Thermodynamic and Kinetic Study of the Fibrillization of a Family of Tetrapeptides and Its Application to Self-Sorting. What Takes So Long?

Insight is provided into the thermodynamics and kinetics of peptide fibrillization, a process of particular interest in biomedicine due to its direct relationship to amyloid diseases. The studied isomeric tetrapeptides are terminally end-capped and present two hydrophobic phenylalanine residues and two ionizable hydrophilic aspartic acid residues. These compounds form gels in water which are composed of self-assembled fibrillar networks. Potentiometric titrations have afforded acid-base and solubility constants associated with the fibrillization process. A remarkable pK(a) shift of the peptide side chains is observed linked to aggregation, allowing for fiber formation at pH values around neutrality. The magnitude of the pK(a) shift is directly related to the solubility of the tetrapeptides, namely, to the free energy change associated with fibrillization. Therefore, potentiometric titration emerges as a simple tool to evaluate the thermodynamic parameters of the process. Additionally kinetic measurenients with NMR, fluorescence spectroscopy, and SANS reveal that initial peptide dimerization is most likely to be the fibrillization rate-determining step. The aggregation process in all cases presents a relatively long lag time of ca. 1-3 h and takes more than 8 h to complete. No correlation is observed between kinetic and thermodynamic parameters. Finally, kinetically controlled self-sorting of a mixture of two isomeric tetrapeptides is described.