Thermodynamic analysis of stacking hybridization of oligonucleotides with DNA template

RNA Complexes with Nicks and Gaps: Thermodynamic and Kinetic Effects of Coaxial Stacking and Dangling Ends

Multiple RNA strands can interact in solution and assume a large variety of configurations dictated by their potential for base pairing. Although duplex formation from two complementary oligonucleotides has been studied in detail, we still lack a systematic characterization of the behavior of higher order complexes. Here, we focus on the thermodynamic and kinetic effects of an upstream oligonucleotide on the binding of a downstream oligonucleotide to a common template, as we vary the sequence and structure of the contact interface. We show that coaxial stacking in RNA is well correlated with but much more stabilizing than helix propagation over an analogous intact double helix step (median ΔΔG°37 °C ≈ 1.7 kcal/mol). Consequently, approximating coaxial stacking in RNA with the helix propagation term leads to large discrepancies between predictions and our experimentally determined melting temperatures, with an offset of ≈10 °C. Our kinetic study reveals that the hybridization of the downstream probe oligonucleotide is impaired (lower kon) by the presence of the upstream oligonucleotide, with the thermodynamic stabilization coming entirely from an extended lifetime (lower koff) of the bound downstream oligonucleotide, which can increase from seconds to months. Surprisingly, we show that the effect of nicks is dependent on the length of the stacking oligonucleotides, and we discuss the binding of ultrashort (1-4 nt) oligonucleotides that are relevant in the context of the origin of life. The thermodynamic and kinetic data obtained in this work allow for the prediction of the formation and stability of higher-order multistranded complexes.

Single-molecule analysis of DNA base-stacking energetics using patterned DNA nanostructures

The DNA double helix structure is stabilized by base-pairing and base-stacking interactions. However, a comprehensive understanding of dinucleotide base-stacking energetics is lacking. Here we combined multiplexed DNA-based point accumulation in nanoscale topography (DNA-PAINT) imaging with designer DNA nanostructures and measured the free energy of dinucleotide base stacking at the single-molecule level. Multiplexed imaging enabled us to extract the binding kinetics of an imager strand with and without additional dinucleotide stacking interactions. The DNA-PAINT data showed that a single additional dinucleotide base stacking results in up to 250-fold stabilization for the DNA duplex nanostructure. We found that the dinucleotide base-stacking energies vary from -0.95 ± 0.12 kcal mol-1 to -3.22 ± 0.04 kcal mol-1 for C|T and A|C base-stackings, respectively. We demonstrate the application of base-stacking energetics in designing DNA-PAINT probes for multiplexed super-resolution imaging, and efficient assembly of higher-order DNA nanostructures. Our results will aid in designing functional DNA nanostructures, and DNA and RNA aptamers, and facilitate better predictions of the local DNA structure.

A new approach to precise thermodynamic characterization of hybridization properties of modified oligonucleotides: Comparative studies of deoxyribo- and glycine morpholine pentaadenines

The development of new derivatives and analogues of nucleic acids for the purposes of molecular biology, biotechnology, gene diagnostics, and medicine has been a hotspot for the last two decades. Methylenecarboxamide (glycine) morpholine oligomer analogues (gM) seem to be promising therapeutic candidates because of the ability to form sequence specific complexes with DNA and RNA. In this paper we describe new approaches to the determination of thermodynamic parameters for hybridization of tandem oligonucleotide complexes with the complementary template. It makes possible to determine changes in enthalpy and entropy corresponding to the binding of an individual oligomer with the template, and to the formation of cooperative contact at the helix-helix interface of two neighboring duplex fragments (in the nick). We have experimentally analyzed the series of model tandem complexes of different length at various oligomer concentrations, ionic strength, and pH. The analysis of thermodynamic parameters of complex formation for native and modified oligomers revealed higher Gibbs free energy values of hybridization and cooperative interaction of morpholine-containing complexes at the helix-helix interface under standard conditions (1M NaCl, pH7.2). Further comparative analysis of the hybridization properties of modified oligomers at ionic strength and pH allows us to determine the charge state of the morpholine backbone and the thermodynamic origin of the effects observed. It was found that the decrease in pH to 5.5 led to the protonation of internal morpholine nitrogens. The obtained results prove the veracity of the proposed model and the possibility to evaluate thermodynamic parameters of short native and modified oligomers with high accuracy.

