Diastereochemically controlled porphyrin dimer formation on a DNA duplex scaffold

DNA-porphyrin conjugates were designed and synthesized for the preparation of the conformationally controlled porphyrin dimer structures constructed on a d(GCGTATACGC)2. Porphyrin derivatives were introduced to the central TATpA sequence where p represents the phosphoramidate for the attachment of the free-base porphyrin (FbP) and zinc-coordinated porphyrin (ZnP), which allows contact of the two porphyrins in the minor groove. The porphyrin dimers were characterized using CD, UV-vis, steady-state, and time-resolved fluorescence spectroscopies, indicating that the porphyrins form face-to-face conformations. Also the co-facial conformation was confirmed by comparison with spectra of the non-self-complementary duplex containing one porphyrin moiety. Introduction of zinc into porphyrin moiety destabilized the duplex formation. Two diastereomers showed different thermal stabilities and affected the conformations of porphyrin dimers. The temperature-dependent assembly and the conformational change of the porphyrin dimer on the duplex DNA were observed in the UV-vis spectra, indicating that the dynamic movement of the porphyrin dimer occurs on the duplex. The results indicate that the porphyrin dimers of DNA-FbP conjugates are overlapped clockwise and are located in the minor groove of the usual B-form DNA backbone. The interaction and conformation of two porphyrin moieties are controlled by the following three factors: (1) temperature change during and after formation of the duplex porphyrins at lower temperature; (2) diastereochemistry of the phosphoramidates where porphyrins are connected via a linker; and (3) zinc ion coordination that destabilizes the interaction of porphyrins as well duplex formation.     

Synthesis of functionalised nucleosides for incorporation into nucleic acid-based serine protease mimics

The synthesis of nucleosides modified with an extra imidazole, carboxyl and hydroxyl group is described. These nucleosides can be incorporated into an oligonucleotide duplex, thus generating a novel type of serine protease mimic.     

Porphyrin-DNA conjugates: porphyrin induced adenine-guanine homoduplex stabilization and interduplex assemblies

DNA has found widespread uses as a nanosized scaffold for assembly of patterned multichomophoric nanostructures. Herein we report the synthesis, self-assembly, stability, and spectroscopic studies of short alternating non-self-complementary DNA sequences 5'-(dGdA)(4) and 5'-(dAdG)(4) with non-charged tetraarylporphyrins covalently linked to the 5' position of deoxyadenosine or deoxyguanosine via a phosphate or amide linker. The linker, the metal in the porphyrin coordination center, and the neighboring nucleobase have very distinct effects on the duplex formation of porphyrin-deoxyguanosine-deoxyadenosine oligodeoxynucleotides. At ionic strength between 5 mM and 40 mM, free base trispyridylphenylporphyrin appended to the 5' termini of 5'-(dAdG)(4) oligonucleotide via short non-polar amide linker served as a hydrophobic molecular cap inducing deoxyadenosine-deoxyguanosine antiparallel homoduplex. At ionic strength of ≥60 mM, the free base porphyrin functioned as a molecular 'glue' and induced the formation of porphyrin-DNA inter-homoduplex assemblies with characteristic tetrasignate CD Cotton effects in the porphyrin Soret band region. When the porphyrin cap was covalently attached to 5' position of deoxyguanosine or deoxyadenosine via charged phosphate linker, no significant deoxyadenosine-deoxyguanosine hybridization was observed even at elevated ionic strengths.     

Programmable conformational regulation of porphyrin dimers on geometric scaffold of duplex DNA

DNA-porphyrin conjugates were designed and synthesized in which free-base and Zn-coordinated porphyrins were introduced to the N⁶-position of the internal adenosine. Conformations of the porphyrin dimer in the major groove of duplex DNA were controlled by changing the orientation and the distance between the two porphyrin moieties. The porphyrin dimers formed a thermally favorable face-to-face conformation on the duplex DNA. In the disadvantageous geometry for porphyrin dimer formation on the duplex, the porphyrins induced the DNA duplex structures to the Z-form conformation. These results indicate that the interaction of the two porphyrins and the conformation of duplex DNA are controlled by the program of DNA sequences.     

