We have used Brillouin spectroscopy to study the velocities and attenuation of acoustic phonons in wet‐spun films of Na‐DNA and Li‐DNA as a function of the degree of hydration at room temperature. Our data for the longitudinal acoustic (LA) phonon velocity vs water content display several interesting features and reveal effects that we can model at the atomic level as interhelical bond softening and relaxation of the hydration shell. The model for interhelical softening makes use of other physical parameters of these films, which we have determined by gravimetric, x‐ray, and optical microscopy studies. We extract intrinsic elastic constants for hydrated Na‐DNA molecules of c11 ≃ 8.0 × 1010 dynes/cm2 and c33 ≃ 5.7 × 1010 dynes/cm2, which corresponds to a Young's modulus, E ≃ 1.1 × 1010 dynes/cm2 (with Poisson's ratio, σ = 0.44). The negative velocity anisotropy of the LA phonons indicates that neighboring DNA molecules are held together by strong interhelical bonds in the solid state. The LA phonon attenuation data can be understood by the relaxational model in which the acoustic phonon is coupled to a relaxation mode of the water molecules. Na‐DNA undergoes the A to B phase transition at a relative humidity (rh) of 92% while Li‐DNA (which remains in the B form in this range) decrystallizes at an rh of 84%. We find that our Brillouin results for Na‐ and Li‐DNA are remarkably similar, indicating that the A to B phase transition does not play an important role in determining the acoustic properties of these two types of DNA.
ASJC Scopus subject areas
- Organic Chemistry