Why do DNA helical models exhibit wedge rolls? A mathematical rationale

Abstract

The base-pair parameters wedge-tilt and wedge-roll are extremely useful for describing curved DNA structures. The wedge effect has generally been assumed to be due to the differences between the base-pair tilts and rolls of two successive base-pairs in a DNA molecule. A mathematical formalism has been established here, which shows that, wedgc-roll is a function of basepair tilt and helical twist, similarly the wedge-tilt is a function of base-pair roll and helical twist. This provides a straightforward explanation for the reported wedge-roll in A-DNA helical model of 12° and gives a value of -3.8° for B-DNA fiber model. The wedge-rolls and wedgetilts calculated for the oligonucleotide crystal structures, using these relations, are in excellent agreement with the values reported earlier, which were calculated directly from the coordinates, using a global helix axis.