Fluorescence‐detected circular dichroism of dinucleoside phosphates. A study of solution conformations and the two‐state model

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Abstract
Three fluorescent dinucleoside phosphates containing 1,N6‐ethenoadenosine (εA), εApεA, εApεC, and εApU were studied using fluorescence‐detected circular dichroism (FDCD), circular dichroism (CD), and absorption measurements. The FDCD data indicate that εApεC and εApU can be described as two‐state systems consisting of a fluorescent species and a stacked, nonfluorescent species. Thermodynamic stacking parameters are calculated for these molecules using the van't Hoff equation. εApεA is found to be a more complicated system with a fluorescent CD which is different in shape, but comparable in magnitude, to the conventional CD of the dimer. This molecule, unlike the other two dinucleoside phosphates, cannot be characterized as a two‐state system; it is described as consisting of at least three states at temperatures above 35°C. The CD data were subjected to a linear analysis in order to determine the minimum number of states present. In agreement with the FDCD data, εApεC and εApU are found to consist of a minimum of two states, while εApεA is indicated to have at least three. The more complicated behavior of the latter dimer is also indicated by the values of the unstacked CD obtained in the van't Hoff analysis.