-Sheet-based assemblies have attracted a considerable amount of attention from researchers of various disciplines because of their association with a variety of diseases and their emerging potential in material sciences and biotechnology. [1] Protein misfolding and self-assembly into highly ordered b-sheet-rich fibrillar assemblies that are known as amyloid fibrils are common features of a growing class of systemic and neurodegenerative diseases, which include Alzheimer s, Parkinson s, and Huntington s diseases, senile systemic amyloidoses, as well as type II diabetes. [2] Although there is strong evidence that implicates amyloid formation in the pathogenesis of these diseases, the precise mechanisms of amyloid formation and clearance in vivo, as well as the structural basis of amyloid toxicity, remain unknown. The lack of tools to monitor and/or control the initial structural transitions associated with protein misfolding, amyloid formation, and/or dissociation is the main cause of this gap in knowledge. Significant efforts have been devoted to study proteins and small peptides that self-assemble into amyloidlike fibrillar structures as model systems to investigate amyloid formation or to generate materials with interesting physical properties. However, knowledge of the mechanical and structural dynamics within b-sheet assemblies such as amyloid fibrils remains limited. Early assumptions that bsheet assemblies, which include amyloid fibrils, occupy a global minimum of free energy that is lower than that of the native state [3] led to a greater emphasis on the understanding and inhibition of amyloid formation, rather than that of amyloid dissociation and clearance. Despite the stability of bsheet-rich amyloid fibrils against proteases, acids, and chemical denaturants, increasing evidence from human [4] and in vitro studies indicates that a dynamic structure exists within amyloid fibrils and suggests that the process of amyloid formation is reversible. [5a,b] These findings, along with the fact that strategies aimed at the destabilization of amyloid fibrils and/or the acceleration of their clearance seem to reverse the disease phenotype, [6][7] suggest that a detailed understanding of the stability and dynamic behavior of amyloid fibrils is of critical importance to the development of therapeutic strategies for amyloid diseases.Our research group has previously shown [8][9][10] that the incorporation of molecular switches into polypeptides, based on an in situ intramolecular O!N acyl group migration, [11] allows for the controlled induction or reversal of secondary structural transitions [12a,b,c] and self-assembly of small peptide chains. Herein we describe a switch peptide that is designed to disrupt amyloid-like b-sheet assemblies through the controlled induced transition from a b-sheet to an a-helix structure (Figure 1). The experimental results illustrate the potential of switch peptides as a tool to investigate the structural dynamics of amyloid fibrils, to provide an insight into the structural basis o...