Circular dichroism of adenosine dinucleotides.

The circular dichroism and absorption spectra of (3'-5')ApA, (5'-5')AppA, and (5'-5')ApcpA [diadenosine(5'5') pyrophosphonate] under various ionic coditions and at various temperatures are reported. Temperature studies reveal that under the same ionic strength and solvation conditions the probability of base-base interaction is ApA greater than AppA greater than ApcpA. It is found that increasing the molar refractivity of the solvent decreases the base-base interaction for both ApA and AppA and that both compounds proceed to a monomeric state in solvents of high molar refractivity. Solvent effect studies also indicate that a major percentage of the driving force for the base-base interaction can be accounted for by London dispersion forces. Low ionic strength appears to favor the base-base interacted conformation. All compounds tend toward the same noninteracted form at high temperatures. Plots of [theta]lambda against X (where X is an experimental parameter) were constructed in order to obtain information concerning which transitions might be undergoing oscillator coupling. To a first approximation, transitions coupled to each other should have very similar deta[theta]lambda/detaX for the experimental range of X. Applying this technique to the experimental data yielded information concerning possible transition coupling.