Solution structure of a wedge-shaped synthetic molecule at a two-base bulge site in DNA

The solution structure of the complex formed between an oligonucleotide containing a two-base bulge (5'-CACGCAGTTCGGAC.5'-GTCCGATGCGTG) and DDI, a designed synthetic agent, has been elucidated using high-resolution NMR spectroscopy and restrained molecular dynamic simulation. DDI, which has been found to modulate DNA strand slippage synthesis by DNA polymerase I [Kappen, L. S., Xi, Z., Jones, G. B., and Goldberg, I. H. (2003) Biochemistry 42, 2166-2173], is a wedge-shaped spirocyclic molecule whose aglycone structure closely resembles that of the natural product, NCSi-gb, which strongly binds to an oligonucleotide containing a two-base bulge. Changes in chemical shifts of the DNA upon complex formation and intermolecular NOEs between DDI and the bulged DNA duplex indicate that agent specifically binds to the bulge site of DNA. The benzindanone moiety of DDI intercalates via the minor groove into the G7-T8-T9.A20 pocket, which consists of a helical base pair and two unpaired bulge bases, stacking with the G7 and A20 bases. On the other hand, the dihydronaphthalenone and aminoglycoside moieties are positioned in the minor groove. The aminoglycoside, which is attached to spirocyclic ring, aligns along the A20T21G22 sequence of the nonbulged strand, while the dihydronaphthalenone, which is restrained by the spirocyclic structure, is positioned near the G7-T8-T9 bulge site. The aminoglycoside is closely aligned with the dihydronaphthalenone, preventing its intercalation into the bulge site. In the complex, the unpaired purine (G7) is intrahelical and stacks with the intercalating moiety of DDI, whereas the unpaired pyrimidine (T8) is extrahelical. The structure of the complex formed by binding of the synthetic agent to the two-base bulged DNA reveals a binding mode that differs in important details from that of the natural product, explaining the different binding specificity for the bulge sites of DNA. The structure of the DDI-bulged DNA complex provides insight into the structure-binding affinity relationship, providing a rational basis for the design of specific, high-affinity probes of the role of bulged nucleic acid structures in various biological processes.     

Stereochemical control of small molecule binding to bulged DNA: comparison of structures of spirocyclic enantiomer-bulged DNA complexes

The solution structure of the complex formed between an oligonucleotide containing a two-base bulge (5'-CACGCAGTTCGGAC.5'-GTCCGATGCGTG) and ent-DDI, a designed synthetic agent, has been elucidated using high-resolution NMR spectroscopy and restrained molecular dynamic simulation. Ent-DDI is a left-handed wedge-shaped spirocyclic molecule whose aglycone portion is an enantiomer of DDI, which mimics the spirocyclic geometry of the natural product, NCSi-gb, formed by base-catalyzed activation of the enediyne antibiotic neocarzinostatin. The benzindanone moiety of ent-DDI intercalates between the A6.T21 and the T9.A20 base pairs, overlapping with portions of the purine bases; the dihydronaphthalenone moiety is positioned in the minor groove along the G7-T8-T9 bulge sequence; and the aminoglycoside is in the middle of the minor groove, approaching A20 of the nonbulged strand. This alignment of ent-DDI along the DNA helical duplex is in the reverse direction to that of DDI. The aminoglycoside moiety of ent-DDI is positioned in the 3' direction from the bulge region, whereas that of the DDI is positioned in the 5' direction from the same site. This reverse binding orientation within the bulge site is the natural consequence of the opposite handedness imposed by the spirocyclic ring junction and permits the aromatic ring systems of the two spirocyclic enantiomers access to the bulge region. NMR and CD data indicate that the DNA in the DDI-bulged DNA complex undergoes a larger conformational change upon complex formation in comparison to the ent-DDI-bulged DNA, explaining the different binding affinities of the two drugs to the bulged DNA. In addition, there are different placements of the bulge bases in the helical duplex in the two complexes. One bulge base (G7) stacks inside the helix, and the other one (T8) is extrahelical in the DDI-bulged DNA complex, whereas both bulge bases in the ent-DDI-bulged DNA complex prefer extrahelical positions for drug binding. Elucidation of the detailed binding characteristics of the synthetic spirocyclic enantiomers provides a rational basis for the design of stereochemically controlled drugs for bulge binding sites.     

