Non-natural Bases Research Articles

Unveiling structural and energetic characterization of the emissive RNA alphabet anchored in the methylthieno[3,4-d]pyrimidine heterocycle core.

This study presents a comprehensive theoretical exploration of the fluorescent non-natural emissive nucleobases- mthA, mthG, mthC, and mthU derived from the methylthieno[3,4-d]pyrimidine heterocycle. Our calculations, aligning with experimental findings, reveal that these non-natural bases exert minimal influence on the geometry of classical Watson-Crick base pairs within an RNA duplex, maintaining H-bonding akin to natural bases. In terms of energy, the impact of the modified bases, but for mthG, is also found to be little significant. We delved into an in-depth analysis of the photophysical properties of these non-natural bases. This investigation unveiled a correlation between their absorption/emission peaks and the substantial impact of the modification on the energy levels of the highest unoccupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbital (LUMO). Notably, this alteration in energy levels resulted in a significant reduction of the HOMO-LUMO gap, from approximately 5.4-5.5 eV in the natural bases, to roughly 3.9-4.7 eV in the modified bases. This shift led to a consequential change in absorption and emission spectra towards longer wavelengths, elucidating their bathochromic shift.

High-Order Quantum-Mechanical Analysis of Hydrogen Bonding in Hachimoji and Natural DNA Base Pairs.

High-order quantum chemistry is applied to hydrogen-bonded natural DNA nucleobase pairs [adenine:thymine (A:T) and guanine:cytosine (G:C)] and non-natural Hachimoji nucleobase pairs [isoguanine:1-methylcytosine (B:S) and 2-aminoimidazo[1,2a][1,3,5]triazin-4(1H)-one:6-amino-5-nitropyridin-2-one (P:Z)] to see how the intermolecular interaction energies and their energetic components (electrostatics, exchange-repulsion, induction/polarization, and London dispersion interactions) vary among the base pairs. We examined the Hoogsteen (HG) geometries in addition to the traditional Watson-Crick (WC) geometries. Coupled-cluster theory through perturbative triples [CCSD(T)] extrapolated to the complete basis set (CBS) limit and high-order symmetry-adapted perturbation theory (SAPT) at the SAPT2+(3)(CCD)δMP2/aug-cc-pVTZ level are used to estimate highly accurate noncovalent interaction energies. Electrostatic interactions are the most attractive component of the interaction energies, but the sum of induction/polarization and London dispersion is nearly as large, for all base pairs and geometries considered. Interestingly, the non-natural Hachimoji base pairs interact more strongly than the corresponding natural base pairs, by -21.8 (B:S) and -0.3 (P:Z) kcal mol-1 in the WC geometries, according to CCSD(T)/CBS. This is consistent with the H-bond distances being generally shorter in the non-natural base pairs. The natural base pairs are energetically more stabilized in their Hoogsteen geometries than in their WC geometries. The Hoogsteen geometry makes the A:T base pair slightly more stable, by -0.8 kcal mol-1, and it greatly stabilizes the G:C+ base pair, by -15.3 kcal mol-1. The G:C+ stabilization is mainly due to the fact that C has typically added a proton when found in Hoogsteen geometries. By contrast, Hoogsteen geometries are substantially less favorable than WC geometries for non-natural Hachimoji base pairs, by 17.3 (B:S) and 13.8 (P:Z) kcal mol-1.

Site-Directed Mutagenesis for In Vitro and In Vivo Experiments Exemplified with RNA Interactions in Escherichia Coli.

Site-directed mutagenesis is a technique used to introduce specific mutations in DNA to investigate the interaction between small non-coding ribonucleic acid (sRNA) molecules and target messenger RNAs (mRNAs). In addition, site-directed mutagenesis is used to map specific protein binding sites to RNA. A 2-step and 3-step PCR based introduction of mutations is described. The approach is relevant to all protein-RNA and RNA-RNA interaction studies. In short, the technique relies on designing primers with the desired mutation(s), and through 2 or 3 steps of PCR synthesizing a PCR product with the mutation. The PCR product is then used for cloning. Here, we describe how to perform site-directed mutagenesis with both the 2- and 3-step approach to introduce mutations to the sRNA, McaS, and the mRNA, csgD, to investigate RNA-RNA and RNA-protein interactions. We apply this technique to investigate RNA interactions; however, the technique is applicable to all mutagenesis studies (e.g., DNA-protein interactions, amino-acid substitution/deletion/addition). It is possible to introduce any kind of mutation except for non-natural bases but the technique is only applicable if a PCR product can be used for downstream application (e.g., cloning and template for further PCR).

