Biophysical studies of DNA modified with conformationally constrained nucleotides: comparison of 2'-exo (north) and 3'-exo (south) 'locked' templates

Synthesis of Fluorinated Nucleosides/Nucleotides and Their Antiviral Properties

The FDA has approved several drugs based on the fluorinated nucleoside pharmacophore, and numerous drugs are currently in clinical trials. Fluorine-containing nucleos(t)ides offer significant antiviral and anticancer activity. The insertion of a fluorine atom, either in the base or sugar of nucleos(t)ides, alters its electronic and steric parameters and transforms the lipophilicity, pharmacodynamic, and pharmacokinetic properties of these moieties. The fluorine atom restricts the oxidative metabolism of drugs and provides enzymatic metabolic stability towards the glycosidic bond of the nucleos(t)ide. The incorporation of fluorine also demonstrates additional hydrogen bonding interactions in receptors with enhanced biological profiles. The present article discusses the synthetic methodology and antiviral activities of FDA-approved drugs and ongoing fluoro-containing nucleos(t)ide drug candidates in clinical trials.

Triaza-tricyclanos - synthesis of a new class of tricyclic nucleoside analogues by stereoselective cascade cyclocondensation

Herein, we report a stereoselective synthesis of a novel type of conformationally constrained nucleoside analogue in which the sugar part is replaced by a new symmetrical tricycle consisting of a morpholine ring condensed with two imidazolidines. 1,5-Dialdehydes obtained from trityl- and dimethoxytrityl-protected uridine, ribothymidine, inosine, cytidine, adenosine and guanosine by metaperiodate oxidation were reacted with N¹,N³-dibenzyl-1,2,3-triaminopropane; the latter reactant was produced using a new method that avoids explosive intermediates. Reactions of dialdehydes with propane-triamine via cascade tricyclization resulted in the corresponding triaza-tricyclic derivatives bearing three new stereogenic centers in high yields. Out of the eight possible diastereoisomers, one stereoisomer was formed in each case due to the chiral control of the starting nucleoside-dialdehydes and the steric constraint of the condensed ring system. The absolute configuration of the new stereotriad was determined by X-ray diffraction and NMR experiments. A mechanistic study performed under reductive conditions to trap the presumed bicyclic intermediate showed that the triamine reactant first attacks the 2'-aldehyde group, followed by a rapid bicyclization to form the imidazolidino-morpholine unit.

The Medicinal Chemistry of Artificial Nucleic Acids and Therapeutic Oligonucleotides

Nucleic acids play a central role in human biology, making them suitable and attractive tools for therapeutic applications. While conventional drugs generally target proteins and induce transient therapeutic effects, nucleic acid medicines can achieve long-lasting or curative effects by targeting the genetic bases of diseases. However, native oligonucleotides are characterized by low in vivo stability due to nuclease sensitivity and unfavourable physicochemical properties due to their polyanionic nature, which are obstacles to their therapeutic use. A myriad of synthetic oligonucleotides have been prepared in the last few decades and it has been shown that proper chemical modifications to either the nucleobase, the ribofuranose unit or the phosphate backbone can protect the nucleic acids from degradation, enable efficient cellular uptake and target localization ensuring the efficiency of the oligonucleotide-based therapy. In this review, we present a summary of structure and properties of artificial nucleic acids containing nucleobase, sugar or backbone modifications, and provide an overview of the structure and mechanism of action of approved oligonucleotide drugs including gene silencing agents, aptamers and mRNA vaccines.

