Raman study of CaDNA films as a function of water content and excess CaCl$_{2}$ concentration: Stability of the B conformation.

Highly oriented, wet-spun films of CaDNA expand in the direction perpendicular to the helical axis as the hydration of the film is increased. CaDNA films with a high CaCl$_{2}$ content show an unexpected shrinkage at a relative humidity of about 93$\%$. We have performed Raman experiments on CaDNA films as a function of both water content and excess CaCl$_{2}$ concentration in order to determine if this unexpected shrinkage might be related to a conformational transition of the DNA molecules. We find that the DNA molecules remain in the B conformation for all salt contents down to a relative humidity of 59$\%$.