Synthetic, Functional Thymidine-Derived Polydeoxyribonucleotide Analogues from a Six-Membered Cyclic Phosphoester.

A grand challenge that crosses synthetic chemistry and biology is the scalable production of functional analogues of biomacromolecules. We have focused our attention on the use of deoxynucleoside building blocks bearing non-natural bases to develop a synthetic methodology that allows for the construction of high molecular weight deoxynucleotide polymers. Our six-membered cyclic phosphoester ring-opening polymerization strategy is demonstrated, herein, by an initial preparation of novel polyphosphoesters, comprised of butenyl-functionalized deoxyribonucleoside repeat units, connected via 3′,5′-backbone linkages. A thymidine-derived bicyclic monomer, 3′,5′-cyclic 3-(3-butenyl) thymidine ethylphosphate, was synthesized in two steps directly from thymidine, via butenylation and diastereoselective cyclization promoted by N,N-dimethyl-4-aminopyridine. Computational modeling of the six-membered 3′,5′-cyclic phosphoester ring derived from deoxyribose indicated strain energies at least 5.4 kcal/mol higher than those of the six-membered monocyclic phosphoester, 2-ethoxy-1,3,2-dioxaphosphinane 2-oxide. These calculations supported the hypothesis that the strained 3′,5′-cyclic monomer can promote ring-opening polymerization to afford the resulting poly(3′,5′-cyclic 3-(3-butenyl) thymidine ethylphosphate)s with low dispersities (Đ < 1.10). This advanced design combines the merits of natural product-derived materials and functional, degradable polymers to provide a new platform for functional, synthetically derived polydeoxyribonucleotide-analogue materials.

Post-SELEX optimization of aptamers.

Aptamers are functional single-stranded DNA or RNA oligonucleotides, selected in vitro by SELEX (Systematic Evolution of Ligands by Exponential Enrichment), which can fold into stable unique three-dimensional structures that bind their target ligands with high affinity and specificity. Although aptamers show a number of favorable advantages such as better stability and easier modification when compared with the properties of antibodies, only a handful of aptamers have entered clinical trials and only one, pegaptanib, has received US Food and Drug Administration approval for clinical use. The main reasons that limit the practical application of aptamers are insufficient nuclease stability, bioavailability, thermal stability, or even affinity. Some aptamers obtained from modified libraries show better properties; however, polymerase amplification of nucleic acids containing non-natural bases is currently a primary drawback of the SELEX process. This review focuses on several post-SELEX optimization strategies of aptamers identified in recent years. We describe four common methods in detail: truncation, chemical modification, bivalent or multivalent aptamer construction, and mutagenesis. We believe that these optimization strategies should improve one or more specific properties of aptamers, and the type of feature(s) selected for improvement will be dependent on the application purpose.

Synthesis of Nonnatural Oligonucleotides Made Exclusively of Alkynyl C-Nucleosides with Nonnatural Bases.

This unit describes detailed procedures for the preparation of nonnatural C-nucleosides comprising seven types of nonnatural nucleobases attached to 1'-position of 2'-deoxyribose through an acetylene bond with the β-configuration. In addition, derivatization of these alkynyl C-nucleosides into the corresponding phosphoramidites and the subsequent oligonucleotide synthesis are also presented. The processes are depicted in three parts. The first basic protocol deals with the synthesis of a key intermediate, 5-O-DMTr-protected 1-ethynyl-2-deoxy-β-D-ribofuranoside. The second basic protocol mentions the procedures of the preparation of nonnatural C-nucleosides by a palladium-catalyzed coupling reaction of the ethynyl intermediate and halogen-attached nonnatural nucleobases. The synthetic procedures of the corresponding nonnatural phosporamidites are also described. The third basic protocol presents the solid-phase, automated synthesis of nonnatural oligonucleotides composed exclusively of the nonnatural C-nucleotides.

