Counterion Effects on the Structure and Dynamics of Solid DNA

We have measured the counterion species and concentration dependence of the low-frequency Raman spectrum, speed of sound, and the $A-B$ transition in solid DNA. We find that both the frequency of the lowest Raman active mode, and the water content at the $A-B$ phase transition depend strongly on ion size and concentration. Longitudinal elastic constants show only a small dependence. These results indicate that ion-mediated interhelical forces control several important properties of solid DNA.