2'-O-[2-(guanidinium)ethyl]-modified oligonucleotides: stabilizing effect on duplex and triplex structures

[structure: see text] Oligonucleotides with a novel 2'-O-[2-(guanidinium)ethyl] (2'-O-GE) modification have been synthesized using a novel protecting group strategy for the guanidinium group. This modification enhances the binding affinity of oligonucleotides to RNA as well as duplex DNA (DeltaT(m) 3.2 degrees C per modification). The 2'-O-GE modified oligonucleotides exhibited exceptional resistance to nuclease degradation. The crystal structure of a palindromic duplex formed by a DNA oligonucleotide with a single 2'-O-GE modification was solved at 1.16 A resolution.     

Porphyrins conjugated to DNA as CD reporters of the salt-induced B to Z-DNA transition

The porphyrin chromophore incorporated at the 5'-position of an oligonucleotide allows the simultaneous detection of the B- to Z-DNA transition via the porphyrin Soret band circular dichroism exciton couplet signal around 420 nm and the oligonucleotide CD region below 300 nm, at micromolar concentrations.     

Duplex and triplex formation of mixed pyrimidine oligonucleotides with stacking of phenyl-triazole moieties in the major groove

5-(1-Phenyl-1,2,3-triazol-4-yl)-2'-deoxycytidine was synthesized from a modified CuAAC protocol and incorporated into mixed pyrimidine oligonucleotide sequences together with the corresponding 5-(1-phenyl-1,2,3-triazol-4-yl)-2'-deoxyuridine. With consecutive incorporations of the two modified nucleosides, improved duplex formation with a complementary RNA and improved triplex formation with a complementary DNA duplex were observed. The improvement is due to π-π stacking of the phenyl-triazole moieties in the major groove. The strongest stacking and most pronounced positive influence on thermal stability was found in between the uridine analogues or with the cytidine analogue placed in the 3' direction to the uridine analogue. Modeling indicated a different orientation of the phenyl-triazole moieties in the major groove to account for the difference between the two nucleotides. The modified oligonucleotides were all found to be significantly stabilized toward nucleolytic degration.     

Automated synthesis of 3'-metalated oligonucleotides

We report the first synthesis of a metallonucleoside bound to a solid support and subsequent oligonucleotide synthesis with this precursor. Large-scale syntheses of metal-containing oligonucleotides are achieved using a solid support modified with [Ru(bpy)(2)(impy')](2+) (bpy is 2,2'-bipyridine; impy' is 2'-iminomethylpyridyl-2'-deoxyuridine). A duplex formed with the metal-containing oligonucleotide exhibits superior thermal stability when compared to the corresponding unmetalated duplex (T(m) = 50 degrees C vs T(m) = 48 degrees C). Electrochemical (E(1/2) = 1.3 V vs NHE), absorption (lambda(max) = 480 nm), and emission (lambda(max) = 720 nm, tau = 44 ns, Phi = 0.11 x 10(-)(3)) data for the ruthenium-modified oligonucleotides indicate that the presence of the oligonucleotide does not perturb the electronic properties of the ruthenium complex. The absence of any change in the emission properties upon duplex formation suggests that the [Ru(bpy)(2)(impy)](2+) chromophore will be a valuable probe for DNA-mediated electron-transfer studies. Despite the relatively high Ru(III/II) reduction potential, oxidative quenching of photoexcited [Ru(bpy)(2)(impy)](2+) does not lead to oxidative damage of guanine or other DNA bases.     

A porphyrin C-nucleoside incorporated into DNA

[reaction: see text] A free porphyrin coupled on 2-deoxy-D-ribose was synthesized and incorporated into DNA via phosphoramidite chemistry. Substitution at the ends of a 5'-modified self-complementary duplex was found to be thermally and thermodynamically stabilizing. The porphyrin moiety strongly intercalates in the duplex when located near the center, and retains its fluorescence properties in DNA.     

Molecular dynamics simulation and binding energy calculation for estimation of oligonucleotide duplex thermostability in RNA-based therapeutics

For oligonucleotide-based therapeutics, a thorough understanding of the thermodynamic properties of duplex formation is critical to developing stable and potent drugs. For unmodified small interfering RNA (siRNA), DNA antisense oligonucleotide (AON) and locked nucleic acid (LNA), DNA/LNA modified oligonucleotides, nearest neighbor (NN) methods can be effectively used to quickly and accurately predict duplex thermodynamic properties such as melting point. Unfortunately, for chemically modified olignonucleotides, there has been no accurate prediction method available. Here we describe the potential of estimating melting temperature (T(m)) for nonstandard oligonucleotides by using the correlation of the experimental T(m) with the calculated duplex binding energy (BE) for oligonucleotides of a given length. This method has been automated into a standardized molecular dynamics (MD) protocol through Pipeline Pilot (PP) using the CHARMm component in Discovery Studio (DS). Results will be presented showing the correlation of the predicted data with experiment for both standard and chemically modified siRNA and AON.     

