Abstract
The CD spectra of films of the lithium salt of E. coli and calf thymus DNA, and alternating dโAT : 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 Aโform CD spectra. A possible explanation for the small magnitude of the 260โnm band of the Aโform film spectra compared to doubleโstranded 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 โ5 to โ35; dโAT : AT films show magnitudes to โ300. CD spectra of this type are designated ฮจ spectra. Similar spectra have been reported from reconstituted complexes of DNA and polylysine or fโ1 histone. If the origins of the film and proteinโDNA 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 ฮจ 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 longโrange periodicity. It is likely that the observed CD spectra of fโ1 complexes are artifacts of DNA aggregation. The possibility that some other previously published spectra of proteinโDNA complexes also reflect artifacts is suggested.