Induced formation of a DNA bulge structure by a molecular wedge ligand-postactivated neocarzinostatin chromophore

Our previous structure elucidation of the complexes of DNA and postactivated neocarzinostatin chromophore (NCS-chrom) compounds revealed two distinctly different binding modes of this antitumor molecule. A thorough understanding of these results will provide the molecular basis for the binding and DNA chain cleavage properties of NCS-chrom. NCSi-gb is one of the postactivated mimics of NCS-chrom which is formed under thiol-free conditions and is able to bind to DNA. This report describes the structure refinement of the NCSi-gb-bulge-DNA complex [Stassinopoulos, A., Jie, J., Gao, X., and Goldberg, I. H. (1996) Science 272, 1943-1946] and the NMR characterization of the free bulge-DNA and free NCSi-gb. These results reveal that the formation of the complex involves conformational changes in both the DNA and the ligand molecule. Of mechanistic importance for the NCS-chrom-DNA interaction, the two ring systems of the drug are brought closer to each other in the complex. This conformation correlates well with the previously observed marked enhancement of the formation of a DNA bulge cleaving species in the presence of bulge-DNA sequences, due to the promotion of the intramolecular radical quenching of the activated NCS-chrom. Interestingly, the binding of NCSi-gb promotes the formation of a bulge binding pocket; this was not found in the unbound DNA. NCS-chrom is unique among the enediyne antibiotics in its ability to undergo two different mechanisms of activation to form two different DNA binding and cleaving species. The two corresponding DNA complexes are compared. One, the bulge-DNA binder NCSi-gb, involves the major groove, and the second, the duplex binder NCSi-glu which is generated by glutathione-induced activation, involves the minor groove. Since the two NCS-chrom-related ligand molecules contain some common chemical structural elements, such as the carbohydrate ring, the striking differences in their DNA recognition and chain cleavage specificity provide insights into the fundamental principles of DNA recognition and ligand design.     

Interaction of bulged DNA with leucine-containing mimics of NCS-chrom

Synthesis of chiral spirocyclic helical compounds containing leucine that mimic the molecular architecture of the potent DNA bulge binder obtained from the natural product metabolite NCSi-gb has been accomplished. The interaction between the compounds and DNA was studied by circular dichroism (CD) method. The results suggested that the two synthetic diastereoisomers specifically targeted the bulge site of DNA and induced conformational change of bulged DNA greatly.     

Solution structure of an O6-[4-oxo-4-(3-pyridyl)butyl]guanine adduct in an 11 mer DNA duplex: evidence for formation of a base triplex

The pyridyloxobutylating agents derived from metabolically activated tobacco-specific nitrosamines can covalently modify guanine bases in DNA at the O(6) position. The adduct formed, O(6)-[4-oxo-4-(3-pyridyl)butyl]guanine ([POB]dG), results in mutations that can lead to tumor formation, posing a significant cancer risk to humans exposed to tobacco smoke. A combined NMR-molecular mechanics computational approach was used to determine the solution structure of the [POB]dG adduct within an 11mer duplex sequence d(CCATAT-[POB]G-GCCC).d(GGGCCATATGG). In agreement with the NMR results, the POB ligand is located in the major groove, centered between the flanking 5'-side dT.dA and the 3'-side dG.dC base pairs and thus in the plane of the modified [POB]dG.dC base pair, which is displaced slightly into the minor groove. The modified base pair in the structure adopts wobble base pairing (hydrogen bonds between [POB]dG(N1) and dC(NH4) amino proton and between [POB]dG(NH2) amino proton and dC(N3)). A hydrogen bond appears to occur between the POB carbonyl oxygen and the partner dC's second amino proton. The modified guanine purine base, partner cytosine pyrimidine base, and POB pyridyl ring form a triplex via this unusual hydrogen-bonding pattern. The phosphodiester backbone twists at the lesion site, accounting for the unusual phosphorus chemical shift differences relative to those for the control DNA duplex. The helical distortions and wobble base pairing induced by the covalent binding of POB to the O(6)-position of dG help explain the significant decrease of 17.6 degrees C in melting temperature of the modified duplex relative to the unmodified control.     

