Observed CD Spectra Research Articles

Circular dichroism of films of polynucleotides.

AbstractThe 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.

Circular dichroism of an optically active benzene chromophore-1-methylindan

The gas phase circular dichroism and absorption spectra of a conformationally stable, optically active benzene chromophore, S-(+)-1-methylindan, has been measured from 280 to 170 nm. The structure of the CD spectrum differs from that of the absorption in the B1u region and this observation is interpreted as evidence for the presence of a σ−π* or π−σ* E1g state in addition to the π−π* B1u state. Also, the CD spectrum near 190 nm requires the conclusion that an additional electronic state is located between the analogs of the B1u and E1u states. This state is assigned as E2g, in agreement with consistent theoretical predictions. The observed CD spectra can be accounted for in a consistent manner on the basis of a scheme which uses the results of parametrized calculations of coupled-oscillator contributions and one-electron term calculation results due to Caldwell and Kauzmann for the molecule studied.