Fibre X-ray study of the helix-helix transitions of poly(dG-dC).poly(dG-dC)

Poly(dG-dC).poly(dG-dC) and poly(dG-m(5)dC).poly(dG-m5dC) present helix-hel ix transitions which are commonly assumed to be changes between the right-h anded A- or B-DNA double helices and the left-handed Z-DNA structure. The m echanisms for such transconformations are highly improbable especially when they are supposed to be active in long polynucleotide chains organised in semicrystalline fibres. The present alternative possibility assumes that ra ther than the Z-DNA it is a right-handed double helix (S-DNA) which actuall y takes part in these form transitions. Two molecular models of this S form , in good agreement with X-ray measurements, are proposed. They present alt ernating C(2')-endo and C(3')-endo sugar puckering. Dihedral angles, sets o f atomic co-ordinates and stereo views of the two S-DNA structures are give n together with curves of calculated diffracted intensities. (C) 1999 Acade mie des sciences/Editions scientifiques et medicales Elsevier SAS.