Sequence Recognition in the Minor Groove of DNA by Covalently Linked Formamido Imidazole−Pyrrole−Imidazole Polyamides: Effect of H-Pin Linkage and Linker Length on Selectivity and Affinity

Thermodynamics and site stoichiometry of DNA binding by a large antiviral hairpin polyamide

PA1 (dIm-PyPyβPyPyPy-γ-PyPyβPyPyPyPyβ-Ta) is a large (14-ring) hairpin polyamide that was designed to recognize the DNA sequence 5'-W₂GW₇-3', where W is either A or T. As is common among the smaller 6-8-ring hairpin polyamides (PAs), it binds its target recognition sequence with low nM affinity. However, in addition to its large size, it is distinct from these more extensively characterized PAs in its high tolerance for mismatches and antiviral properties. In ongoing attempts to understand the basis for these distinctions, we conducted thermodynamics studies of PA1-DNA interactions. The temperature dependence of binding affinity was measured using TAMRA-labeled hairpin DNAs containing a single target sequence. PA1 binding to either an ATAT/TATA or an AAAA/TTTT pattern is consistently entropically driven. This is in contrast to the A/T pattern-dependent driving forces for DNA binding by netropsin, distamycin, and smaller hairpin polyamides. Analysis of the salt dependence of PA1-DNA binding reveals that within experimental error, there is no dependence on ionic strength, indicating that the polyelectrolyte effect does not contribute to PA1-DNA binding energetics. This is similar to that observed for smaller PAs. PA1-DNA recognition sequence binding stoichiometries were determined at both nM (fluorescence) and μM (circular dichroism) concentrations. With all sequences and under both conditions, multiple PA1 molecules bind the small DNA hairpin that contains only a single recognition sequence. Implications for these observations are discussed.

Sequence-specific DNA binding Pyrrole-imidazole polyamides and their applications

Pyrrole-imidazole polyamides (Py-Im polyamides) are cell-permeable compounds that bind to the minor groove of double-stranded DNA in a sequence-specific manner without causing denaturation of the DNA. These compounds can be used to control gene expression and to stain specific sequences in cells. Here, we review the history, structural variations, and functional investigations of Py-Im polyamides.

Novel diamino imidazole and pyrrole-containing polyamides: Synthesis and DNA binding studies of mono- and diamino-phenyl-ImPy*Im polyamides designed to target 5'-ACGCGT-3'

Pyrrole- and imidazole-containing polyamides are widely investigated as DNA sequence selective binding agents that have potential use as gene control agents. The key challenges that must be overcome to realize this goal is the development of polyamides with low molar mass so the molecules can readily diffuse into cells and concentrate in the nucleus. In addition, the molecules must have appreciable water solubility, bind DNA sequence specifically, and with high affinity. It is on this basis that the orthogonally positioned diamino/dicationic polyamide Ph-ImPy*Im 5 was designed to target the sequence 5'-ACGCGT-3'. Py* denotes the pyrrole unit that contains a N-substituted aminopropyl pendant group. The DNA binding properties of diamino polyamide 5 were determined using a number of techniques including CD, ΔT(M), DNase I footprinting, SPR and ITC studies. The effects of the second amino moiety in Py* on DNA binding affinity over its monoamino counterpart Ph-ImPyIm 3 were assessed by conducting DNA binding studies of 3 in parallel with 5. The results confirmed the minor groove binding and selectivity of both polyamides for the cognate sequence 5'-ACGCGT-3'. The diamino/dicationic polyamide 5 showed enhanced binding affinity and higher solubility in aqueous media over its monoamino/monocationic counterpart Ph-ImPyIm 3. The binding constant of 5, determined from SPR studies, was found to be 1.5 × 10(7)M(-1), which is ∼3 times higher than that for its monoamino analog 3 (4.8 × 10(6)M(-1)). The affinity of 5 is now approaching that of the parent compound f-ImPyIm 1 and its diamino equivalent 4. The advantages of the design of diamino polyamide 5 over 1 and 4 are its sequence specificity and the ease of synthesis compared to the N-terminus pyrrole analog 2.

