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Base-pair parameters in the crystal structures of the U-helix (•) and the WC-helix (○): (A) Buckle, (B) propeller twist, (C) shear, (D) the relation between the change in base-pair shear and twist angle between adjacent base pairs. The line was fit to the U-helix data (R2 = 0.97). The base sequence (5′ to 3′) is along the x-axis for the U-helix (top) and WC-helix (bottom). Buckle and shear are inverted in sign about the molecular dyad in the center of the double helix (A,C).