Solution structures of aminofluorene [AF]-stacked conformers of the syn [AF]-C8-dG adduct positioned opposite dC or dA at a template-primer junction.

A solution structural study has been undertaken on the aminofluorene-C8-dG ([AF]dG) adduct located at a single-strand-double-strand d(A1-A2-C3-[AF]G4-C5-T6-A7-C8-C9-A10-T11-C12-C13). d(G14-G15-A16-T17-G18-G19-T20- A21-G22-N23) 13/10-mer junction (N = C or A) using proton-proton distance restraints derived from NMR data in combination with intensity-based relaxation matrix refinement computations. This single-strand-double-strand junction models one arm of a replication fork composed of a 13-mer template strand which contains the [AF]dG modification site and a 10-mer primer strand which has been elongated up to the modified guanine with either its complementary dC partner or a dA mismatch. The solution structures establish that the duplex segment retains a minimally perturbed B-DNA conformation with Watson-Crick hydrogen-bonding retained up to the dC5.dG22 base pair. The guanine ring of the [AF]dG4 adduct adopts a syn glycosidic torsion angle and is displaced into the major groove when positioned opposite dC or dA residues. This base displacement of the modified guanine is accompanied by stacking of one face of the aminofluorene ring of [AF]dG4 with the dC5.dG22 base pair, while the other face of the aminofluorene ring is stacked with the purine ring of the nonadjacent dA2 residue. By contrast, the dC and dA residues opposite the junctional [AF]dG4 adduct site adopt distinctly different alignments. The dC23 residue positioned opposite the adduct site is looped out into the minor groove by the aminofluorene ring. The syn displaced orientation of the modified dG with stacking of the aminofluorene and the looped out position of the partner dC could be envisioned to cause polymerase stalling associated with subsequent misalignment leading to frameshift mutations in appropriate sequences. The dA23 residue positioned opposite the adduct site is positioned in the major groove with its purine ring aligned face down over the van der Waals surface of the major groove and its amino group directed toward the T6.A21 base pair. The Hoogsteen edge of the modified guanine of [AF]dG4 and the Watson-Crick edge of dA23 positioned opposite it are approximately coplanar and directed toward each other but are separated by twice the hydrogen-bonding distance required for pairing. This structure of [AF]dG opposite dA at a model template-primer junctional site can be compared with a previous structure of [AF]dG opposite dA within a fully paired duplex [Norman, D., Abuaf, P., Hingerty, B. E., Live, D. , Grunberger, D., Broyde, S., and Patel, D. J. (1989) Biochemistry 28, 7462-7476]. The alignment of the Hoogsteen edge of [AF]dG (syn) positioned opposite the Watson-Crick edge of dA (anti) has been observed for both systems with the separation greater in the case of the junctional alignment in the model template-primer system. However, the aminofluorene ring is positioned in the minor groove in the fully paired duplex while it stacks over the junctional base pair in the template-primer system. This suggests that the syn [AF]dG opposite dA junctional alignment can be readily incorporated within a duplex by a translation of this entity toward the minor groove.     

