Structure of poly (dT)center dot poly (dA)center dot poly (dT)

The molecular structure of poly (dT).poly (dA).poly (dT) has been determine d and refined using the continuous x-ray intensity data on layer lines in t he diffraction pattern obtained from an oriented fiber of the DNA. The fina l R-value for the preferred structure is 0.29 significantly lower than that for plausible alternatives. The molecule forms a 12-fold right-handed trip le-helix of pitch 38.4 Angstrom and each base tripler is stabilized by a se t of four Crick-Watson-Hoogsteen hydrogen bonds. The deoxyribose rings in a ll the three strands have C2'-endo conformations. The grooveless cylindrica l shape of the triple-helix is consistent with the lack of lateral organiza tion in the fiber.