Significant controversy surrounds the estimation of the translational and rotational entropy of a macromolecule in aqueous solutions. To address this issue, the entropy of intersubunit cross-linking of a complex was determined in this study by comparing the calorimetrically measured unfolding entropies of a synthetic two-stranded a-helical coiled-coil both with and without a disulfide bond between the two polypeptide chains. The experimental cross-linking enthalpy and entropy are equivalent. In contrast, ideal gas statistics predict that the translational and rotational enthalpy and entropy differ by an order of magnitude. After the vibrational component is subtracted from the experimental cross-linking entropy, the resultant translation and 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 than an order of magnitude smaller than estimates in the literature based on statistical mechanics of ideal gas models, which are 90-100 eu. This makes it doubtful that the translational and rotational entropy of macromolecules in aqueous solutions can be treated using ideal gas statistics.