Synthesis of Metallointercalator−DNA Conjugates on a Solid Support

Metallointercalator−DNA conjugates were prepared by amide bond formation between active esters on the nonintercalating ligands of transition metal complexes and primary amines presented at the 5‘ or the 3‘ termini of oligonucleotides attached to solid supports. The conjugates were liberated from the support by aminolysis and purified by HPLC on C18 or C4 stationary phases, which separates the two diastereomeric forms of the conjugates containing either the Λ or the Δ enantiomer of the octahedral metal complex. The coupling reaction proceeds with ∼75% conversion of the amino-terminated oligonucleotide into the conjugate; the isolated yield is ∼200 nmol for syntheses initiated on DNA-synthesis columns with a loading of 2 μmol. The conjugates were characterized by ultraviolet−visible and circular dichorism absorption spectroscopy, electrospray ionization mass spectrometry, enzymatic digestion, and polyacrylamide gel electrophoresis (PAGE). Oligonucleotides bearing [Rh(phi)_2(bpy‘)]^(3+) (phi = 9,10-phenanthrene quinone diimine; bpy‘ = 4-butyric acid-4‘-methyl bipyridyl) form 1:1 duplexes with the complementary strand, and the electrophoretic mobility under nondenaturating PAGE of duplexes containing Δ-Rh is notably different from duplexes containing Λ-Rh. High-resolution PAGE of DNA photocleavage reactions initiated by irradiation of the tethered Rh complexes reveal intercalation of the complex only near the tethered end of the duplex. Analogous DNA-binding properties were observed with [Rh(phi)_2(bpy‘)]^(3+) tethered to the 3‘ terminus. By combining the 3‘ and 5‘ modification strategies, a mixed-metal DNA conjugate containing both [Os(phen)(bpy‘)(Me_2-dppz)]^(2+) (Me_2-dppz = 7,8-dimethyldipyridophenazine) on the 3‘ terminus and [Rh(phi)_2(bpy‘)]^(3+) on the 5‘ terminus was prepared and isolated. Taken together, these strategies for preparing metallointercalator−DNA conjugates offer a useful approach to generate chemical assemblies to probe long-range DNA-mediated charge transfer where the redox initiator is confined to and intercalated in a well-defined binding site

Oxidative Charge Transfer To Repair Thymine Dimers and Damage Guanine Bases in DNA Assemblies Containing Tethered Metallointercalators

Potent oxidants which intercalate in DNA serve as tools to probe DNA-mediated electron-transfer reactions. A photoexcited rhodium intercalator, Rh(phi)_2DMB^(3+) (phi = 9,10-phenanthrenequinone diimine and DMB = 4,4‘-dimethyl-2,2‘-bipyridine), tethered to DNA, promotes both oxidative damage to 5‘-GG-3‘ doublets in DNA and the repair of thymine dimers from a remote site on the DNA duplex. DNA-mediated repair of a thymine dimer lesion by charge transfer from the tethered rhodium intercalator is quantitative, albeit with low photoefficiency, occurs in an intraduplex reaction over long range (36 Å), and requires that the intervening bases be paired. When both oxidative reactions, repair and oxidative damage, are monitored on the same duplex, competition is evident; the presence of both a 5‘-GG-3‘ site and the thymine dimer diminished the dimer repair efficiency by 20−40% and decreased damage at the 5‘-GG-3‘ sites 2-fold compared to similar sequences lacking either the guanine doublet or thymine dimer, respectively. In addition to damage at the 5‘-G of 5‘-GG-3‘ sites, we also observe oxidation at the 3‘-G of the 5‘-GTTG-3‘ tetrad only in the presence of thymine dimer. Overall, the yield of repaired thymine strand was at least 10 times higher than the yield of oxidized guanine in the same sequences. While the 5-GG-3‘ may represent the thermodynamically favored site for oxidative reaction, repair of the thymine dimer appears to be kinetically more favorable. Dipyridophenanzine (dppz) complexes of ruthenium(III), less potent oxidants which intercalate in DNA, oxidize 5‘-GG-3‘ doublets efficiently but cannot trigger the repair of the thymine dimer lesion. Oxidative damage to DNA from a distance, mediated by the DNA base pair stack, can, however, be utilized to probe the disruption in the base stack generated by the thymine dimer. The presence of the dimer does not diminish oxidation by a Ru(III) intercalator at a distal guanine doublet, suggesting that the disruption caused by the dimer does not block charge transfer through the DNA duplex. DNA-mediated electron-transfer reactions of metallointercalators therefore serve to illustrate important aspects of radical migration and its consequence with respect to reactions at a distance through the DNA base pair stack