New complex of post-activated neocarzinostatin chromophore with DNA: bulge DNA binding from the minor groove

Neocarzinostatin (NCS-chrom), a natural enediyne antitumor antibiotic, undergoes either thiol-dependent or thiol-independent activation, resulting in distinctly different DNA cleavage patterns. Structures of two different post-activated NCS-chrom complexes with DNA have been reported, revealing strikingly different binding modes that can be directly related to the specificity of DNA chain cleavage caused by NCS-chrom. The third structure described herein is based on recent studies demonstrating that glutathione (GSH) activated NCS-chrom efficiently cleaves DNA at specific single-base sites in sequences containing a putative single-base bulge. In this structure, the GSH post-activated NCS-chrom (NCSi-glu) binds to a decamer DNA, d(GCCAGAGAGC), from the minor groove. This binding triggers a conformational switch in DNA from a loose duplex in the free form to a single-strand, tightly folded hairpin containing a bulge adenosine embedded between a three base pair stem. The naphthoate aromatic moiety of NCSi-glu intercalates into a GG step flanked by the bulge site, and its substituent groups, the 2-N-methylfucosamine carbohydrate ring and the tetrahydroindacene, form a complementary minor groove binding surface, mostly interacting with the GCC strand in the duplex stem of DNA. The bulge site is stabilized by the interactions involving NCSi-glu naphthoate and GSH tripeptide. The positioning of NCSi-glu is such that only single-chain cleavage via hydrogen abstraction at the 5'-position of the third base C (which is opposite to the putative bulge base) in GCC is possible, explaining the observed single-base cleavage specificity. The reported structure of the NCSi-glu-bulge DNA complex reveals a third binding mode of the antibiotic and represents a new family of minor groove bulge DNA recognition structures. We predict analogue structures of NCSi-R (R = glu or other substituent groups) may be versatile probes for detecting the existence of various structures of nucleic acids. The NMR structure of this complex, in combination with the previously reported NCSi-gb-bulge DNA complex, offers models for specific recognition of DNA bulges of various sizes through binding to either the minor or the major groove and for single-chain cleavage of bulge DNA sequences.     

Solution structure of the N-(deoxyguanosin-8-yl)-1-aminopyrene ([AP]dG) adduct opposite dA in a DNA duplex.

Solution structural studies have been undertaken on the aminopyrene-C(8)-dG ([AP]dG) adduct in the d(C5-[AP]G6-C7). d(G16-A17-G18) sequence context in an 11-mer duplex with dA opposite [AP]dG, using proton-proton distance and intensity restraints derived from NMR data in combination with distance-restrained molecular mechanics and intensity-restrained relaxation matrix refinement calculations. The exchangeable and nonexchangeable protons of the aminopyrene and the nucleic acid were assigned following analysis of two-dimensional NMR data sets on the [AP]dG.dA 11-mer duplex in H2O and D2O solution. The broadening of several resonances within the d(G16-A17-G18) segment positioned opposite the [AP]dG6 lesion site resulted in weaker NOEs, involving these protons in the adduct duplex. Both proton and carbon NMR data are consistent with a syn glycosidic torsion angle for the [AP]dG6 residue in the adduct duplex. The aminopyrene ring of [AP]dG6 is intercalated into the DNA helix between intact Watson-Crick dC5.dG18 and dC7.dG16 base pairs and is in contact with dC5, dC7, dG16, dA17, and dG18 residues that form a hydrophobic pocket around it. The intercalated AP ring of [AP]dG6 stacks over the purine ring of dG16 and, to a lesser extent dG18, while the looped out deoxyguanosine ring of [AP]dG6 stacks over dC5 in the solution structure of the adduct duplex. The dA17 base opposite the adduct site is not looped out of the helix but rather participates in an in-plane platform with adjacent dG18 in some of the refined structures of the adduct duplex. The solution structures are quite different for the [AP]dG.dA 11-mer duplex containing the larger aminopyrene ring (reported in this study) relative to the previously published [AF]dG.dA 11-mer duplex containing the smaller aminofluorene ring (Norman et al., Biochemistry 28, 7462-7476, 1989) in the same sequence context. Both the modified syn guanine and the dA positioned opposite it are stacked into the helix with the aminofluorene chromophore displaced into the minor groove in the latter adduct duplex. By contrast, the aminopyrenyl ring participates in an intercalated base-displaced structure in the present study of the [AP]dG.dA 11-mer duplex and in a previously published study of the [AP]dG.dC 11-mer duplex (Mao et al., Biochemistry 35, 12659-12670, 1996). Such intercalated base-displaced structures without hydrogen bonding between the [AP]dG adduct and dC or mismatched dA residues positioned opposite it, if present at a replication fork, may cause polymerase stalling and formation of a slipped intermediate that could produce frameshift mutations, the most dominant mutagenic consequence of the [AP]dG lesion.     