Polymerase incorporation of a 2'-deoxynucleoside-5'-triphosphate bearing a 4-hydroxy-2-mercaptobenzimidazole nucleobase analogue.

Here, we describe the enzymatic construction of a new larger base pair formed between adenine (A) and a 4-hydroxy-2-mercaptobenzimidazole (SB) nucleobase analogue. We investigated the enzymatic incorporation of 2'-deoxynucleoside-5'-triphosphate bearing a SB nucleobase analogue (dSBTP) into oligonucleotides (ONs) by DNA polymerases. dSBTP could be effectively incorporated at the site opposite a dA in a DNA template by several B family DNA polymerases. These findings provide new insights into various aspects of biotechnology, including the design of non-natural base pairs.

Synthesis and complementary self-association of novel lipophilic π-conjugated nucleoside oligomers.

A series of lipophilic nucleosides comprising natural and non-natural bases that are π-conjugated to a short oligophenylene-ethynylene fragment has been synthesized. These bases comprise guanosine, isoguanosine, and 2-aminoadenosine as purine heterocycles, and cytidine, isocytosine and uridine as complementary pyrimidine bases. The hydrogen-bonding dimerization and association processes between complementary bases were also studied by (1)H NMR and absorption spectroscopy in order to obtain the relevant association constants.

A firmly hybridizable, DNA-like architecture with DAD/ADA- and ADD/DAA-type nonnatural base pairs as an extracellular genetic candidate.

We describe artificial DNA molecules exclusively consisting of four types of alkynyl C-nucleotides with nonnatural bases. The artificial DNA exhibited almost the same characteristics as natural DNA, such as in regard to the stepwise duplex and triplex formation and the right-handed higher-order structure with an antiparallel alignment fashion.

Ethenoguanines undergo glycosylation by nucleoside 2'-deoxyribosyltransferases at non-natural sites.

Deoxyribosyl transferases and functionally related purine nucleoside phosphorylases are used extensively for synthesis of non-natural deoxynucleosides as pharmaceuticals or standards for characterizing and quantitating DNA adducts. Hence exploring the conformational tolerance of the active sites of these enzymes is of considerable practical interest. We have determined the crystal structure at 2.1 Å resolution of Lactobacillus helveticus purine deoxyribosyl transferase (PDT) with the tricyclic purine 8,9-dihydro-9-oxoimidazo[2,1-b]purine (N 2,3-ethenoguanine) at the active site. The active site electron density map was compatible with four orientations, two consistent with sites for deoxyribosylation and two appearing to be unproductive. In accord with the crystal structure, Lactobacillus helveticus PDT glycosylates the 8,9-dihydro-9-oxoimidazo[2,1-b]purine at N7 and N1, with a marked preference for N7. The activity of Lactobacillus helveticus PDT was compared with that of the nucleoside 2′-deoxyribosyltransferase enzymes (DRT Type II) from Lactobacillus leichmannii and Lactobacillus fermentum, which were somewhat more effective in the deoxyribosylation than Lactobacillus helveticus PDT, glycosylating the substrate with product profiles dependent on the pH of the incubation. The purine nucleoside phosphorylase of Escherichia coli, also commonly used in ribosylation of non-natural bases, was an order of magnitude less efficient than the transferase enzymes. Modeling based on published active-site structures as templates suggests that in all cases, an active site Phe is critical in orienting the molecular plane of the purine derivative. Adventitious hydrogen bonding with additional active site residues appears to result in presentation of multiple nucleophilic sites on the periphery of the acceptor base for ribosylation to give a distribution of nucleosides. Chemical glycosylation of O 9-benzylated 8,9-dihydro-9-oxoimidazo[2,1-b]purine also resulted in N7 and N1 ribosylation. Absent from the enzymatic and chemical glycosylations is the natural pattern of N3 ribosylation, verified by comparison of spectroscopic and chromatographic properties with an authentic standard synthesized by an unambiguous route.