Oxidative Thymine Dimer Repair in the DNA Helix

The metallointercalator Rh(phi)_2DMB^(3+) (phi, 9,10-phenanthrenequinone diimine; DMB, 4,4′-dimethyl-2,2′-bipyridine) catalyzed the repair of a thymine dimer incorporated site-specifically in a 16-base pair DNA duplex by means of visible light. This repair could be accomplished with rhodium noncovalently bound to the duplex and at long range (16 to 26 angstroms), with the rhodium intercalator tethered to either end of the duplex assembly. This long-range repair was mediated by the DNA helix. Repair efficiency did not decrease with increasing distance between intercalated rhodium and the thymine dimer, but it diminished with disruption of the intervening π-stack

Definitive Metabolite Identification Coupled with Automated Ligand Identification System (ALIS) Technology: A Novel Approach to Uncover Structure–Activity Relationships and Guide Drug Design in a Factor IXa Inhibitor Program

A potent and selective Factor IXa (FIXa) inhibitor was subjected to a series of liver microsomal incubations, which generated a number of metabolites. Using automated ligand identification system-affinity selection (ALIS-AS) methodology, metabolites in the incubation mixture were prioritized by their binding affinities to the FIXa protein. Microgram quantities of the metabolites of interest were then isolated through microisolation analytical capabilities, and structurally characterized using MicroCryoProbe heteronuclear 2D NMR techniques. The isolated metabolites recovered from the NMR experiments were then submitted directly to an <i>in vitro</i> FIXa enzymatic assay. The order of the metabolites’ binding affinity to the Factor IXa protein from the ALIS assay was completely consistent with the enzymatic assay results. This work showcases an innovative and efficient approach to uncover structure–activity relationships (SARs) and guide drug design via microisolation–structural characterization and ALIS capabilities

Discovery of Selective RNA-Binding Small Molecules by Affinity-Selection Mass Spectrometry

Recent advances in understanding the relevance of noncoding RNA (ncRNA) to disease have increased interest in drugging ncRNA with small molecules. The recent discovery of ribocil, a structurally distinct synthetic mimic of the natural ligand of the flavin mononucleotide (FMN) riboswitch, has revealed the potential chemical diversity of small molecules that target ncRNA. Affinity-selection mass spectrometry (AS-MS) is theoretically applicable to high-throughput screening (HTS) of small molecules binding to ncRNA. Here, we report the first application of the Automated Ligand Detection System (ALIS), an indirect AS-MS technique, for the selective detection of small molecule–ncRNA interactions, high-throughput screening against large unbiased small-molecule libraries, and identification and characterization of novel compounds (structurally distinct from both FMN and ribocil) that target the FMN riboswitch. Crystal structures reveal that different compounds induce various conformations of the FMN riboswitch, leading to different activity profiles. Our findings validate the ALIS platform for HTS screening for RNA-binding small molecules and further demonstrate that ncRNA can be broadly targeted by chemically diverse yet selective small molecules as therapeutics

Definitive Metabolite Identification Coupled with Automated Ligand Identification System (ALIS) Technology: A Novel Approach to Uncover Structure–Activity Relationships and Guide Drug Design in a Factor IXa Inhibitor Program

A potent and selective Factor IXa (FIXa) inhibitor was subjected to a series of liver microsomal incubations, which generated a number of metabolites. Using automated ligand identification system-affinity selection (ALIS-AS) methodology, metabolites in the incubation mixture were prioritized by their binding affinities to the FIXa protein. Microgram quantities of the metabolites of interest were then isolated through microisolation analytical capabilities, and structurally characterized using MicroCryoProbe heteronuclear 2D NMR techniques. The isolated metabolites recovered from the NMR experiments were then submitted directly to an <i>in vitro</i> FIXa enzymatic assay. The order of the metabolites’ binding affinity to the Factor IXa protein from the ALIS assay was completely consistent with the enzymatic assay results. This work showcases an innovative and efficient approach to uncover structure–activity relationships (SARs) and guide drug design via microisolation–structural characterization and ALIS capabilities

Novel Antibacterial Class

We report the discovery and characterization of a novel ribosome inhibitor (NRI) class that exhibits selective and broad-spectrum antibacterial activity. Compounds in this class inhibit growth of many gram-positive and gram-negative bacteria, including the common respiratory pathogens Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus, and Moraxella catarrhalis, and are nontoxic to human cell lines. The first NRI was discovered in a high-throughput screen designed to identify inhibitors of cell-free translation in extracts from S. pneumoniae. The chemical structure of the NRI class is related to antibacterial quinolones, but, interestingly, the differences in structure are sufficient to completely alter the biochemical and intracellular mechanisms of action. Expression array studies and analysis of NRI-resistant mutants confirm this difference in intracellular mechanism and provide evidence that the NRIs inhibit bacterial protein synthesis by inhibiting ribosomes. Furthermore, compounds in the NRI series appear to inhibit bacterial ribosomes by a new mechanism, because NRI-resistant strains are not cross-resistant to other ribosome inhibitors, such as macrolides, chloramphenicol, tetracycline, aminoglycosides, or oxazolidinones. The NRIs are a promising new antibacterial class with activity against all major drug-resistant respiratory pathogens