Experimental support for a right-handed vertical double helix

The experimentally observed geometry of a miniature double helix of high anti (χCN fixed ? 120°) nucleic acid structure indicates substantial interstrand base stacking with little intrastrand base stacking. This geometry is consistent with the right-handed vertical double helix in which the base planes are parallel to helical axis proposed by Olson [(1977) Proc. Natl. Acad. Sci. USA 74 , 1775] from theoretical calculations. The experimental data do not agree with the left-handed model in which the base planes are perpendicular to helical axis that has been proposed by Yathindra and Sundaralingam [(1976) Nucl. Acids Res. 3 , 729]. The theoretically computed chemical shift changes for the various double-helical configurations show that for the system examined, only the vertical model can explain the experimentally observed shift trends. The melting curve for the helix–coil transition for high anti dinucleoside monophosphate has been observed for the first time. Even though the experimental data support vertical double helices when χCN is fixed at 120°, data on naturally occurring nucleic acid structures indicate that they have no proclivity to enter into vertically stabilized double-helical arrays.