Structure of Py.Pu.Py DNA triple helices. Fourier transforms of fiber-type x-ray diffraction of single crystals

Well-formed hexagonal crystals of oligomeric DNA triple helices exhibit fiber-type x-ray diffraction patterns [cf., Liu et al. (1994) Nature Struct. Biol. 1, 11], which can be interpreted in terms of Fourier transforms of these helices. Precession photographs of a triplex formed of dA and dT chains show that it has 13 residues per turn. In contrast, a sequence containing the four natural bases A, G, C, and T has 12 residues per turn. In this sense the triple helices exhibit a sequence-dependent polymorphism, though both have C2'-endo sugar pucker and B rather than A conformation. New models are constructed, using constraints from x-ray diffraction, and Fourier transforms of the models are calculated. Good agreement in the amplitudes and positions of the calculated and observed diffraction intensities confirms the structures for both triple helices. These are the first stereochemically satisfactory DNA triple helices for which coordinates based on adequate experimental data were provided. Sequences for crystallization are designed to achieve unique base alignments and are screened for the presence of sharp bands on gel electrophoresis to assure the absence of multiple species caused by strand slippage. Despite intensive efforts to observe normal crystal diffraction by varying sequences and conditions, all crystals exhibited only fiber-type diffraction. We suggest that this behavior may be an intrinsic property of triple helices and discuss possible reasons for the results. Spectroscopic and chemical experiments establish that the oligonucleotides exist in solution as triple helices under the conditions of crystallization.