Evidence for heterogeneity in DNA‐associated solvent mobility from acridine phosphorescence spectra

The mobility of solvent associated with native DNA in comparison with that of the bulk solvent is monitored from the temperature‐dependent red shift in the phosphorescence spectra of acridines bound to DNA and free in glycol–buffer mixtures. Over the temperature range for which the red shift occurs the phosphorescence decay changes with emission wavelength, indicating the time‐dependent nature of the process. Moreover, at these temperatures, emission anisotropy measurements establish that motions of the dye itself are not involved. Correspondence between perturbations to the solvent that influence the temperature at which the red shift occurs for free acridine with those for the DNA‐bound dye confirm that “bound solvent” is responsible for the spectral changes. For the DNA‐bound acridines the extent of the red shift is smaller and the midpoint T 1/2 of the transition is warmer. The reduction in the red shift reveals that the bound dye is less exposed to solvent and varies as 9‐aminoacridine < acridine orange ∼ proflavin, i.e., 9‐amino‐acridine is less exposed to solvent. On the other hand, the warmer T 1/2 indicates that DNA‐associated solvent is considerably less mobile than bulk solvent. T 1/2 varies for proflavin bound to DNA, poly[d(AT)], poly[d(GC)], and poly(dG): poly(dC), and for proflavin, acridine orange, and 9‐aminoacrine bound to DNA. These observations suggest that there is a heterogeneity in the mobility of DNA‐associated solvent.