Le concept switch-peptides : le potentiel de la migration S,N acylique dans l'induction de l'activité biologique et la rupture de structures secondaires in situ

Conformational transitions have found broad interest due to their impact on protein misfolding and self-assembly as key events leading to the development of neurodegenerative diseases. However, investigation of these dynamic events has been limited so far mainly due to the intrinsic tendency of the involved polypeptides for self-association and aggregation. Consequently, the elucidation of β-sheet and fibril-forming processes related to degenerative diseases remains a difficult task, often giving contradictory results. In order to overcome these difficulties and to investigate early steps of misfolding and subsequent fibril and plaque formation, a new concept termed "Switch-peptides" is presented in this thesis, opening new perspectives for the rational design of drugs preventing Alzheimer's disease. Figure. Elaboration of the "Switch-peptides" concept via S to N acyl migration. As shown in the Figure, the switch-peptides represent stable, self-contained folding precursors, in which folding and self-assembly is blocked by the presence of the Ser-, Thr-, Cys- derived switch elements S dissecting the regular peptide backbone by an (thio)ester and a flexible C-C bond (Soff). Removal of the protecting group Y by enzymatic cleavage or by change of pH, triggers X,N acyl migration restoring the native peptide backbone in situ, and setting off the folding process in the nascent state ("in statu nascendi", ISN) of the parent molecule. These resulting conformational changes i.e. from a random coil to a folded state, can be used to study the onset of biological activity (A) onset (B) and disruption (C) of secondary and tertiary structures. The work described in the present thesis elaborates the structural and chemical foundations of the concept, notably in the induction of structural and conformational properties of peptides. Special attention is focused on the S to N acyl migration in elaborating an in situ ligation methodology. For potential applications in vitro and in vivo, we explore a new thioester solubilizing group to realize efficient native chemical ligations at physiological conditions (Figure). Besides the synthetic access of switch-peptides thioesters via solid-phase techniques, the impact of internal (steric hindrance) and external pH factors on the kinetics of the intramolecular acyl migration is evaluated by analytical HPLC. The major applications of the switch-peptides concept are explored via three main axis : In order to settle the foundations of strategy B, our study was based on the possibility of inducing in situ the formation of secondary and tertiary structures. For this purpose, an amphiphilic model peptide is designed to mimic conformational transitions relevant in degenerative disease, i.e. transitions of type random coil to β sheet. The role of the switch element S in the disruption of secondary structures due to the conformational decoupling of σ on P (Soff), is demonstrated by CD.The induced conformational transition to β sheet