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Abstract
The molecular structure of poly (I)•poly (A)•poly (I) has been determined and refined using the continuous intensity data on layer lines in the x-ray diffraction pattern obtained from an oriented fiber of this polymorphic RNA complex. The polymer forms a 12-fold right-handed triple-helix of pitch 39.7Å and each base-triplet is stabilized by quasi Crick-Watson-Hoogsteen hydrogen bonds. The ribose rings in all the three strands have C3′-endo conformations. The final R-value for this best structure is 0.24 and the x-ray fit is significantly superior to all the alternative structures where the different chains might have different furanose conformations. This all-purine triple-helix, counter-intuitively, has a diameter roughly 3Å shorter than that of DNA and RNA triple-helices containing a homopurine and two complementary homopyrimidine strands. Its compact, grooveless cylindrical shape is consistent with the lack of lateral organization.