Conformation Locking of the Pentose Ring in Nucleotide Monophosphate Coordination Polymers via π-π Stacking and Metal-Ion Coordination

The conformation of the pentose ring in nucleotides is extremely important and a basic problem in biochemistry and pharmaceutical chemistry. In this study, we used a strategy to regulate the conformation of pentose rings of nucleotides via the synergistic effect of metal-ion coordination and π-π stacking. Seven types of coordination complexes were developed and characterized using Fourier transform infrared spectroscopy, elemental analysis, thermogravimetric analysis, powder X-ray diffraction, ultraviolet-visible spectroscopy, ¹H nuclear magnetic resonance spectroscopy, electrospray ionization mass spectrometry, and single-crystal X-ray diffraction. On the basis of two conformational parameters obtained from single-crystal structure analysis, i.e., the pseudorotation phase angle and degree of puckering, the exact conformation of the furanose ring in these coordination polymers was unequivocally determined. Crystallographic studies demonstrate that a short bridging ligand (4,4'-bipyridine) is conducive to the formation of a twist form, and long auxiliary ligands [1,2-bis(4-pyridyl)ethene and 4,4'-azopyridine] induce the formation of an envelope conformation. However, the longest auxiliary ligands [1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene] cannot limit the flexibility of a nucleotide. Our results demonstrated that the proposed strategy is universal and controllable. Moreover, the chirality of these coordination polymers was examined by combining the explanation of their crystal structures with solid-state circular dichroism spectroscopy measurements.

Coupling between conformational dynamics and catalytic function at the active site of the lead-dependent ribozyme

In common with other self-cleaving RNAs, the lead-dependent ribozyme (leadzyme) undergoes dynamic fluctuations to a chemically activated conformation. We explored the connection between conformational dynamics and self-cleavage function in the leadzyme using a combination of NMR spin-relaxation analysis of ribose groups and conformational restriction via chemical modification. The functional studies were performed with a North-methanocarbacytidine modification that prevents fluctuations to C2'-endo conformations while maintaining an intact 2'-hydroxyl nucleophile. Spin-relaxation data demonstrate that the active-site Cyt-6 undergoes conformational exchange attributed to sampling of a minor C2'-endo state with an exchange lifetime on the order of microseconds to tens of microseconds. A conformationally restricted species in which the fluctuations to the minor species are interrupted shows a drastic decrease in self-cleavage activity. Taken together, these data indicate that dynamic sampling of a minor species at the active site of this ribozyme, and likely of related naturally occurring motifs, is strongly coupled to catalytic function. The combination of NMR dynamics analysis with functional probing via conformational restriction is a general methodology for dissecting dynamics-function relationships in RNA.

5'- vs. 3'-end sugar conformational control in shaping up dinucleotides

The 5'-end N-sugar puckering is currently believed to govern the intramolecular dinucleotide stacking. We demonstrate that if this 5'-conformation is indeed important in shaping up dinucleotide structures, the 3'-end sugar conformation can either potentiate or cancel the stacking capacity induced by the 5'-end N-sugar conformation.

Synthesis of a North-methanocarba-thymidine (N-MCT) analog

A detailed protocol for the synthesis of North-methanocarba-thymidine (N-MCT), a potent antiviral nucleoside with a restricted bicyclo[3.1.0]hexane pseudosugar conformation, is presented. The process is described in two parts. The first basic protocol deals with the synthesis of the carbobicyclic pseudosugar precursor that can be utilized in the syntheses of other bicyclo[3.1.0]hexane nucleosides with natural and non-natural nucleobases. The second basic protocol describes the specific construction of the thymine base in a linear fashion from the carbobicyclic intermediate.

The effect on quadruplex stability of North-nucleoside derivatives in the loops of the thrombin-binding aptamer

Modified thrombin-binding aptamers (TBAs) carrying uridine (U), 2'-deoxy-2'-fluorouridine (FU) and North-methanocarbathymidine (NT) residues in the loop regions were synthesized and analyzed by UV thermal denaturation experiments and CD spectroscopy. The replacement of thymidines in the TGT loop by U and FU results in an increased stability of the antiparallel quadruplex structure described for the TBA while the presence of NT residues in the same positions destabilizes the antiparallel structure. The substitution of the thymidines in the TT loops for U, FU and NT induce a destabilization of the antiparallel quadruplex, indicating the crucial role of these positions. NMR studies on TBAs modified with uridines at the TGT loop also confirm the presence of the antiparallel quadruplex structure. Nevertheless, replacement of two Ts in the TT loops by uridine gives a more complex scenario in which the antiparallel quadruplex structure is present along with other partially unfolded species or aggregates.

