At low temperature and low salt concentration, both imino proton and 31p-nmr spectra of DNA complexes with the intercalators ethidium and propidium are in the slowexchange region. Increasing temperature and/or increasing salt concentration results in an increase in the site exchange rate. Ring-current effects from the intercalated phenanthridinium ring of ethidium and propidium cause upfield shifts of the imino protons of A . T and G . C base pairs, which are quite similar for the two intercalators.
The limiting induced chemical s h i h for propidium and ethidium at saturation of DNA binding sites are approximately 0.9 ppm for A . T and 1.1 ppm for G . C base pairs. The similarity of the shifts for ethidium and propidium, in both the slow-and fastexchange regions over the entire titration of DNA, shows that a binding model for propidium with neighborexclusion binding and negative ligand cooperativity is correct. The fact that a unique chemical shift is obtained for imino protons at intercalated sites over the entire titration and that no unshifted imino proton peaks remain at saturation binding of ethidium and propidium supports a neighborexclusion binding model with intercalators bound at alternating sites rather than in clusters on the double helix. Addition of ethidium and propidium to DNA results in downfield shifts in "P-nmr spectra. At saturation ratios of intercalator to DNA base pairs in the titration, a downfield shoulder (approximately -2.7 ppm) is apparent, which accounts for approximately 15% of the spectral area. The main peak is at -3.9 to -4.0 ppm relative to -4.35 in uncomplexed DNA. The simplest neighbor-binding model predicts a downfield peak with approximately 50% of the spectral area and an upfield peak, near the chemical shift for uncomplexed DNA, with 50% of the area. This is definitely not the case with these intercalators. The observed chemical shifts and areas for the DNA complexes can be explained by models, for example, that involve spreading the intercalation-induced unwinding of the double helix over several base pairs and/or a DNA sequence-and conformation-dependent heterogeneity in intercalation-induced chemical shifts and resulting exchange rates.