Minor groove binding of Co(III)meso-tetrakis(N-methylpyridinium-4-yl)porphyrin to various duplex and triplex polynucleotides

The binding site and the geometry of Co(III)meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (CoTMPyP) complexed with double helical poly(dA).poly(dT) and poly(dG).poly(dC), and with triple helical poly(dA).[poly(dT)](2) and poly(dC).poly(dG).poly(dC)(+) were investigated by circular and linear dichroism (CD and LD). The appearance of monomeric positive CD at a low [porphyrin]/[DNA] ratio and bisignate CD at a high ratio of the CoTMPyP-poly(dA).poly(dT) complex is almost identical with its triplex counterpart. Similarity in the CD spectra was also observed for the CoTMPyP-poly(dG).poly(dC) and -poly(dC).poly(dG).poly(dC)(+) complex. This observation indicates that both monomeric binding and stacking of CoTMPyP to these polynucleotides occur at the minor groove. However, different binding geometry of CoTMPyP, when bind to AT- and GC-rich polynucleotide, was observed by LD spectrum. The difference in the binding geometry may be attributed to the difference in the interaction between polynucleotides and CoTMPyP: in the GC polynucleotide case, amine group protrude into the minor groove while it is not present in the AT polynucleotide.     

Probing DNA bulges with designed helical spirocyclic molecules

Because bulged structures (unpaired bases) in nucleic acids are of general biological significance, it has been of interest to design small molecules as specific probes of bulge function. On the basis of our earlier work with the specific DNA bulge-binding metabolite obtained from the enediyne antitumor antibiotic neocarzinostatin chromophore (NCS-chrom), we have prepared three small helical spirocyclic molecules that most closely mimic the natural product. These wedge-shaped molecules resemble the natural product in having the sugar residue attached to the same five-membered ring system. In one instance, the sugar is aminoglucose in beta-glycosidic linkage, and in the other, two enantiomers have the natural sugar N-methylfucosamine in alpha-glycosidic linkage. All three analogues were found to interfere with bulge-specific cleavage by NCS-chrom and the ability of bulged DNA to serve as a template for DNA polymerase 1 in accordance with their binding affinities for DNA containing a two-base bulge. Comparable results were obtained with the analogues for the less efficiently cleaved three-base bulge DNA structures. In each situation, the enantiomers possessing the natural sugar in alpha-glycosidic linkage are the most potent inhibitors of the cleavage reaction. In the DNA polymerase reactions, again, the closest natural product mimics were the most effective in selectively impeding nucleotide extension at the bulge site, presumably by complex formation. These results demonstrate the potential usefulness of bulge-binding compounds in modifying DNA structure and function and support efforts to design and prepare reactive species of these molecules that can covalently modify bulged DNA.     

Solution structure of a five-adenine bulge loop within a DNA duplex.