Oligonucleotides containing new fluorescent 1-phenylethynylpyrene and 9,10-bis(phenylethynyl)anthracene uridine-2′-carbamates: synthesis and properties

The synthesis of two novel fluorescent uridine-2′-carbamate phosphoramidites is described. The reagents carrying fluorescent polyaromatic hydrocarbons 1-phenylethynylpyrene (PEPy) or 9,10-bis(phenylethynyl)anthracene (BPEA) are suitable for oligonucleotide synthesis. Prepared oligonucleotide conjugates show strong dye emissions at 401 and 485 nm, but low FRET rate when located in the oligonucleotide duplex. The dyes show considerable compensation of the usual carbamate duplex destabilization. The possible explanation of both effects is binding of PEPy and BPEA to the minor groove of the DNA duplex.     

Functionalization of 2″‐C‐(Piperazinomethyl)‐2′,3′‐BcNA (Bicyclic Nucleic Acids) with Pyren‐1‐ylcarbonyl Units

Herein, we describe the incorporation of 2″‐C‐(piperazinomethyl)‐2′,3′‐BcNA (Bicyclic Nucleic Acids) into oligonucleotides via phosphoramidite chemistry and their subsequent solid‐phase functionalization with pyren‐1‐ylcarbonyl units after oligonucleotide synthesis. Thermal denaturation measurements showed that one modification led to increased thermal stability of the resulting duplex, and that two modifications could be incorporated in close proximity without decreasing the duplex stability (compared to the duplex stability of unmodified RNA). Fluorescence studies of the modified duplexes revealed that the structure and intensity of the fluorescence spectra were largely sequence‐dependent. Furthermore, molecular‐modeling studies showed that the pyrene moieties are placed in the major groove, and that the configuration at C(2″) is important for the thermal stability of the duplex.     

Stabilizing or destabilizing oligodeoxynucleotide duplexes containing single 2'-deoxyuridine residues with 5-alkynyl substituents

The 5-position of pyrimidines in DNA duplexes offers a site for introducing alkynyl substituents that protrude into the major groove and thus do not sterically interfere with helix formation. Substituents introduced at the 5-position of the deoxyuridine residue of dU:dA base pairs may stabilize duplexes and reinforce helices weakened by a low G/C content, which would otherwise lead to false negative results in DNA chip experiments. Here we report on a method for preparing oligonucleotides with a 5-alkynyl substituent at a 2'-deoxyuridine residue by on-support Sonogashira coupling involving the fully assembled oligonucleotide. A total of 25 oligonucleotides with 5-alkynyl substituents were prepared. The substituents either decrease the UV melting point of the duplex with the complementary strand or increase it by up to 7.1 degrees C, compared with that of the unmodified control duplex. The most duplex-stabilizing substituent, a pyrenylbutyramidopropyne moiety, is likely to intercalate but does not prevent sequence-specific base pairing of the modified deoxyuridine residue or the neighboring nucleotides. It also increases the signal for a target strand when employed on a small oligonucleotide microarray. The ability to tune the melting point of a DNA dodecamer duplex with a single side chain over a temperature range of >11 degrees C may prove useful when developing DNA sequences for biomedical applications.     

Characterization of noncovalent complexes formed between minor groove binding molecules and duplex DNA by electrospray ionization-mass spectrometry

The noncovalent complex formed in solution between minor groove binding molecules and an oligonucleotide duplex was investigated by electrospray ionization-mass spectrometry (ESI-MS). The oligonucleotide duplex formed between two sequence-specific 14-base pair oligonucleotides was observed intact by ESI-MS and in relatively high abundance compared to the individual single-stranded components. Only sequence-specific A:B duplexes were observed, with no evidence of random nonspecific aggregation (i.e., A:A or B:B) occurring under the conditions utilized. Due to the different molecular weights of the two 14-base pair oligonucleotides, unambiguous determination of each oligonucleotide and the sequence-specific duplex was confirmed through their detection at unique mass-to-charge ratios. The noncovalent complexes formed between the self-complementary 5′-dCGCAAATTTGCG-3′ oligonucleotide and three minor groove binding molecules (distamycin A, pentamidine, and Hoechst 33258) were also observed. Variation of several electrospray ionization interface parameters as well as collision-induced dissociation methods were utilized to characterize the nature and stability of the noncovalent complexes. The noncovalent complexes upon collisional activation dissociated into single-stranded oligonucleotides and single-stranded oligonucleotides associated with a minor groove binding molecule. ESI-MS shows potential for the study of small molecule-oligonucleotide duplex interactions and determination of small molecule binding stoichiometry.     