The solution structure of double helical arabino nucleic acids (ANA and 2'F-ANA): effect of arabinoses in duplex-hairpin interconversion

We report here the first structure of double helical arabino nucleic acid (ANA), the C2′-stereoisomer of RNA, and the 2′-fluoro-ANA analogue (2′F-ANA). A chimeric dodecamer based on the Dickerson sequence, containing a contiguous central segment of arabino nucleotides, flanked by two 2′-deoxy-2′F-ANA wings was studied. Our data show that this chimeric oligonucleotide can adopt two different structures of comparable thermal stabilities. One structure is a monomeric hairpin in which the stem is formed by base paired 2′F-ANA nucleotides and the loop by unpaired ANA nucleotides. The second structure is a bimolecular duplex, with all the nucleotides (2′F-ANA and ANA) forming Watson–Crick base pairs. The duplex structure is canonical B-form, with all arabinoses adopting a pure C2′-endo conformation. In the ANA:ANA segment, steric interactions involving the 2′-OH substituent provoke slight changes in the glycosidic angles and, therefore, in the ANA:ANA base pair geometry. These distortions are not present in the 2′F-ANA:2′F-ANA regions of the duplex, where the –OH substituent is replaced by a smaller fluorine atom. 2′F-ANA nucleotides adopt the C2′-endo sugar pucker and fit very well into the geometry of B-form duplex, allowing for favourable 2′F···H8 interactions. This interaction shares many features of pseudo-hydrogen bonds previously observed in 2′F-ANA:RNA hybrids and in single 2′F-ANA nucleotides.

The conformationally constrained N-methanocarba-dT analogue adopts an unexpected C4'-exo sugar pucker in the structure of a DNA hairpin

Incorporation of a bicyclo[3.1.0]hexane scaffold into the nucleoside sugar was devised to lock the embedded cyclopentane ring in conformations that mimic the furanose North and South sugar puckers. To analyze the effects of North-methanocarba-2'-deoxythymidine (N-MCdT) on the B-form DNA, we crystallized d(CGCGAA[mcTmcT]CGCG) with two N-MCdTs. Instead of a duplex, the 12mer forms a tetraloop hairpin, whereby loop N-MCdTs adopt the C4'-exo pucker (NE; P = 50°). Thus, the bicyclic framework does not limit the pucker to the anticipated C2'-exo range (NNW; P = -18°).

Conformationally rigid nucleoside probes help understand the role of sugar pucker and nucleobase orientation in the thrombin-binding aptamer

Modified thrombin-binding aptamers carrying 2′-deoxyguanine (dG) residues with locked North- or South-bicyclo[3.1.0]hexane pseudosugars were synthesized. Individual 2′-deoxyguanosines at positions dG5, dG10, dG14 and dG15 of the aptamer were replaced by these analogues where the North/anti and South/syn conformational states were confined. It was found that the global structure of the DNA aptamer was, for the most part, very accommodating. The substitution at positions 5, 10 and 14 with a locked South/syn-dG nucleoside produced aptamers with the same stability and global structure as the innate, unmodified one. Replacing position 15 with the same South/syn-dG nucleoside induced a strong destabilization of the aptamer, while the antipodal North/anti-dG nucleoside was less destabilizing. Remarkably, the insertion of a North/anti-dG nucleoside at position 14, where both pseudosugar conformation and glycosyl torsion angle are opposite with respect to the native structure, led to the complete disruption of the G-tetraplex structure as detected by NMR and confirmed by extensive molecular dynamics simulations. We conclude that conformationally locked bicyclo[3.1.0]hexane nucleosides appear to be excellent tools for studying the role of key conformational parameters that are critical for the formation of a stable, antiparallel G-tetrad DNA structures.

Crystal structure of tricyclo-DNA: an unusual compensatory change of two adjacent backbone torsion angles

The crystal structure of a DNA duplex with tricyclo-DNA (tc-DNA) residues explains the increased RNA affinity of tc-DNA relative to DNA and tc-DNA's superior resistance to nucleases.