Rational design and studies of excimer forming novel dual probes to target RNA

In this paper, we report structure-based rational design and physico-chemical and biological studies of novel pyrene excimer forming dual probes for visualization of intracellular RNAs. Herein, the probes based on 2'-O-methyl RNA with linkers of different structure and length between pyrene moiety and ribose are studied with respect to their hybridization and spectral properties. We found optimal linkers that provide more intense excimer emission (at ∼480nm) of RNA-bound probes; particularly, the length of the linker arm of the 3'-component of dual probes plays a key role in formation of pyrene excimer. Calculated molecular dynamics trajectories and probability distributions of pyrene-pyrene dimer formation upon hybridization of the dual probes with RNA target are in agreement with the obtained fluorescence spectroscopy data for the corresponding duplexes. Our study demonstrates the excellent binding properties of new dual probes to structured RNA and their feasibility for the visualization of intracellular RNA targets.

Bridged oligonucleotides as molecular probes for investigation of enzyme-substrate interaction and allele-specific analysis of DNA

The efficiency of enzymatic conversion of DNA complexes containing non-nucleotide inserts has been studied. T4 DNA ligase and Taq DNA polymerase have been included in the study as examples of widely used DNA-dependent enzymes. A series of substrate DNA complexes have been formed using native oligonucleotides and bridged ones bearing non-nucleotide inserts based on phosphodiesters of di-, tetra-, or hexaethylene glycol, 1,5-pentanediol, 1,10-decanediol, and 3-hydroxy-2(hydroxymethyl)-tetrahydrofuran. The perturbation in DNA located far from the site of the enzyme action had almost no influence on the substrate properties of the complex, while insertion near this site significantly deteriorated them. The use of a series of modified duplexes allows one to locate the position of the enzyme-binding site on DNA substrate with the accuracy of 1-2 nucleotides. The presence of a non-nucleotide insert in the complex has been also shown to enhance the efficiency of single mismatch discrimination upon both template-directed ligation and extension of oligonucleotides.

Kinetics of base stacking-aided DNA hybridization

The association and dissociation rate constants (k(a) and k(d)) of DNA hybridizations involving dual, single or no stacking with different base-pairing sizes were measured, which reveals the advantage of stacking hybridization in both the kinetic and steady state.

Enhanced cellular binding of concatemeric oligonucleotide complexes

Interaction of oligonucleotides condensed into long concatemeric complexes with cancer cells was investigated. Pairs of 24- and 25-mer oligodeoxyribonucleotides were designed so that they could hybridize and form concatemeric structures. Pre-assembling of the oligonucleotides into concatemers considerably enhanced their ability to bind to human embryo kidney 293 cells and neuroblastoma IMR-32 cells as compared to free oligonucleotides. Efficiency of concatemers binding to the cells is improved with increase of the length and concentration of concatemeric complexes. The obtained results suggest incorporation of pharmacologically active oligonucleotides into concatemeric complexes as an approach to improvement of their cellular interaction.

Base-stacking and base-pairing contributions into thermal stability of the DNA double helix

Two factors are mainly responsible for the stability of the DNA double helix: base pairing between complementary strands and stacking between adjacent bases. By studying DNA molecules with solitary nicks and gaps we measure temperature and salt dependence of the stacking free energy of the DNA double helix. For the first time, DNA stacking parameters are obtained directly (without extrapolation) for temperatures from below room temperature to close to melting temperature. We also obtain DNA stacking parameters for different salt concentrations ranging from 15 to 100 mM Na⁺. From stacking parameters of individual contacts, we calculate base-stacking contribution to the stability of A•T- and G•C-containing DNA polymers. We find that temperature and salt dependences of the stacking term fully determine the temperature and the salt dependence of DNA stability parameters. For all temperatures and salt concentrations employed in present study, base-stacking is the main stabilizing factor in the DNA double helix. A•T pairing is always destabilizing and G•C pairing contributes almost no stabilization. Base-stacking interaction dominates not only in the duplex overall stability but also significantly contributes into the dependence of the duplex stability on its sequence.