The Family “Diverse” but not Natural: The Case of Albania

The question and mostly the debate on marriage between persons of the same sex touches two fundamental principles: the equal sign of the rights and the rejection of any form of discrimination, in particular, as it pertains to personal identity. The approach to the intolerant phenomenon is of ancient date in the Albanian juridical and social environment. Furthermore, it is an approach that has nothing to do and therefore should not be identified with the political history that has living Albanian society. First, this was intolerance as mentality, as an act, then become legal precept. First, this was intolerance as mentality, as an act, then become a normative rule. Initially, it was homophobia and after the rule of law. In other words, it was a mix of ultra-conservative traditions with orient-atheist foundations and the ius naturalist vision of the rights, which classifies what is natural and non-natural base on legal topics. So there are two questions to be resolved: 1-from a legal standpoint, is it justifiable the rejection of gay marriage?; 2- do homosexual couples , enjoy the right to have a recognized civil family? DOI: 10.5901/mjss.2013.v4n10p76

Easy Design of Colorimetric Logic Gates Based on Nonnatural Base Pairing and Controlled Assembly of Gold Nanoparticles

Utilizing the principles of metal-ion-mediated base pairs (C-Ag-C and T-Hg-T), the pH-sensitive conformational transition of C-rich DNA strand, and the ligand-exchange process triggered by DL-dithiothreitol (DTT), a system of colorimetric logic gates (YES, AND, INHIBIT, and XOR) can be rationally constructed based on the aggregation of the DNA-modified Au NPs. The proposed logic operation system is simple, which consists of only T-/C-rich DNA-modified Au NPs, and it is unnecessary to exquisitely design and alter the DNA sequence for different multiple molecular logic operations. The nonnatural base pairing combined with unique optical properties of Au NPs promises great potential in multiplexed ion sensing, molecular-scale computers, and other computational logic devices.

Dynamics and efficiency of photoinduced charge transport in DNA: Toward the elusive molecular wire

Experimental investigations of photoinduced charge transport in synthetic DNA capped hairpins possessing electron acceptor and donor stilbene chromophores at either end have established the mechanism, dynamics, and efficiency of charge transport in DNA. The mechanism for charge transport in repeating A-T base pairs (A-tracts) was found to change from single-step superexchange at short distances to multistep incoherent hole hopping at longer distances. The rate constants for base-to-base hole hopping in longer A- and G-tract sequences are 1.2 × 109 s–1 and 4.3 × 109 s–1, respectively, considerably slower than the rate constants associated with molecular wires. Even slower rate constants are observed for alternating or random base sequences such as those encountered in natural DNA. The efficiency of charge separation in capped hairpins with A-tract sequences is also low as a consequence of the competition of hole hopping with charge recombination. Significantly higher efficiencies for charge separation are possible using diblock purine base sequences consisting of two or three adenines followed by a larger number of guanines. The short A-block serves as a molecular rectifier, slowing down charge recombination. More efficient charge separation can also be achieved using non-natural bases or by using the triplet acceptor anthraquinone for hole injection.

Computational investigation of thermochemical properties of non-natural C-nucloebases: different hydrogen-bonding preferences for non-natural Watson–Crick base pairs

In the present density functional theory study, we have compared intrinsic properties of non-natural nucleobases (acA, acG, acC, and acT nucleobases) such as proton affinities, gas phase acidities, tautomerization, and hydrogen-bonding properties with those in normal Watson–Crick nucleobases (A, G, C, T nucleobases). The hydrogen-bonding interactions in non-natural and Watson–Crick base pairs were studied at B3LYP/6-311++G (d,p) level regarding their geometries, energies, and topological features of the electron density. The quantum theory of atoms-in-molecule (QTAIM) and natural bond orbital (NBO) analyses were employed to elucidate the interaction characteristics in base pairs. The electron density ρ(r) as well as its Laplacian \( \nabla^{2} \) ρ(r) at the hydrogen bond critical point predicted by QTAIM is strongly correlated with hydrogen bond structural parameter and the second-order perturbation energies in NBO scheme. Our results show that most of hydrogen bonds in normal Watson–Crick and non-natural base pairs must be considered as electrostatic interactions. Results of calculations revealed that energetic values of hydrogen bonds in T–A base pair are more than those in ac T–ac A base pair, while values of hydrogen bonds in C–G base pair and ac C–ac G base pair are almost the same. These results confirmed stability order of stabilization energies of these base pairs.

