title: Raison d'être and structural model for the B-Z transition of poly d(G-C).poly d(G-C).
abstract: In DNA oligonucleotides crystallized in the A form, the nucleotides adopt standard conformation except for steps 5'-CpG-3' where reduced base-pair twist and a sliding motion of the base pairs along their long axes causes pronounced interstrand guanine-guanine overlap. As a consequence, torsion angles alpha, beta and gamma are consistently trans, trans, trans instead of the common-gauche, trans, +gauche. This conformation significantly increases the intraresidue distance between the guanine base and the 5'-phosphate group. A molecular model of poly d(G-C).poly d(G-C) built with these structural characteristics in the A form, which we call A2-DNA, shows that rotation of the guanosine sugar into the syn orientation is easily achieved and pushes the base pair across the helix axis. If successive guanosines are changed this way, a smooth transformation occurs to the left-handed Z-DNA. We suggest that A- and A2-DNA forms of poly d(G-C).poly d(G-C) are metastable and that the actual transition is B in equilibrium (A in equilibrium A2) in equilibrium Z-DNA.
authors.affiliation: Institüt für Kristallographie, Freie Universität Berlin, Germany.
authors.forename: W
authors.initials: W
authors.lastname: Saenger
authors.forename: U
authors.initials: U
authors.lastname: Heinemann
mesh_terms: D003461:Crystallography; D004247:DNA; D008958:Models, Molecular; D009038:Motion; D009690:Nucleic Acid Conformation; D011089:Polydeoxyribonucleotides
chemical_list: D011089:Polydeoxyribonucleotides; C019530:poly(dC-dG); D004247:DNA