The three-dimensional solution structure of a DNA molecule of the sequence 5'-d(GCATCGAAAAAGCTACG)-3' paired with 5'-d(CGTAGCCGATGC)-3' containing a five-adenine bulge loop (dA(5)-bulge) between two double helical stems was determined by 2D (1)H and (31)P NMR, infrared, and Raman spectroscopy. The DNA in both stems adopt a classical B-form double helical structure with Watson-Crick base pairing and C2'-endo sugar conformation. In addition, the two dG/dC base pairs framing the dA(5)-bulge loop are formed and are stable at least up to 30 degrees C. The five adenine bases of the bulge loop are localized at intrahelical positions within the double helical stems. Stacking on the double helical stem is continued for the first four 5'-adenines in the bulge loop. The total rise (the height) of these four stacked adenines roughly equals the diameter of the double helical stem. The stacking interactions are broken between the last of these four 5'-adenines and the fifth loop adenine at the 3'-end. This 3'-adenine partially stacks on the other stem. The angle between the base planes of the two nonstacking adenines (A10 and A11) in the bulge loop reflects the kinking angle of the global DNA structure. The neighboring cytosines opposite the dA(5)-bulge (being parts of the bulge flanking base pairs) do not stack on one another. This disruption of stacking is characterized by a partial shearing of these bases, such that certain sequential NOEs for this base step are preserved. In the base step opposite the loop, an extraordinary hydrogen bond is observed between the phosphate backbone of the 5'-dC and the amino proton of the 3'-dC in about two-thirds of the conformers. This hydrogen bond probably contributes to stabilizing the global DNA structure. The dA(5)-bulge induces a local kink into the DNA molecule of about 73 degrees (+/-11 degrees ). This kinking angle and the mutual orientation of the two double helical stems agree well with results from fluorescence resonance energy transfer measurements of single- and double-bulge DNA molecules.     

Solution structure of the (+)-cis-anti-benzo[a]pyrene-dA ([BP]dA) adduct opposite dT in a DNA duplex.

Minor adducts, derived from the covalent binding of anti-benzo[a]pyrene-7,8-dihydroxy-9,10-epoxide to cellular DNA, may play an important role in generating mutations and initiating cancer. We have applied a combined NMR-computational approach including intensity based refinement to determine the solution structure of the minor (+)-cis-anti-[BP]dA adduct positioned opposite dT in the d(C1-T2-C3-T4-C5-[BP]A6-C7-T8-T9-C10-C11). (d(G12-G13-A14-A15-G16-T17-G18-A19-G20+ ++-A21-G22) 11-mer duplex. The BP ring system is intercalated toward the 5'-side of the [BP]dA6 lesion site without disrupting the flanking Watson-Crick dC5.dG18 and [BP]dA6.dT17 base pairs. This structure of the (+)-cis-anti-[BP]dA.dT 11-mer duplex, containing a bay region benzo[a]pyrenyl [BP]dA adduct, is compared with the corresponding structure of the (+)-trans-anti-[BPh]dA.dT 11-mer duplex (Cosman et al., Biochemistry 32, 12488-12497, 1993), which contains a fjord region benzo[c]phenanthrenyl [BPh]dA adduct with the same R stereochemistry at the linkage site. The carcinogen intercalates toward the 5'-direction of the modified strand in both duplexes (the adduct is embedded within the same sequence context) with the buckling of the Watson-Crick [BP]dA6.dT17 base pair more pronounced in the (+)-cis-anti-[BP]dA.dT 11-mer duplex compared to its Watson-Crick [BPh]dA.dT17 base pair in the (+)-trans-anti-[BPh]dA.dT 11-mer duplex. The available structural studies of covalent polycyclic aromatic hydrocarbon (PAH) carcinogen-DNA adducts point toward the emergence of a general theme where distinct alignments are adopted by PAH adducts covalently linked to the N(6) of adenine when compared to the N(2) of guanine in DNA duplexes. The [BPh]dA and [BP]dA N(6)-adenine adducts intercalate their polycyclic aromatic rings into the helix without disruption of their modified base pairs. This may reflect the potential flexibility associated with the positioning of the covalent tether and the benzylic ring of the carcinogen in the sterically spacious major groove. By contrast, such an intercalation without modified base pair disruption option appears not to be available to [BP]dG N(2)-guanine adducts where the covalent tether and the benzylic ring are positioned in the more sterically crowded minor groove. In the case of [BP]dG adducts, the benzopyrenyl ring is either positioned in the minor groove without base pair disruption, or if intercalated into the helix, requires disruption of the modified base pair and displacement of the bases out of the helix.     