On the mechanism of dynamic polymerization via recycled ss-DNA templated assembly of non-natural bases

We report on the mechanism of DNA templated dynamic polymerization and show that DNA of sufficient length can serve as a template for the polymerization of non-natural base building blocks in which the template can be used again by a simple heating–cooling step.

Theoretical Studies on the Intermolecular Interactions of Potentially Primordial Base‐Pair Analogues

Recent experimental studies on the Watson-Crick type base pairing of triazine and aminopyrimidine derivatives suggest that acid/base properties of the constituent bases might be related to the duplex stabilities measured in solution. Herein we use high-level quantum chemical calculations and molecular dynamics simulations to evaluate the base pairing and stacking interactions of seven selected base pairs, which are common in that they are stabilized by two N-H...O hydrogen bonds separated by one N-H...N hydrogen bond. We show that neither the base pairing nor the base stacking interaction energies correlate with the reported pK(a) data of the bases and the melting points of the duplexes. This suggests that the experimentally observed correlation between the melting point data of the duplexes and the pK(a) values of the constituent bases is not rooted in the intrinsic base pairing and stacking properties. The physical chemistry origin of the observed experimental correlation thus remains unexplained and requires further investigations. In addition, since our calculations are carried out with extrapolation to the complete basis set of atomic orbitals and with inclusion of higher electron correlation effects, they provide reference data for stacking and base pairing energies of non-natural bases.

Multivalent DNA Aptamer-Based Therapeutic Agents for Lymphoma and Leukemia.

Abstract 2711Poster Board II-687Non-Hodgkins lymphomas affect 450,000 patients in the United States and even with recent advances in antibody-based therapies; more than 20,000 people will die of their disease annually. The goal of this work was to develop a high affinity, stable aptamer selective for B-cell leukemias and lymphomas. Aptamers are small DNA molecules that have the ability to bind to proteins with high affinity and specificity. They are also ideal candidates as therapeutic carriers. Aptamer binding is based on the ability of small oligonucleotide polymers (typically 20–80mers) to fold into unique three-dimensional structures that can interact with a specific target. Based on nature of this interaction, aptamers could be considered to be antibody analogs. Compared to antibodies however, one of the inherent properties of aptamers is that their small size (typically 10–20,000 daltons for aptamers vs 150,000 daltons for antibodies,) might address some difficult pharmacologic issues of antibodies, which penetrate slowly into tumors and clear the blood slowly. Recently, the TDO5 aptamer was identified and it was found to bind to membrane bound human IgM, a component of the BCR complex in B-Cell neoplasms. In contrast to currently available monoclonal antibodies, TD05 binds to membrane bound human IgM only and not with soluble IgM, eliminating the possibility of competitive inhibition by soluble IgM in the serum. The specificity of the aptamer was confirmed by screening with 24 cell lines and fresh clinical leukemia samples. Out of 24 cell lines, the IgM-negative cells, including T-cell leukemias and solid tumor lines such as breast, kidney, and colon and ovarian, showed no binding with TD05 indicating there is no non-specific adhesion with cell lines. One of the challenges of using the current form of TD05 as a drug carrier is that it is not yet suitable for use in vivo because of low avidity (>10uM) and stability (t ½= 1min) at physiological conditions. In order to increase the affinity of this aptamer, a new truncated multivalent and nuclease stable aptamer was designed. First, truncation of the original version of TD05 was considered because reduced size may lead to more efficient chemical syntheses and better pharmacologic properties. The resulting TD05.1 has a 5-fold increased affinity compared to original version of TD05. Second, bivalent (BV) aptamers utilizing TD05.1 with various polyethylene glycol (PEG) linker lengths were designed. Linker length is critical in designing multivalent aptamers to avoid loss of binding due to steric hindrance and to optimize the binding geometry, both of which would affect binding affinity. An optimal linker length of ∼16nm was chosen after empiric binding studies. Nuclease stability was also addressed by the introduction of chemical functionalities into TD05.1. The introduction of non-natural bases such as LNA bases (Locked Nucleic Acids), have been demonstrated to be effective in this regard. The stability of LNA stems from the bicyclic furanose unit locked in a sugar conformation. In order to retain the specific recognition and 3-dimensional nature of the aptamer's folding, LNA analogues were only introduced at the regions that are not involved in binding. Additionally, to further improve nuclease resistance, increase circulation time in vivo and to prevent non-specific adhesion to serum proteins and cells, the LNA modified BV aptamer TD05.1 was modified with polyethylene glycol at the 3' and 5' ends. The introduction of LNA and polyethylene glycol further stabilized the secondary structure, increasing the affinity and nuclease resistance in serum from one minute to seven hours(t ½> 420 min). We then constructed a trivalent analog of the aptamer with multiple functionalities, including chelators and fluorophores, which showed ∼25-fold higher affinity compared to monomeric aptamer at physiological conditions. In conclusion, this study demonstrates the development of a trivalent, high affinity aptamer selective for the membrane bound human IgM found in B-cell leukemias and lymphoma. Studies to assess the biological activity and the use of this construct as a drug carrier to treat B-NHL and B-CLL are in progress. Disclosures:No relevant conflicts of interest to declare.

