Structure of poly d(ai)·poly d(ct) in two different packing arrangements

Rengaswami Chandrasekaran, Akella Radha, Hye Shin Park

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7 Scopus citations


X-ray diffraction analysis of poly d(AI)poly·d(CT) in oriented and polycrystalline fibers has revealed the DNA structure to be a 10-fold, right-handed, antiparallel, Watson-Crick base paired double helix in two distinct packing arrangements corresponding to one and two helices, respectively, in the unit cell. The helix pitch is 32.1 Å and 32.4 Å in the two cases, almost 1.5 Å shorter than in classical B-DNA. The resulting B-DNA geometry, described in terms of a tetranucleotide repeat which is conformationally similar to B-DNA, has its minor groove closely shut and major groove correspondingly widened, thus striking a sharp morphological contrast to B-DNA. According to difference electron density maps, a spine of hydration along the minor groove connects both strands and provides structural stability; ordered sodium ions and water molecules are actively involved in bridging the phosphate groups of neighboring helices. The crystallographic R-values for these two allomorphs are 0.26 and 0.20, respectively, for data up to 3. 0 Å resolution.

Original languageEnglish (US)
Pages (from-to)285-305
Number of pages21
JournalJournal of Biomolecular Structure and Dynamics
Issue number2
StatePublished - Oct 1997

ASJC Scopus subject areas

  • Structural Biology
  • Molecular Biology


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