Circular dichroism of films of polynucleotides

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Abstract
The CD spectra of films of the lithium salt of E. coli and calf thymus DNA, and alternating d‐AT : AT were measured as a function of relative humidity. Films of the ammonium acetate salt of DNA were also measured. The ammonium films yield the previously reported A‐form CD spectra. A possible explanation for the small magnitude of the 260‐nm band of the A‐form film spectra compared to double‐stranded RNA spectra is that the film DNA is in a different conformation than RNA within the A family of conformations.At relative humidities of 92% or lower, a negative nonconservative CD spectrum with negative minima near 270 and 210 nm is observed with the lithium films. The magnitude of the minima varies from film to film. In films of DNA the magnitude ranges from a delta epsilon of −5 to −35; d‐AT : AT films show magnitudes to −300. CD spectra of this type are designated Ψ spectra. Similar spectra have been reported from reconstituted complexes of DNA and polylysine or f‐1 histone. If the origins of the film and protein–DNA complex spectra are similar, the complex spectra are not the result of specific secondary structural changes induced in the DNA by the protein fraction. Theoretical analysis suggests that Ψ spectra are not the result of changes in the secondary or tertiary structure of DNA. Instead, the previously proposed explanation based on liquid crystals is favored. The DNA could form asymmetric structures with long‐range periodicity. It is likely that the observed CD spectra of f‐1 complexes are artifacts of DNA aggregation. The possibility that some other previously published spectra of protein–DNA complexes also reflect artifacts is suggested.