Study on coacervation of the repeat pentapeptide model of tropoelastin: effect of cations

The effects of various cations on coacervation of the polypentapeptide (Val-Pro-Gly-Val-Gly)n, a sequential model of tropoelastin, were studied by following the turbidity at 300 nm and the fluorescence intensity due to the interaction between the polypentapeptide and a hydrophobic probe (4-benzoylamido-4'-aminostilbene-2,2'-disulfonic acid). In the absence of cations, as the polypentapeptide concentration was increased, the turbidity curves shifted to lower temperatures and became much steeper, while the fluorescence intensity increased markedly. This suggests that a conformational change is induced by the association of the polypentapeptide molecules and indicates that coacervation occurs predominantly by intermolecular hydrophobic association. In the presence of Mg2+, the temperature profile of the coacervation curve of the polypentapeptide obtained by turbidity measurement moved to higher temperature and the fluorescence intensity decreased significantly. This suggests that Mg2+ inhibits the hydrophobic interaction of the polypentapeptide molecules on coacervation but does not induce conformational change on association of the molecules. In the presence of Ca2+, Na+, and K+, the temperature profile of the coacervation curve of the polypentapeptide was not affected appreciably, but the fluorescence intensity was decreased by these cations in the order of Na+, K+, and Ca2+. Circular dichroism spectra of the polypentapeptide in 80% trifluoroethanol in the presence of cations showed a less ordered state of the polypentapeptide. This less ordered state implies inhibition by the cations of the hydrophobic interaction between the molecules.