Targeting Nucleic Acids using Dynamic Combinatorial Chemistry

Dynamic combinatorial chemistry (DCC) is a powerful method for the identification of novel ligands for the molecular recognition of receptor molecules. The method relies on self-assembly processes to generate libraries of compounds under reversible conditions, allowing a receptor molecule to select the optimal binding ligand from the mixture. However, while DCC is now an established field of chemistry, there are limited examples of the application of DCC to nucleic acids. The requirement to conduct experiments under physiologically relevant conditions, and avoid reaction with, or denaturation of, the target nucleic acid secondary structure, limits the choice of the reversible chemistry, and presents restrictions on the building block design. This review will summarize recent examples of applications of DCC to the recognition of nucleic acids. Studies with duplex DNA, quadruplex DNA, and RNA have utilized mainly thiol disulfide libraries, although applications of imine libraries, in combination with metal coordination, have been reported. The use of thiol disulfide libraries produces lead compounds with limited biostability, and hence design of stable analogues or mimics is required for many applications.

Targeting the ICB2 site of the topoisomerase IIalpha promoter with a formamido-pyrrole-imidazole-pyrrole H-pin polyamide

The synthesis, DNA binding characteristics and biological activity of an N-formamido pyrrole- and imidazole-containing H-pin polyamide (f-PIP H-pin, 2) designed to selectively target the ICB2 site on the topoIIalpha promoter, is reported herein. Thermal denaturation, circular dichroism, isothermal titration calorimetry, surface plasmon resonance and DNase I footprinting studies demonstrated that 2 maintained the selectivity of the unlinked parent monomer f-PIP (1) and with a slight enhancement in binding affinity (K(eq)=5 x 10(5)M(-1)) to the cognate site (5'-TACGAT-3'). H-pin 2 also exhibited comparable ability to inhibit NF-Y binding to 1, as demonstrated by gel shift studies. However, in stark contrast to monomer 1, the H-pin did not affect the up-regulation of topoisomerase IIalpha (topoIIalpha) in cells (Western blot), suggesting that the H-pin does not enter the nucleus. This study is the first to the authors' knowledge that reports such a markedly different cellular response between two compounds of almost identical binding characteristics.

[Design, synthesis and evaluation of polyamide-nucleoside hybrids and oligonucleotides conjugated hybrid as a novel gene expression control compound]

On the basis of reports that a minor groove binder pyrrolepolyamide can interfere with gene expression by the sequence-specific recognition of DNA, we expected that nucleoside bearing a pyrrolepolyamide would be able to regulate gene expression. Therefore, we designed and synthesized the pyrrolepolyamide-adenosine (Hybrid 1) and -2'-deoxyguanosine hybrids (Hybrid 2 and Hybrid 3) as lead compounds for gene expression control compounds. The pyrrolepolyamide frame of Hybrid 2 and Hybrid 3 combines at the 2-exocyclic amino group of the 2'-deoxyguanosine by a linker and the 2-exocyclic amino group of guanine exists in the minor groove side of the duplex. Hybrid 2 is the 2'-deoxyguanosine-pyrrolepolyamide hybrid using the 3-aminopropionyl linker, while Hybrid 3 uses the 3-aminopropyl linker. An evaluation of the DNA binding sequence selectivity was performed by analysis of T(m) values and CD spectra, using distamycin A as a contrast. Hybrid 3 has provided more excellent sequence-distinguishable ability than other hybrids and Distamycin A. Moreover, on the basis of these results, we synthesized oligonucleotides conjugated to Hybrid 4, which is stable under conditions of DNA oligonucleotide solid phase synthesis, arranged from Hybrid 3. From T(m) values and CD spectral analysis, it was found that oligonucleotides conjugating Hybrid 4 possess high recognition ability and very high binding ability for the DNA that includes the pyrrolepolyamide binding sequence.

Design, Synthesis, and Analysis of Minor Groove Binder Pyrrolepolyamide-2′-Deoxyguanosine Hybrids

Pyrrolepolyamide-2′-deoxyguanosine hybrids (Hybrid 2 and Hybrid 3) incorporating the 3-aminopropionyl or 3-aminopropyl linker were designed and synthesized on the basis of previously reported results of a pyrrolepolyamide-adenosine hybrid (Hybrid 1). Evaluation of the DNA binding sequence selectivity of pyrrolepolyamide-2′-deoxyguanosine hybrids was performed by CD spectral and T_m analyses. It was shown that Hybrid 3 possessed greater binding specificity than distamycin A, Hybrid 1 and Hybrid 2.