Solution composition and thermal denaturation for the production of single-stranded PCR amplicons: Piperidine-induced destabilization of the DNA duplex?

Strategies to produce single-stranded PCR amplicons for detection by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) were investigated using modified electrospray solutions and by thermally denaturing the duplex structures with a resistively heated electrospray ionization source. A synthetic 20-mer oligonucleotide annealed to its complementary strand was used as a model system for initial experiments. Electrospray solutions were altered by varying the relative proportion of aqueous phase in efforts to induce destabilization of the double helix. When the electrospray solution contains a 25% aqueous content, the 20-mer oligonucleotide is detected in its double-stranded form. Increasing the proportion of aqueous phase in the electrospray solution to 60% destabilized the double helix, resulting in the detection of only single-stranded species. This strategy was extended to an 82-bp polymerase chain reaction (PCR) product derived from the human tyrosine hydroxylase gene (HUMTH01). In efforts to destabilize the 82-bp PCR product, electrospray solutions reaching 70% aqueous content were necessary to promote the detection of only single-stranded amplicons. Implementation of the resistively heated transfer line and an electrospray solution in which the oligonucleotide is on the threshold of duplex stability allowed for double-stranded and single-stranded species to be generated from the same ESI solutions at both ambient and elevated transfer line temperatures, respectively, without disruption of the electrospray process. The volatile base piperidine, present at 20 mM concentrations in the electrospray solution, was found to play a critical role in the formation of single-stranded species at the higher aqueous percentages and a duplex destabilization mechanism has been proposed.     

Incorporation of porphyrin acetylides into duplexes of the simplified nucleic acid GNA

A porphyrin-acetylide-modified GNA (glycol nucleic acid) phosphoramidite building block was synthesized in an economical fashion starting from (S)-glycidyl-4,4'- dimethoxytrityl ether in just 4 steps with an overall yield of 48%. The porphyrin acetylide nucleotide was incorporated into GNA duplexes opposite ethylene glycol abasic sites and the duplexes were analyzed by UV-melting, UV-vis, fluorescence spectroscopy, and circular dichroism. The modified GNA duplexes display lower thermal stabilities, however, the stabilities of the duplexes can be modulated by the incorporation of Zn(2+) (further destabilization) or Ni(2+) (stabilization relative to the uncomplexed porphyrin). Uncomplexed as well as Ni(2+)-coordinated porphyrins intercalate into the GNA duplex whereas Zn(2+)-coordinated porphyrins are most likely located outside the base stack. Adjacent porphyrins in opposite strands of GNA duplexes show an electronic interaction with each other which might be exploited in the future for the design of photoelectrical devices.     

DNA as supramolecular scaffold for porphyrin arrays on the nanometer scale

Tetraphenyl porphyrin substituted deoxyuridine was used as a building block to create discrete multiporphyrin arrays via site specific incorporation into DNA. The successful covalent attachment of up to 11 tetraphenyl porphyrins in a row onto DNA shows that there is virtually no limitation in the amount of substituents, and the porphyrin arrays thus obtained reach the nanometer scale (approximately 10 nm). The porphyrin substituents are located in the major groove of the dsDNA and destabilize the duplex by deltaT(m) 5-7 degrees C per porphyrin modification. Force-field structure minimization shows that the porphyrins are either in-line with the groove in isolated modifications or aligned parallel to the nucleobases in adjacent modifications. The CD signals of the porphyrins are dominated by a negative peak arising from the intrinsic properties of the building block. In the single strands, the porphyrins induce stabilization of a secondary helical structure which is confined to the porphyrin modified part. This arrangement can be reproduced by force-field minimization and reveals an elongated helical arrangement compared to the double helix of the porphyrin-DNA. This secondary structure is disrupted above approximately 55 degrees C (T(p)) which is shown by various melting experiments. Both absorption and emission spectroscopy disclose electronic interactions between the porphyrin units upon stacking along the outer rim of the DNA leading to a broadening of the absorbance and a quenching of the emission. The single-stranded and double-stranded form show different spectroscopic properties due to the different arrangement of the porphyrins. Above T(p) the electronic properties (absorption and emission) of the porphyrins change compared to room temperature measurements due to the disruption of the porphyrin stacking at high temperature. The covalent attachment of porphyrins to DNA is therefore a suitable way of creating helical stacks of porphyrins on the nanometer scale.     