Design of Sequence-Specific DNA Binding Ligands That Use a Two-Stranded Peptide Motif for DNA Sequence Recognition

Abstract

The design and DNA binding activity of β-structure-forming peptides and netropsin-peptide conjugates are reported. It is found that a pair of peptides - S,S'-bis(Lys-Gly-Val-Cys-Val- NH-NH-Dns) - bridged by an S-S bond binds at least 10 times more strongly to poly(dG)?poly(dC) than to poly(dA)?poly(dT). This peptide can also discriminate between 5′-GpG-3′ and 5′-GpC-3′ steps in the DNA minor groove. Based on these observations, new synthetic ligands, bis-netropsins, were constructed in which two netropsin-like fragments were attached by means of short linkers to a pair of peptides - Gly-Cys-Gly- or Val-Cys-Val - bridged by S-S bonds. These compounds possess a composite binding specificity: the peptide chains recognize 5′-GpG-3′ steps on DNA, whereas the netropsin-like fragments bind preferentially to tuns of 4 AT base pairs. Our data indicate that combining the AT-base-pair specific properties of the netropsin-type structure with the 5′-GpG-3′-specific properties of certain oligopeptides offers a new approach to the synthesis of ligands capable of recognizing mixed sequences of AT- and GC-base pairs in the DNA minor groove. These compounds are potential models for DNA-binding domains in proteins which specifically recognize base pair sequences in the minor groove of DNA.     

Minor groove binding of Co(III)meso-tetrakis(N-methylpyridinium-4-yl)porphyrin to various duplex and triplex polynucleotides

The binding site and the geometry of Co(III)meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (CoTMPyP) complexed with double helical poly(dA)·poly(dT) and poly(dG)·poly(dC), and with triple helical poly(dA)·[poly(dT)]₂ and poly(dC)·poly(dG)·poly(dC)⁺ were investigated by circular and linear dichroism (CD and LD). The appearance of monomeric positive CD at a low [porphyrin]/[DNA] ratio and bisignate CD at a high ratio of the CoTMPyP-poly(dA)·poly(dT) complex is almost identical with its triplex counterpart. Similarity in the CD spectra was also observed for the CoTMPyP-poly(dG)·poly(dC) and -poly(dC)·poly(dG)·poly(dC)⁺ complex. This observation indicates that both monomeric binding and stacking of CoTMPyP to these polynucleotides occur at the minor groove. However, different binding geometry of CoTMPyP, when bind to AT- and GC-rich polynucleotide, was observed by LD spectrum. The difference in the binding geometry may be attributed to the difference in the interaction between polynucleotides and CoTMPyP: in the GC polynucleotide case, amine group protrude into the minor groove while it is not present in the AT polynucleotide.     

Stimulation on DNA triplet repeat strand slippage synthesis by the designed spirocycles

The designed simpler chiral spirocyclic helical compounds that mimic the molecular architecture of the DNA bulge binder NCSi-gb have been prepared. It has been found that the synthesized spirocyclic compounds have strong stimulation effect on DNA slippage synthesis. Their stimulation activities on DNA strand slippage suggest that they may bind to or induce the formation of a non Watson-Crick structure during in vitro replication of DNA triplet repeats.     

Interaction of the nonintercalative antitumour drugs SN-6999 and SN-18071 with DNA: influence of ligand structure on the binding specificity

Binding to DNA's of the non-intercalative ligands SN-6999 and SN-18071 has been studied by means of circular dichroism, UV absorption, thermal melting and for SN-6999 by viscosity measurements. Both antitumour drugs show a preference for dA.dT rich DNA's, but the base pair selectivity of SN-18071 is lower as indicated by some affinity to dG.dC containing duplex DNA. The dA.dT base pair specificity of SN-6999 is comparable to that of netropsin. It forms very stable complexes with dA.dT containing duplex DNA and competes with netropsin binding on DNA. The ligands SN-18071 and pentamidine are totally released from their complexes with poly(dA-dT).poly(dA-dT) by competitive netropsin binding. The results demonstrate that hydrogen bonding capacity of the ligand in addition to other factors strongly contribute to the base sequence specificity in the recognition process of the ligand with DNA. A binding model of SN-6999 with five dA.dT pairs in the minor groove of B-DNA is suggested.     

