The optical properties of Li‐ and Na‐DNA films

We have measured the refractive indices of highly crystalline Li‐ and Na‐DNA wet‐spun films as a function of their water content using an immersion technique. We calculated the molecular polarizabilities of a DNA base pair using the Lorentz–Lorenz relation for anisotropic materials, the measured water contents, and densities corrected for void formation. For Li‐DNA, the polarizabilities are independent of the relative humidity, whereas for Na‐DNA, there are large changes at the A‐B transition and also at low humidities. The average polarizability of A‐Na‐DNA is in agreement with that calculated from refractive index increments and also as calculated by a simple addition of bond polarizabilities, whereas the values for Li‐ and B‐Na‐DNA are about 30% larger than the calculated values. We propose that these anomalous values are due to nonlinear polarizabilities of the phosphate group.