Gas-phase stability of double-stranded oligodeoxynucleotides and their noncovalent complexes with DNA-binding drugs as revealed by collisional activation in an ion trap

The intrinsic (gas-phase) stabilities of duplex, self-complementary oligonucleotides were measured in a relative way by subjecting the duplex precursor ions to increasing amounts of collision energy during the collisional-activated decomposition (CAD) events in an ion-trap mass spectrometer. The results are displayed as a dissociation profile, an s-shaped curve that shows the dependence of the relative abundance of the duplex on the applied collision energy. The total number of charges, the total number of base pairs, and the location of the high proton-affinity bases (i.e., G and C) are the main factors that affect the intrinsic stability of the duplex oligonucleotides. If the charge state is the same, the stability, as measured as a half-wave collision energy, E1/2, correlates well with the total number of H bonds for the duplex. The intrinsic stabilities of noncovalent complexes between duplex oligonucleotide and some DNA-binding drugs were also measured by using the newly developed method. Although duplexes are stabilized in the gas phase when they bind to drug molecules, correlations between gas-phase stabilities and the solution-binding affinities have not yet been obtained. Complexes in which the drug is bound in the minor groove must be joined tightly because they tend to dissociate in the gas phase by breaking covalent bonds of the oligonucleotide to give base loss and small sequence-ion formation. Complexes in which the drug is known to favor intercalation dissociate by breaking weak, noncovalent bonds to form single-stranded oligonucleotides although cleavage of covalent bonds of the oligonucleotide also occurs.     

Liquid chromatography/electrospray mass spectrometric analysis of metabolites from an inhibitory RNA duplex

Liquid chromatography/mass spectrometry (LC/MS) was used as a method for analyzing the metabolites of a model short interfering RNA (siRNA) duplex. The model siRNA duplex incorporated oligonucleotide stabilizing and protecting chemistries as these have been shown to increase the half-life of oligonucleotides. Two complementary 23 nucleotide single strands were joined to form the duplex. The intact duplex was analyzed using ion-pair reversed-phase chromatography coupled to electrospray ionization mass spectrometry (ESI-MS). The method used a hexafluoroisopropanol/triethylamine ion-pairing buffer with a methanol gradient to separate single-stranded oligonucleotide components from the duplex. This buffer system with ESI also preserved the duplex in the gas phase for analysis by a triple quadrupole mass spectrometer. Using this methodology, in vitro and in vivo metabolites from urine and rabbit ocular vitreous humor were determined and a pattern of duplex siRNA degradation was established. The masses of the metabolites were determined by ESI-MS and used with the known sequence of the siRNA duplex to identify the metabolites. Over the time course of the metabolism experiments it was shown that the breakdown products of the siRNA are consistent with the nuclease protection given by chemical modifications and that the duplex structure adds additional stability compared to the single strands alone. This study demonstrates that the ability of LC/MS to analyze duplex oligonucleotides has unique benefits for the study of siRNA metabolism.     

Structural characterization and biological evaluation of small interfering RNAs containing cyclohexenyl nucleosides

CeNA is an oligonucleotide where the (deoxy)ribose sugars have been replaced by cyclohexenyl moieties. We have determined the NMR structure of a CeNA:RNA duplex and have modeled this duplex in the crystal structure of a PIWI protein. An N puckering of the ribose nucleosides, a 2H3 conformation of the cyclohexenyl nucleosides, and an A-like helix conformation of the backbone, which deviates from the standard A-type helix by a larger twist and a smaller slide, are observed. The model of the CeNA:RNA duplex bound to the PIWI protein does not show major differences in the interaction of the guide CeNA with the protein when compared with dsRNA, suggesting that CeNA modified oligonucleotides might be useful as siRNAs. Incorporation of one or two CeNA units in the sense or antisense strands of dsRNA led to similar or enhanced activity compared to unmodified siRNAs. This was tested by targeting inhibition of expression of the MDR1 gene with accompanying changes in P-glycoprotein expression, drug transport, and drug resistance.