The influence of nearest neighbours on the efficiency of coaxial stacking at contiguous stacking hybridization of oligodeoxyribonucleotides

Contiguous stacking hybridization of oligodeoxyribonucleotides with a stem of preformed minihairpin structure of a DNA template was studied with the use of UV-melting technique. It was shown that the free-energy of the coaxial stacking interaction (deltaG degrees ST at 37 degrees C, 1 M NaCl, pH 7.4) at the complementary interface XA*pTY/ZATV (an asterisk stands for a nick) strongly depends on the type of nearest neighbor bases X and Y flanking the nicked dinucleotide step. The maximum efficiency of the coaxial stacking was observed for the PuA*pTPy/PuATPy interface, whereas the minimum efficiency was obtained for the PyA*pTPu/PyATPu interface. A 5'-phosphate residue in the nick enhances the coaxial stacking. In dependence on duplex structure the observed efficiency of A*T/AT coaxial stacking varied from (- 0.97 kcal/mol) for unphosphorylated TA*TA/TATA interface to three-fold higher value (- 2.78 kcal/mol) for GA*pTT/AATC interface.

The influence of the non-nucleotide insert on the hybridization properties of oligonucleotides

The effect of different non-nucleotide inserts incorporated into oligonucleotide chains on their hybridization properties was studied by the method of thermal denaturation. Various types of alkyldiols and oligoethylene glycols were used as inserts modifying oligonucleotide backbone. Such modification of oligonucleotides caused the destabilization of their complementary complexes. It was shown that the hybridization properties of the modified oligonucleotides depend on several features of inserts: the type, number, length of insertions, and positions of interrupted dinucleotide steps in oligonucleotide chain.

Efficiency of coaxial stacking depends on the DNA duplex structure

Thermodynamic parameters of coaxial stacking at complementary helix-helix interfaces GX*pYG/CZVC (X,Y=A,C,T,G;*-nick) created by contiguous oligonucleotide hybridization were determined. The data obtained were compared to the thermodynamic parameters of coaxial stacking at the interfaces CX*pYC/GZVG. Multiple linear regression analysis has revealed that the free-energy increments of interaction for the contacts GX*pYG/CZVC and CX*pYC/GZVG can be described by a set of uniform Delta G degrees(X*pY/ZV) values. The difference in the observed free-energy of the coaxial stacking between the two sets is defined by the contribution from the factors reflecting structural differences between compared DNA duplexes.

Evaluating the contribution of base stacking during translesion DNA replication

Despite the nontemplating nature of the abasic site, dAMP is often preferentially inserted opposite the lesion, a phenomenon commonly referred to as the "A-rule". We have evaluated the molecular mechanism accounting for this unique behavior using a thorough kinetic approach to evaluate polymerization efficiency during translesion DNA replication. Using the bacteriophage T4 DNA polymerase, we have measured the insertion of a series of modified nucleotides and have demonstrated that increasing the size of the nucleobase does not correlate with increased insertion efficiency opposite an abasic site. One analogue, 5-nitroindolyl-2'-deoxyriboside triphosphate, was unique as it was inserted opposite the lesion with approximately 1000-fold greater efficiency compared to that for dAMP insertion. Pre-steady-state kinetic measurements yield a kpol value of 126 s(-1) and a Kd value of 18 microM for the insertion of 5-nitroindolyl-2'-deoxyriboside triphosphate opposite the abasic site. These values rival those associated with the enzymatic formation of a natural Watson-Crick base pair. These results not only reiterate that hydrogen bonding is not necessary for nucleotide insertion but also indicate that the base-stacking and/or desolvation capabilities of the incoming nucleobase may indeed play the predominant role in generating efficient DNA polymerization. A model accounting for the increase in catalytic efficiency of this unique nucleobase is provided and invokes pi-pi stacking interactions of the aromatic moiety of the incoming nucleobase with aromatic amino acids present in the polymerase's active site. Finally, differences in the rate of 5-nitroindolyl-2'-deoxyriboside triphosphate insertion opposite an abasic site are measured between the bacteriophage T4 DNA polymerase and the Klenow fragment. These kinetic differences are interpreted with regard to the differences in various structural components between the two enzymes and are consistent with the proposed model for DNA polymerization.