Redesigning the Architecture of the Base Pair: Toward Biochemical and Biological Function of New Genetic Sets

Recognition of the nucleic acid bases within the DNA scaffold comprises the basis for transmission of genetic information, dictating protein and cell assembly, organismal development, and evolution. Driven in part by the need to test our current understanding of this information transfer, chemists have begun to design and synthesize nonnatural bases and base pair structures to mimic the function of DNA without relying on Nature's purine-pyrimidine base pair scaffold. Multiple examples have been recently described that self-assemble stably and sequence specifically in vitro, and some isolated unnatural base pairs can be replicated in vitro as well. Moreover, recent experiments with unnatural bases in bacterial cells have demonstrated surprisingly efficient reading of the chemical information. This suggests the future possibility of redesigning and replacing the chemical information of an evolving cell while retaining biological function.

Synthesis and Hybridization Properties of Modified Oligodeoxynucleotides Carrying Non‐Natural Bases

The impact of the presence of nonnatural bases on the properties of oligodeoxynucleotides has been studied. First, oligodeoxynucleotides carrying 2'-deoxyzebularine were prepared, and the stability of duplexes carrying this analogue was determined by DNA melting experiments. Melting temperatures and thermodynamic data indicated the preference of 2'-deoxyzebularine for 2'-deoxyguanosine, which behaves as a 2'-deoxycytidine analogue, forming a less stable base pair due to the absence of the amino group at position 4. Moreover, the duplex-hairpin equilibrium of a self-complementary oligodeoxynucleotide carrying several natural and nonnatural bases including 2'-deoxyzebularine as a central mispair, was studied. Depending on the base present in the middle of the sequence, it is possible to affect the stability of the bimolecular duplex modulating the duplex-hairpin equilibrium. Magnesium ions were shown to stabilize preferentially the bimolecular duplex form. The results indicate the importance of the modifications and the role of cations in shifting the structural equilibrium.

Artificial DNA Made Exclusively of Nonnatural C-Nucleosides with Four Types of Nonnatural Bases

We describe a new class of DNA-like oligomers made exclusively of nonnatural, stable C-nucleosides. The nucleosides comprise four types of nonnatural bases attached to a deoxyribose through an acetylene bond with the beta-configuration. The artificial DNA forms right-handed duplexes and triplexes with the complementary artificial DNA. The hybridization occurs spontaneously and sequence-selectively, and the resulting duplexes have thermal stabilities very close to those of natural duplexes. The artificial DNA might be applied to a future extracellular genetic system with information storage and amplifiable abilities.