Under physiological conditions B-form DNA is an exceedingly stable structur
e. However, experimental evidences obtained through nuclear magnetic resona
nce and fluorescence anisotropy suggest that the structure of the double he
lix fluctuates substantially. We describe photoacoustic phase modulation fr
equency measurements of ethidium bromide (Eb) with calf thymus, DNA. As in
fluorescence phase modulation measurements, we used an intercalating dye as
a probe; however, we monitored the triplet excited state lifetime at diffe
rent ionic strengths. The triplet lifetime of Eb varied from about 0.30 ms,
with no DNA present, to 20 ms, (at a DNA:Eb molar ratio of 5). With salt t
itration, this value falls, to about 2.0 ms. This result suggests, a strong
coupling between the phenantridinium ring of the ethidium and the base pai
rs because of the stacking movement of the DNA molecule under salt effect.
This, effect may be understood considering DNA as a polyelectrolyte. The co
unterions, in the solution shield the phosphate groups, reducing the electr
ostatic repulsion force between them, hence compacting the DNA molecule. Th
e results from Fourier transform infrared demonstrated two important bands:
3187 cm(-1) corresponding to the symmetric stretching of the NH group of t
he bases, and 1225 cm(-1) corresponding to the asymmetric stretching of pho
sphate groups shifted toward higher wavenumbers, suggesting a proximity bet
ween the intercalant and base pairs and a modification of the DNA backbone
state, both induced by salt accretion.