Helix-helix transitions in DNA: fibre X-ray study of the particular cases poly(dG-dC)center dot poly(dG-dC) and poly(dA)center dot 2poly(dT)

The helix-helix transitions which occur in poly(dG-dC) . poly(dG-dC) and in poly (dC-m(5)dC) . poly(dG-m(5)dC) are commonly assumed to be changes betw een the right-handed A- or B-DNA double helices and the left-handed Z-DNA s tructure. The mechanisms for such transconformations are highly improbable, especially when they are supposed to be active in long polynucleotide chai ns organised in semicrystalline fibres. The present alternative possibility assumes that rather than the Z-DNA it is a right-handed double helix (S-DN A) which actually takes part in these form. transitions. Two molecular mode ls of this S form, in good agreement with X-ray measurements, are proposed. They present alternating C(2')-endo and C(3')- endo sugar puckering like t he "alternating B-DNA" put forward some years ago. Dihedral angles, sets of atomic coordinates and stereo views of the two S-DNA structures are given, together with curves of calculated diffracted intensities. Furthermore, we question the possibility of obtaining semicrystalline fibres with triple h elices of poly(dA) . 2poly(dT) in a way which renders X-ray diffraction eff icient. It is suggested that, up to now, only double helices of poly(dA) . poly(dT) can actually be observed by fibre X-ray diffraction measurements.