and used without further purification.
• the pyridinium salt 4 and the benzothiazolium salt 5 were prepared as previously reported. 23
• the 2-aminobenzothiazole 2 (0.3 g, 1.0 mmol) was dissolved in DMSO (2 ml). Methyl iodide (0.25 ml, 2.0 mmol) was added and the mixture was stirred at 110 °C for 17 hours. The mixture was cooled and poured into water. The precipitate formed was collected by filtration and washed with water to give the product as a yellow solid (0.38 g, 86 %).
• 2,4-Dibromo-acetanilide (2.2 g, 7.51 mmol) was dissolved in 10 ml benzene and phosphorus pentasulfide (3.34 g, 7.51 mmol) was added. The mixture was refluxed at 80°C. After a few minutes, a gummy solid was formed at the bottom of the flask. To suspend the solid and make stirring possible, an additional 20 ml of benzene was added and the mixture was swirled vigorously. After refluxing for 5.5 h, thin layer chromatography (TLC) on silica in chloroform suggested complete reaction and the heating was removed.
• TLC thin layer chromatography
• the minor groove-binding, asymmetric cyanine dye BEBO above has been evaluated using real-time PCR and compared with SYBR Green I.
• BEBO did not inhibit the PCR at low concentrations and the fluorescence increase upon binding to dsDNA was sufficient for real-time measurement on the instruments used. Background fluorescence was caused by aggregation and it was approximately twice that of SYBR Green at optimized concentrations.
• the fluorescence increase when binding to DNA was lower than for SYBR Green and caused a retardation of the curves and the Ct was delayed approximately 4 cycles compared to SYBR Green.
• BEBO has been used in this study in real-time PCR and compared with SYBR Green I.
• a dye binding to the minor groove of dsDNA doesn't perturb the DNA duplex like intercalating dyes, which could be useful in for example fluorescence microscopy studies.
• BEBO is an asymmetric cyanine dye and is designed with a curve shape complementary to the convex floor of the minor groove.
• the cyanine chromophore of BEBO is the same as that of BO.
• the shape is similar to other minor groove binding dyes such as Hoecht and DAPI, but BEBO shows a higher fluorescence increase when bound to DNA and absorbs at a higher, more convenient wavelength.
• Most minor groove binders and possibly also BEBO still intercalate in GC-rich regions, while in AT-regions it clearly binds to the minor groove.
• BEBO was supplied in a 5.8 mM stock solution in DMSO.
• Two real-time PCR instruments were used for the investigation: LightCycler from Roche and the Rotorgene from Corbett Research.
• a previously developed and optimized PCR-system was used, amplifying a 240 bp template from a stock of purified PCR-product.
• 100 ⁇ M BEBO and 100X SYBR Green stock solutions were prepared in DMSO. Absorption maximum for BEBO is 467 nm and emission at 492 nm.
• BEBO is an appropriate non-specific dsDNA-binding dye for use in real-time PCR.
• concentration range of optimal use for real-time PCR in the instruments used is 0.1-0.5 ⁇ M. Higher concentrations result in high unwanted background fluorescence while lower concentration than 0.05 ⁇ M does not give enough fluorescence increase.
• BEBO does not give rise to large inhibition the polymerase chain reaction in the lower range of the concentration interval mentioned above.
• a major disturbance of the reaction occurs at concentrations above 1 ⁇ M, where the PCR loses its specificity and only forms short, unspecific products, most likely primer dimers. Inhibition is observed at 0.4 ⁇ M, while 0.2 ⁇ M BEBO doesn't seem to inhibit the PCR to any great extent.
• the background fluorescence is caused by aggregation of BEBO, resulting in spontaneous fluorescence. This aggregation seems to accumulate as the PCR is running, indicated by a linear increase in background signal seen in figures 9, 11 and 13. At high dye concentrations, this phenomenon is also seen with SYBR Green (data not shown). To decrease the aggregation, which is virtually non-existent in ethanol or methanol, 15 % DMSO was added to the reaction. The background decreased significantly, but also resulted in loss of specificity in the PCR.
• Figure 6a-b Absorption spectra of BEBO free in buffer (A) and bound to calf thymus DNA ( ⁇ ) at R value of 0.02.
• FIG. 7 Flow LD spectra of BEBO complexed with: calf thymus DNA (top left), poly[dA-dT] 2 (bottom left), poly [dG-dC] 2 (bottom right), and BO complexed with calf thymus (top right).
• FIG. 9 BEBO dilution, raw data. Triplicates of five different concentrations of BEBO, positive and NTC. From top to bottom (left axis): 5 ⁇ M (brown), 2 ⁇ M (purple), 0.8 ⁇ M (green), 0.2 ⁇ M (blue) and 0.05 ⁇ M (red).
• BEBO vs SYBR Green, raw data. Triplicates with three 100-fold template dilutions. This figure shows the higher background fluorescence level for BEBO and the total fluorescence increase. Note the linear increase in background fluorescence for the BEBO samples.
• BEBO vs SYBR Green, normalized data. BEBO crosses approximately four cycles later than SYBR Green for the same template concentration.

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