We report measurements of the water content and swelling of wet-spun films of Na-, K-, Rb-, and Cs-DNA as a function of relative humidity (rh). The water contents (number of water molecules per base pair of DNA) of these films are found to be quite similar, indicating that the identity of the counterion species is unimportant for the water content. Since the A to B transition for these DNAs occurs at different rhs, the corresponding water contents of the A to B transition are found to be quite different. These films swell as a function of rh in a very similar manner, implying that the changes in the intermolecular bonds in the different DNAs are similar. Dramatic changes are observed in the dimensions of the films above 84% rh for all types of DNA. Combining the water content and swelling measurements yields the dependence of the volume per base pair on water content. The volume per base pair is observed to be a nonlinear function of water content, indicating nonideal mixing of the water with the DNA.

Raman and infrared (IR) spectra were recorded for crystalline cytidine samples at 298, 160, 75 and 10 K. Data were taken over the spectral range 20-4000 cm −1 . The wavenumbers of most vibrational modes were found to increase as the temperature was lowered, but several peaks near 3000 cm −1 showed the opposite trend. A number of peaks split into multiplets at low temperature. No evidence was found of any structural phase transitions in this temperature range. Plausible assignments of all main observed features are suggested.

No abstract

No abstract

Several phase transitions have been observed in SnBr4 up to 23 GPa. Measurements of the Raman spectra show that additional modes are observed in the lattice region at about 4 GPa, suggesting that the symmetry of the unit cell is changing. All lattice modes disappear and the internal vibrational modes lose their splitting in the 13 -15-GPa region, indicating that a crystal-to-amorphous transition occurs in this pressure range. Upon release of pressure, the amorphous phase is stable to about 1 GPa. A crystalline phase is observed below 1 GPa. Optical absorption experiments indicate that the optical band gap displays a continuous (and reversible) closure with increasing pressure. A photoluminescence signal is observed to red shift with increasing pressure and is quenched in the amorphous state.