Switch peptides : a new tool for the study of ß sheet aggregation and the design of novel ß breakers

Incorrect folding of proteins, leading to aggregation and amyloid formation, is associated with a group of degenerative diseases including Alzheimer's disease and late onset diabetes. Amyloid forming proteins are believed to be mainly α-helical in their native conformation, but undergo an α-helical to β-sheet conversion paralleled by fibril and plaque formation. As a most challenging experimental hurdle, the transition from helical / rc conformations to β-sheeted structures is followed by dramatic changes in the physicochemical properties, such as self association and insolubility, thus strongly limiting the experimental access of this molecular key process in neurodegenerative diseases. In 2000, M. Mutter and his group proposed a new concept for resolving some of these intrinsic problems of peptide self-assembly. As a basic idea, peptides with a high potential for β-sheet formation, considered as early event in fibrillogenesis, are prepared as 'folding precursors', in which the folding process is blocked by the insertion of a non native bond (see Figure). These modular switch peptides comprise three distinct elements: A conformational induction unit (σ), a switch element (S), and a target peptide (P). A central feature of the concept of switch-peptides, the switch element S dissects the native polyamide backbone of the peptide P by an ester and a flexible C-C bond, thereby preventing the peptide from folding and separating the conformational impact of σ. The triggering of the switch element by removing the protecting group Y reestablishes the native polyamide backbone via a spontaneous O to N acyl migration reaction, whereupon peptide folding can proceed. The present Ph.D. thesis aims to elaborate this concept in putting conformational transitions from helical or random coil to β-sheet structures in focus. This idea is developed using first potentially β-sheet forming model peptides, and is extended in a second step to amyloid-derived sequences. To this end, a series of model peptides exhibiting high β-sheet potential have been synthesized by SPPS synthesis and the critical chain length for β-sheet formation of oligopeptide (Leu-Ser)n was determined by CD studies. The insertion of a pH inducible S-element allowed for the delineation of the in situ transition from rc (Soff) to β-sheet structure (Son). The kinetic evaluation of these conformational transitions as a function of concentration, pH and temperature served as a base for the host-guest peptides according to the switch concept. Host-guest techniques allow to mimic the structural environment of the peptide segments 'excised' from their polypeptide. Typically, peptide segments (guest sequence) regarded as nucleation centers ('hot spots') for β-sheet and fibril formation are incorporated into a host oligopeptide of high β-sheet potential, e.g. (Leu-Ser)n. The incorporation of S-elements at the C- and N-terminal end of the guest segment results in a rc structure (Soff-state), as monitored by