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Sökning: WFRF:(El Sagheer Afaf)

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  • Bosaeus, Niklas, 1982, et al. (författare)
  • Force-induced melting of DNA-evidence for peeling and internal melting from force spectra on short synthetic duplex sequences
  • 2014
  • Ingår i: Nucleic Acids Research. - 0305-1048 .- 1362-4962. ; 42:12, s. 8083-8091
  • Tidskriftsartikel (refereegranskat)abstract
    • Overstretching of DNA occurs at about 60-70 pN when a torsionally unconstrained double-stranded DNA molecule is stretched by its ends. During the transition, the contour length increases by up to 70% without complete strand dissociation. Three mechanisms are thought to be involved: force-induced melting into single-stranded DNA where either one or both strands carry the tension, or a B-to-S transition into a longer, still base-paired conformation. We stretch sequence-designed oligonucleotides in an effort to isolate the three processes, focusing on force-induced melting. By introducing site-specific inter-strand cross-links in one or both ends of a 64 bp AT-rich duplex we could repeatedly follow the two melting processes at 5 mM and 1 M monovalent salt. We find that when one end is sealed the AT-rich sequence undergoes peeling exhibiting hysteresis at low and high salt. When both ends are sealed the AT sequence instead undergoes internal melting. Thirdly, the peeling melting is studied in a composite oligonucleotide where the same AT-rich sequence is concatenated to a GC-rich sequence known to undergo a B-to-S transition rather than melting. The construct then first melts in the AT-rich part followed at higher forces by a B-to-S transition in the GC-part, indicating that DNA overstretching modes are additive.
  • Bosaeus, Niklas, 1982, et al. (författare)
  • Tension induces a base-paired overstretched DNA conformation
  • 2012
  • Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - 0027-8424 .- 1091-6490. ; 109:38, s. 15179-15184
  • Tidskriftsartikel (refereegranskat)abstract
    • Mixed-sequence DNA molecules undergo mechanical overstretching by approximately 70% at 60-70 pN. Since its initial discovery 15 y ago, a debate has arisen as to whether the molecule adopts a new form [Cluzel P, et al. (1996) Science 271: 792-794; Smith SB, Cui Y, Bustamante C (1996) Science 271: 795-799], or simply denatures under tension [van Mameren J, et al. (2009) Proc Natl Acad Sci USA 106: 18231-18236]. Here, we resolve this controversy by using optical tweezers to extend small 60-64 bp single DNA duplex molecules whose base content can be designed at will. We show that when AT content is high (70%), a force-induced denaturation of the DNA helix ensues at 62 pN that is accompanied by an extension of the molecule of approximately 70%. By contrast, GC-rich sequences (60% GC) are found to undergo a reversible overstretching transition into a distinct form that is characterized by a 51% extension and that remains base-paired. For the first time, results proving the existence of a stretched basepaired form of DNA can be presented. The extension observed in the reversible transition coincides with that produced on DNA by binding of bacterial RecA and human Rad51, pointing to its possible relevance in homologous recombination.
  • Börjesson, Karl, 1982, et al. (författare)
  • Functionalized Nanostructures: Redox-Active Porphyrin Anchors for Supramolecular DNA Assemblies
  • 2010
  • Ingår i: ACS Nano. - 1936-086X .- 1936-0851. ; 4:9, s. 5037-5046
  • Tidskriftsartikel (refereegranskat)abstract
    • We have synthesized and studied a supramolecular system comprising a 39-mer DNA with porphyrin-modified thymidine nucleosides anchored to the surface of large unilamellar vesicles (liposomes). Liposome porphyrin binding characteristics, such as orientation, strength, homogeneity, and binding site size, was determined, suggesting that the porphyrin is well suited as a photophysical and redox-active lipid anchor, in comparison to the inert cholesterol anchor commonly used today. Furthermore, the binding characteristics and hybridization capabilities were studied as a function of anchor size and number of anchoring points, properties that are of importance for our future plans to use the addressability of these redox-active nodes in larger DNA-based nanoconstructs. Electron transfer from photoexcited porphyrin to a lipophilic benzoquinone residing in the lipid membrane was characterized by steady-state and time-resolved fluorescence and verified by femtosecond transient absorption.
  • Börjesson, Karl, 1982, et al. (författare)
  • Nucleic Acid Base Analog FRET-Pair Facilitating Detailed Structural Measurements in Nucleic Acid Containing Systems
  • 2009
  • Ingår i: Journal of the American Chemical Society. - 1520-5126 .- 0002-7863. ; 131:12, s. 4288-4293
  • Tidskriftsartikel (refereegranskat)abstract
    • We present the first nucleobase analog fluorescence resonance energy transfer (FRET)-pair. The pair consists of tC O , 1,3-diaza-2- oxophenoxazine, as an energy donor and the newly developed tC nitro , 7-nitro-1,3-diaza-2-oxophenothiazine, as an energy acceptor. The FRET-pair successfully monitors distances covering up to more than one turn of the DNA duplex. Importantly, we show that the rigid stacking of the two base analogs, and consequently excellent control of their exact positions and orientations, results in a high control of the orientation factor and hence very distinct FRET changes as the number of bases separating tC O and tCnitro is varied. A set of DNA strands containing the FRET-pair at wisely chosen locations will, thus, make it possible to accurately distinguish distance- from orientation-changes using FRET. In combination with the good nucleobase analog properties, this points toward detailed studies of the inherent dynamics of nucleic acid structures. Moreover, the placement of FRET-pair chromophores inside the base stack will be a great advantage in studies where other (biomacro)molecules interact with the nucleic acid. Lastly, our study gives possibly the first truly solid experimental support to the dependence of energy transfer efficiency on orientation of involved transition dipoles as predicted by the Forster theory. © 2009 American Chemical Society.
