Helical parameters of DNA do not change when DNA fibers are wetted: X-ray diffraction study.

We have measured the helical parameters of DNA in concentrated solutions by x-ray fiber diffraction methods. Fibers of the sodium salt of DNA were swollen with water within capillaries; the capillary served to limit water uptake, slowing dissolution. Samples containing up to 80% water gave essentially a B-form diffraction pattern and had virtually the same helical parameters [9.91 base pairs per turn (SD = 0.14); 3.34 A axial rise per residue (SD = 0.019)] as did the initial fibers [9.95 base pairs per turn (SD = 0.15); 3.33 A axial rise per residue (SD = 0.015)]. Hence, under highly solvated conditions in which the interactions between molecules should be greatly decreased, DNA maintains its classical B-form structure.