Sequence specific and high affinity recognition of 5'-ACGCGT-3' by rationally designed pyrrole-imidazole H-pin polyamides: thermodynamic and structural studies

Imidazole (Im) and Pyrrole (Py)-containing polyamides that can form stacked dimers can be programmed to target specific sequences in the minor groove of DNA and control gene expression. Even though various designs of polyamides have been thoroughly investigated for DNA sequence recognition, the use of H-pin polyamides (covalently cross-linked polyamides) has not received as much attention. Therefore, experiments were designed to systematically investigate the DNA recognition properties of two symmetrical H-pin polyamides composed of PyImPyIm (5) or f-ImPyIm (3e, f=formamido) tethered with an ethylene glycol linker. These compounds were created to recognize the cognate 5'-ACGCGT-3' through an overlapped and staggered binding motif, respectively. Results from DNaseI footprinting, thermal denaturation, circular dichroism, surface plasmon resonance and isothermal titration microcalorimetry studies demonstrated that both H-pin polyamides bound with higher affinity than their respective monomers. The binding affinity of formamido-containing H-pin 3e was more than a hundred times greater than that for the tetraamide H-pin 5, demonstrating the importance of having a formamido group and the staggered motif in enhancing affinity. However, compared to H-pin 3e, tetraamide H-pin 5 demonstrated superior binding preference for the cognate sequence over its non-cognates, ACCGGT and AAATTT. Data from SPR experiments yielded binding constants of 1.6x10(8)M(-1) and 2.0x10(10)M(-1) for PyImPyIm H-pin 5 and f-ImPyIm H-pin 3e, respectively. Both H-pins bound with significantly higher affinity (ca. 100-fold) than their corresponding unlinked PyImPyIm 4 and f-ImPyIm 2 counterparts. ITC analyses revealed modest enthalpies of reactions at 298 K (DeltaH of -3.3 and -1.0 kcal mol(-1) for 5 and 3e, respectively), indicating these were entropic-driven interactions. The heat capacities (DeltaC(p)) were determined to be -116 and -499 cal mol(-1)K(-1), respectively. These results are in general agreement with DeltaC(p) values determined from changes in the solvent accessible surface areas using complexes of the H-pins bound to (5'-CCACGCGTGG)(2). According to the models, the H-pins fit snugly in the minor groove and the linker comfortably holds both polyamide portions in place, with the oxygen atoms pointing into the solvent. In summary, the H-pin polyamide provides an important molecular design motif for the discovery of future generations of programmable small molecules capable of binding to target DNA sequences with high affinity and selectivity.

Modifying the N-terminus of polyamides: PyImPyIm has improved sequence specificity over f-ImPyIm

Seven N-terminus modified derivatives of a previously published minor-groove binding polyamide (f-ImPyIm, 1) were synthesized and the biochemical and biophysical chemistry evaluated. These compounds were synthesized with the aim of attaining a higher level of sequence selectivity over f-ImPyIm (1), a previously published strong minor-groove binder. Two compounds possessing a furan or a benzofuran moiety at the N-terminus showed a footprint of 0.5microM at the cognate ACGCGT site (determined by DNase I footprinting); however, the specificity of these compounds was not improved. In contrast, PyImPyIm (4) produced a footprint of 0.5microM but showed a superior specificity using the same technique. When evaluated by thermal melting experiments and circular dichroism using ACGCGT and the non-cognate AAATTT sequence, all compounds were shown to bind in the minor-groove of DNA and stabilize the cognate sequence much better than the non-cognate (except for the non-amido-compound that did not bind either sequence, as expected). PyImPyIm (4) was interesting as the DeltaT(m) for this compound was only 4 degrees C but the footprint was very selective. No binding was observed for this compound with a third DNA (non-cognate, ACCGGT). ITC studies on compound 4 showed exothermic binding with ACGCGT and no heat change was observed for titrating the compound to the other two DNA sequences. The heat capacity (DeltaC(p)) of the PIPI/ACGCGT complex calculated from the hydrophobic interactions and SASA calculations was comparable to the experimental value obtained from ITC (-146calmol(-1)K(-1)). SPR results provided confirmation of the sequence specificity of PyImPyIm (4), with a K(eq) value determined to be 7.1x10(6) M(-1) for the cognate sequence and no observable binding to AAATTT and ACCGGT. Molecular dynamic simulations affirmed that PyImPyIm (4) binds as a dimer in an overlapped conformation, and it fits snugly in the minor-groove of the ACGCGT oligonucleotide. PyImPyIm (4) is an especially interesting molecule, because although the binding affinity is slightly reduced, the specificity with respect to f-ImPyIm (1) is significantly improved.