  • Gorman, Jeffrey, et al. (författare)
  • Deoxyribonucleic Acid Encoded and Size-Defined pi-Stacking of Perylene Diimides
  • 2022
  • Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 144:1, s. 368-376
  • Tidskriftsartikel (refereegranskat)abstract
    • Natural photosystems use protein scaffolds to control intermolecular interactions that enable exciton flow, charge generation, and long-range charge separation. In contrast, there is limited structural control in current organic electronic devices such as OLEDs and solar cells. We report here the DNA-encoded assembly of pi-conjugated perylene diimides (PDIs) with deterministic control over the number of electronically coupled molecules. The PDIs are integrated within DNA chains using phosphoramidite coupling chemistry, allowing selection of the DNA sequence to either side, and specification of intermolecular DNA hybridization. In this way, we have developed a "toolbox" for construction of any stacking sequence of these semiconducting molecules. We have discovered that we need to use a full hierarchy of interactions: DNA guides the semiconductors into specified close proximity, hydrophobic-hydrophilic differentiation drives aggregation of the semiconductor moieties, and local geometry and electrostatic interactions define intermolecular positioning. As a result, the PDIs pack to give substantial intermolecular pi wave function overlap, leading to an evolution of singlet excited states from localized excitons in the PDI monomer to excimers with wave functions delocalized over all five PDIs in the pentamer. This is accompanied by a change in the dominant triplet forming mechanism from localized spin-orbit charge transfer mediated intersystem crossing for the monomer toward a delocalized excimer process for the pentamer. Our modular DNA-based assembly reveals real opportunities for the rapid development of bespoke semiconductor architectures with molecule-by-molecule precision.
  • Hannestad, Jonas, 1981, et al. (författare)
  • Kinetics of Diffusion-Mediated DNA Hybridization in Lipid Monolayer Films Determined by Single-Molecule Fluorescence Spectroscopy
  • 2013
  • Ingår i: ACS Nano. - 1936-086X .- 1936-0851. ; 7:1, s. 308-315
  • Tidskriftsartikel (refereegranskat)abstract
    • We use single-molecule fluorescence microscopy to monitor individual hybridization reactions between membrane-anchored DNA strands, occurring in nanofluidic lipid monolayer films deposited on Teflon AF substrates. The DNA molecules are labeled with different fluorescent dyes, which make it possible to simultaneously monitor the movements of two different molecular species, thus enabling tracking of both reactants and products. We employ lattice diffusion simulations to determine reaction probabilities upon interaction. The observed hybridization rate of the 40-mer DNA was more than 2-fold higher than that of the 20-mer DNA. Since the lateral diffusion coefficient of the two different constructs is nearly identical, the effective molecule radius determines the overall kinetics. This implies that when two DNA molecules approach each other, hydrogen bonding takes place distal from the place where the DNA is anchored to the surface. Strand closure then propagates bidirectionally through a zipper-like mechanism, eventually bringing the lipid anchors together. Comparison with hybridization rates for corresponding DNA sequences in solution reveals that hybridization rates are lower for the lipid-anchored strands and that the dependence on strand length is stronger.
  • Kumar, R., et al. (författare)
  • Template-directed oligonucleotide strand ligation, covalent intramolecular DNA circularization and catenation using click chemistry
  • 2007
  • Ingår i: Journal of the American Chemical Society. - 1520-5126 .- 0002-7863. ; 129:21, s. 6859-6864
  • Tidskriftsartikel (refereegranskat)abstract
    • The copper-catalyzed azide-alkyne cycloaddition reaction has been used for the template-mediated chemical ligation of two oligonucleotide strands, one with a 5'-alkyne and the other with a 3'-azide, to produce a DNA strand with an unnatural backbone at the ligation point. A template-free click-ligation reaction has been used for the intramolecular circularization of a single stranded oligonucleotide which was used as a template for the synthesis of a covalently closed DNA catenane.
  • Lundberg, Erik, 1981, et al. (författare)
  • A new fixation strategy for addressable nano-network building blocks
  • 2010
  • Ingår i: Chemical Communications. - 1359-7345. ; 46:21, s. 3714-3716
  • Tidskriftsartikel (refereegranskat)abstract
    • Rapid controlled self-assembly makes DNA ideal for building nanostructures. A problem using hybridized intermediates in hierarchic assembly is their thermodynamic lability. We demonstrate a click-fixation technology by which robust hexagonal DNA modules can be made. This principle is applicable to a wide variety of DNA nanoconstructs.
  • McGinn, Steven, et al. (författare)
  • New Technologies for DNA analysis-A review of the READNA Project.
  • 2016
  • Ingår i: New Biotechnology. - : Elsevier. - 1876-4347 .- 1871-6784. ; 33:3, s. 311-330
  • Forskningsöversikt (refereegranskat)abstract
    • The REvolutionary Approaches and Devices for Nucleic Acid analysis (READNA) project received funding from the European Commission for 4 1/2 years. The objectives of the project revolved around technological developments in nucleic acid analysis. The project partners have discovered, created and developed a huge body of insights into nucleic acid analysis, ranging from improvements and implementation of current technologies to the most promising sequencing technologies that constitute a 3(rd) and 4(th) generation of sequencing methods with nanopores and in situ sequencing, respectively.
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