Epitaxial Assembly Kinetics of Mutant Amyloid Beta 25-35 Fibrils

Amyloid s25-35 (As25-35) is a toxic fragment of Alzheimer's beta peptide. We have previously shown that As25-35 forms a trigonally oriented network on mica by epitaxial growth mechanisms. To establish chemical reactivity, we use a mutant peptide, As25-34_N27C, in which Cys27 is accessible in the mica-associated fibril. In the present work we explored the kinetics of epitaxial assembly of the mutant fibrils at different peptide and KCl concentrations by using in situ time-resolved AFM. We measured the number and length of As25-25_N27C fibrils per unit scan area as a function of time. At low (<4 mM) KCl concentration both the number and the growth rate of the fibrils increased with increasing peptide concentration. Increasing KCl concentration decreased the number of fibrils bound to the mica surface, and above 32 mM KCl fibril formation was completely abolished even at high peptide concentrations. The epitaxial assembly of As25-35_N27C fibrils is thus a diffusion-limited process inhibited competitively by KCl. The process proceeds via two steps: binding of the peptide to the free fibril end and to the mica surface. While peptide concentration affects both processes, K+ ions compete with peptide binding to the mica surface. By modulating peptide and KCl concentration the complexity of the As25-35_N27C network can be finely tuned.