The oligonucleotides d[(G‐A)7G] and d[(G‐A)12G] self‐associate under physiological conditions (10 mM MgCl2, neutral pH) into a stable double‐helical structure (psRR‐DNA) in which the two polypurine strands are in a parallel orientation in contrast to the antiparallel disposition of conventional B‐DNA. We have characterized psRR‐DNA by gel electrophoresis, UV absorption, vacuum UV circular dichroism, monomer‐excimer fluorescence of oligonucleotides end‐labelled with pyrene, and chemical probing with diethyl pyrocarbonate and dimethyl sulfate. The duplex is stable at pH 4–9, suggesting that the structure is compatible with, but does not require, protonation of the A residues. The data support a model derived from force‐field analysis in which the parallel‐stranded d(G‐A)n helix is right‐handed and constituted of alternating, symmetrical Gsyn.Gsyn and Aanti.Aanti base pairs with N1H…O6 and N6H…N7 hydrogen bonds, respectively. This dinucleotide structure may be the source of a negative peak observed at 190 nm in the vacuum UV CD spectrum, a feature previously reported only for left‐handed Z‐DNA. The related sequence d[(GAAGGA)4G] also forms a parallel‐stranded duplex but one that is less stable and probably involves a slightly different secondary structure. We discuss the potential intervention of psRR‐DNA in recombination, gene expression and the stabilization of genomic structure.
Several crystal structures of RNA fragments, alone or in complex with a specific protein, have been recently solved. In addition, the structures of an artificial ribozyme, the leadzyme, and the cleavage product of a human pathogen ribozyme, have extended the structural diversity of ribozyme architectures. The attained set of folding rules and motifs expand the repertoire seen previously in tRNA structures.
Single crystals of the title compound were obtained from the electrochemical oxidation of ET ( 3 . 5~ lo-' M in a mixture of CH,CI,/CH,CN/H,O 10:2:1) in the presence of the tetrabutylammonium salt of the polyoxoanion [COW,,O,,,-~ 1161 (5 x M in a mixture of CH,CI,/CH,CN 1 :1) in a U-shaped cell with Pt electrodes (0.5 mm diameter) separated by a sintered-glass frit. The intensity of the current was fixed at 1 PA. After 2 or 3 weeks. amber, platelike single crystals of the title compound were collected carefully and air dried.Magnetic measurements were carried out on single crystals with a magnetometer (905 VTS, SEH Corporation) equipped with a SQUID sensor (SQUID = superconducting quantum interference device). EPR measurements of the X band at variable temperature were recorded on a Bruker ER 200D spectrometer equipped with a helium cryostat. In the experiment the static magnetic field is perpendicular to the long axis of the needlelike single crystal. Infrared spectra in the range 400-4700cm-' were made on KBr pellets with a FT-IR interferometer Nicolet MX1. Conductivity measurements were performed using the four-probe method on approximately 4-mm long-needlelike single crystals. O,,] . 5.5: amber platelike crystal: 0.41 x 0.34 x 0.01 mm', Enraf-Nonius CAD4 diffractometer. M = 6081.8, monoclinic.space group f 2 / m with u = 13.971(9), b = 43.117 (7), c =14.042 (5) A, fl = 107.25(3), V = 8078.5 A' . and Z = 2 (pea,cd = 2.496 gem-'; Mo,, radiation, E. = 0.71073 A. p = 96.554 cm-I), structure solved by direct methods and successive Fourier difference synthesis. Refinement of 329 variables with anisotropic thermal parameters (for Co, W and S atoms) gave R ( F ) = 0.068 and Rw(F) = 0.102 by using 2614 absorption corrected reflections with f 2 6rr(I). Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (FRG), on quoting the depository number CSD-57908. the authors, and the journal citation. [8] H. T. Evans, M. T. Pope, Inorg. Chem. 1984, 23, 501. [9] J.Considerable efforts have been made recently in the structural modification of oligonucleotides."] Several properties of the natural 2'-deoxyoligoribonucleotides must be improved for potential therapeutic application of the antisense strategy. For example, the stability of the antisense oligonucleotides towards nucleases must be substantially increased, while the affinity and the specificity for the complementary RNA target is maintained (or even increased). So far, most of the replacements of the phosphodiester linkage that lead to an increased resistance towards nucleases are also connected with a decrease in the affinity for a complementary RNA strand.". ' I Two exceptions to this were recently reported in which either an N-methylhydroxylamine or a thioformacetal unit was used as a PO,We proposed to replace the natural phosphodiester linkage in 1 (Scheme 1) by an amide function (as in 2-4), which offers several advantages compared to previously reported modificat i o n~...
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