Raison d'être and structural model for the B‐Z transition of poly d(G‐C)∗poly d(G‐C)

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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 α,β and γ 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 ⇌ (A ⇌ A2) ⇌ Z‐DNA.