Contribution of translational and rotational entropy to the unfolding of adimeric coiled-coil

Significant controversy surrounds the estimation of the translational and r otational entropy of a macromolecule in aqueous solutions. To address this issue, the entropy of intersubunit cross-linking of a complex was determine d in this study by comparing the calorimetrically measured unfolding entrop ies of a synthetic two-stranded a-helical coiled-coil both with and without a disulfide bond between the two polypeptide chains. The experimental cros s-linking enthalpy and entropy are equivalent. In contrast, ideal gas stati stics predict that the translational and rotational enthalpy and entropy di ffer by an order of magnitude. After the vibrational component is subtracte d from the experimental cross-linking entropy, the resultant translation an d rotational entropy is 5 +/- 8 eu (1 eu = 1 cal.K(-1)mol(-1) = 4.184 J.K(- 1)mol(-1)) at a standard state concentration of 1 M. This value is more tha n an order of magnitude smaller than estimates in the literature based on s tatistical mechanics of ideal gas models, which are 90-100 eu. This makes i t doubtful that the translational and rotational entropy of macromolecules in aqueous solutions can be treated using ideal gas statistics.