Synthesis and chemical transformations of glycol nucleic acid (GNA) nucleosides.

Xeno nucleic acids (XNA) are an increasingly important class of hypermodified nucleic acids with great potential in bioorganic chemistry and synthetic biology. Glycol nucleic acid (GNA) is constructed from a three-carbon 1,2-propanediol (propylene glycol) backbone attached to a nucleobase entity, representing the simplest known XNA. This review is intended to present GNA nucleosides from a synthetic chemistry perspective-a perspective that serves as a starting point for biological studies. Therefore this account focuses on synthetic methods for GNA nucleoside synthesis, as well as their postsynthetic chemical transformations. The properties and biological activity of GNA constituents are also highlighted. A literature survey shows four major approaches toward GNA nucleoside scaffold synthesis. These approaches pertain to glycidol ring-opening, Mitsunobu, SN2, and dihydroxylation reactions. The general arsenal of reactions used in GNA chemistry is versatile and encompasses the Sonogashira reaction, Michael addition, silyl-Hilbert-Johnson reaction, halogenation, alkylation, cyclization, Rh-catalyzed N-allylation, Sharpless catalytic dihydroxylation, and Yb(OTf)3-catalyzed etherification. Additionally, various phosphorylation reactions have enabled the synthesis of diverse types of GNA nucleotides, dinucleoside phosphates, phosphordiamidites, and oligos. Furthermore, recent advances in GNA chemistry have resulted in the synthesis of previously unknown redox-active (ferrocenyl) and luminescent (pyrenyl and phenanthrenyl) GNA nucleosides, which are also covered in this review.

Acyclic (S)-glycol nucleic acid (S-GNA) modification of siRNAs improves the safety of RNAi therapeutics while maintaining potency.

Glycol nucleic acid (GNA) is an acyclic nucleic acid analog connected via phosphodiester bonds. Crystal structures of RNA-GNA chimeric duplexes indicated that nucleotides of the right-handed (S)-GNA were better accommodated in the right-handed RNA duplex than were the left-handed (R)-isomers. GNA nucleotides adopt a rotated nucleobase orientation within all duplex contexts, pairing with complementary RNA in a reverse Watson-Crick mode, which explains the inabilities of GNA C and G to form strong base pairs with complementary nucleotides. Transposition of the hydrogen bond donor and acceptor pairs using novel (S)-GNA isocytidine and isoguanosine nucleotides resulted in stable base-pairing with the complementary G and C ribonucleotides, respectively. GNA nucleotide or dinucleotide incorporation into an oligonucleotide increased resistance against 3'-exonuclease-mediated degradation. Consistent with the structural observations, small interfering RNAs (siRNAs) modified with (S)-GNA had greater in vitro potencies than identical sequences containing (R)-GNA. (S)-GNA is well tolerated in the seed regions of antisense and sense strands of a GalNAc-conjugated siRNA in vitro. The siRNAs containing a GNA base pair in the seed region had in vivo potency when subcutaneously injected into mice. Importantly, seed pairing destabilization resulting from a single GNA nucleotide at position 7 of the antisense strand mitigated RNAi-mediated off-target effects in a rodent model. Two GNA-modified siRNAs have shown an improved safety profile in humans compared with their non-GNA-modified counterparts, and several additional siRNAs containing the GNA modification are currently in clinical development.

The effect of l-thymidine, acyclic thymine and 8-bromoguanine on the stability of model G-quadruplex structures.

BACKGROUND: Guanine-rich oligonucleotides are capable of forming tetrahelical structures known as G-quadruplexes with interesting biological properties. We have investigated the effects of site-specific substitution in the loops and in the tetrads model G-quadruplexes using thymine glycol nucleic acid (GNA) units, l-thymidine and 8-Br-2'-deoxyguanosine. METHODS: Modified oligonucleotides were chemically synthesized and spectroscopic techniques were used to determine the relative stability of the modified G-quadruplex. The double 8-BrdG-modified quadruplexes were further characterized by Nuclear Magnetic Resonance. Binding to thrombin of selected quadruplex was analyzed by gel electrophoresis retention assay. RESULTS: The most interesting results were found with a 8-bromoG substitution that had the larger stabilization of the quadruplex. NMR studies indicate a tight relationship between the loops and the tetrads to accommodate 8-bromoG modifications within the TBA. CONCLUSIONS: The substitutions of loop positions with GNA T affect the TBA stability except for single modification in T7 position. Single l-thymidine substitutions produced destabilization of TBA. Larger changes on quadruplex stability are observed with the use of 8-bromoG finding a single substitution with the highest thermal stabilization found in thrombin binding aptamers modified at the guanine residues and having good affinity for thrombin. Double 8-BrdG modification in anti positions of different tetrads produce a conformational flip from syn to anti conformation of 8-Br-dG to favor loop-tetrad interaction and preserve the overall TBA stability. GENERAL SIGNIFICANCE: Modified guanine-rich oligonucleotides are valuable tools for the search for G-quadruplex structures with higher thermal stability and may provide compounds with interesting protein-nucleic acid binding properties. This article is part of a Special Issue entitled "G-quadruplex" Guest Editor: Dr. Concetta Giancola and Dr. Daniela Montesarchio.