Circular dichroism of adenosine dinucleotides.

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Abstract
The circular dichroism and absorption spectra of (3''-5'')ApA, (5''-5'')AppA, and (5''-5'')ApcpA [diadenosine(5'',5'') pyrophosphonate] under various ionic conditions 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 > AppA > ApcpA. Increasing the molar refractivity of the solvent decreases the base-base interaction for both ApA and AppA and 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 .DELTA.[.theta.].lambda./.DELTA.X for the experimental range of X. Applying this technique to the experimental data yielded information concerning possible transition coupling.