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

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 A 2 ‐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 A 2 ‐DNA forms of poly d(G‐C)·poly d(G‐C) are metastable and that the actual transition is B ⇌ (A ⇌ A 2 ) ⇌ Z‐DNA.