Postulated hydration-dependent packing changes in Na DNA films. As water is added to the DNA film, the interhelix separation remains relatively constant while the solvation shell is filled. Eventually, the volume of hydration water will exceed the volume of the interstitial spaces, causing the lattice to expand. This lattice expansion is consistent with the observation of sample expansion as the DNA is hydrated (12, 13, 37). Upon further hydration, the lattice is expected to begin to dissociate; upon freezing, this DNA precipitates into a new form of aggregate with a packing different from that of the film.