Binding of symmetrical cyanine dyes into the DNA minor groove

Optical methods, such as fluorescence, circular dichroism and linear flow dichroism, were used to study the binding to DNA of four symmetrical cyanine dyes, each consisting of two identical quinoline, benzthiazole, indole, or benzoxazole fragments connected by a trimethine bridge. The ligands were shown to form a monomer type complex into the DNA minor groove. The complex of quinoline-containing ligand with calf thymus DNA appeared to be the most resistant to ionic strength, and it did not dissociate completely even in 1 M NaCl. Binding of cyanine dyes to DNA could also be characterized by possibility to form ligand dimers into the DNA minor groove, by slight preference of binding to AT pairs, as well as by possible intercalation between base pairs of poly(dG)-poly(dC). The correlation found between the binding constants to DNA and the extent of cyanine dyes hydrophobicity estimated as the n-octanol/water partition coefficient is indicative of a significant role of hydrophobic interactions for the ligand binding into the DNA minor groove.     

Molecular probes of DNA bulges: functional assay and spectroscopic analysis

Bulged structures in DNA and RNA have been linked to biomolecular processes involved in numerous diseases, thus probes with affinity for these nucleic acid targets would be of considerable utility to chemical biologists. Herein, we report guided chemical synthesis of small molecules capable of binding to DNA bulges by virtue of their unique (spirocyclic) geometry. The agents, modeled on a natural product congener, show pronounced selectivity for specific bulged motifs and are able to enhance slipped DNA synthesis, a hallmark functional assay of bulge binding. Significantly, bulge-agent complexes demonstrate characteristic fluorescent signatures depending on bulge and flanking sequence in the oligo. It is anticipated that these signature patterns can be harnessed as molecular probes of bulged hotspots in DNA and RNA.     

In vitro expansion of DNA triplet repeats with bulge binders and different DNA polymerases

The expansion of DNA repeat sequences is associated with many genetic diseases in humans. Simple bulge DNA structures have been implicated as intermediates in DNA slippage within the DNA repeat regions. To probe the possible role of bulged structures in DNA slippage, we designed and synthesized a pair of simple chiral spirocyclic compounds [Xi Z, Ouyang D & Mu HT (2006) Bioorg Med Chem Lett 16, 1180-1184], DDI-1A and DDI-1B, which mimic the molecular architecture of the enediyne antitumor antibiotic neocarzinostatin chromophore. Both compounds strongly stimulated slippage in various DNA repeats in vitro. Enhanced slippage synthesis was found to be synchronous for primer and template. CD spectra and UV thermal stability studies supported the idea that DDI-1A and DDI-1B exhibited selective binding to the DNA bulge and induced a significant conformational change in bulge DNA. The proposed mechanism for the observed in vitro expansion of long DNA is discussed.     

A second-generation copper(II)-mediated metallo-DNA-base pair

Metal-dependent pairing of nucleobases represents an alternative DNA base pairing scheme. Our first-generation copper(II)-mediated pyridine-2,6-dicarboxylate (Dipic) and pyridine (Py) metallo-base pair has a stability comparable to the natural base pairs dA:dT and dC:dG but does not have the selectivity of the Watson Crick base pairs. In order to increase the selectivity of base pair formation, a second-generation metallo-base pair was generated consisting of a pyridine-2,6-dicarboxamide (Dipam) and a pyridine (Py) nucleobase. This new metallo-base pair is more stable than the natural base pairs dA:dT and dC:dG and highly selective against mispairing. In addition, incorporation of multiple metallo-base pairs into DNA results in the formation of stable duplexes demonstrating that hydrogen bonding base pairs can efficiently be replaced by metal-dependent base pairs at multiple sites in DNA.