Structure of a B-DNA dodecamer at 16 K.

The crystal structure of the B-DNA dodecamer C-G-C-G-A-A-T-T-C-G-C-G, previously solved and refined at room temperature (290 K), has been analyzed at 16 K (-257 degrees C). The end-to-end winding of the helix does not vary with temperature but remains constant at 10.1 base pairs per turn. Negatively charged phosphate groups throughout the structure do move closer together on cooling, however, probably because of increase in the dielectric constant of water as the temperature is lowered. This has the two-fold effect of reducing the spacing between neighboring double helices from 24.0 to 22.9 A and of narrowing the helix grooves within any isolated molecule. Overall lattice displacements as deduced from crystallographic temperature factors are very much decreased in the 16 K structure, yet displacements at phosphates continue to exceed those of deoxyribose sugars by B = 9 A2 and those of base pairs by B = 22 A2, even at this very low temperature at which practically all thermal motion has been eliminated. These differences, formerly interpreted as evidence for thermal vibration, must now be attributed to static disorder.