Design and Synthesis of Biologically Relevant N-Heteroatom Substituted β-Strand Peptidomimetics

Peptide secondary structures within protein-protein interfaces mediate a wide-array of biological processes and represent an important group of targets for therapeutic intervention. Peptidomimetics capable of recapitulating the structural features with protein domains would thus be of great use as probe molecules for targeting disease-relevant macromolecular interactions. The stabilization of β-sheet secondary structure through peptide backbone modification represents an attractive approach to protein mimicry. Here we present backbone N-heteroatom substitution as a subtle yet remarkably effective approach to β-sheet/strand stabilization. We describe the synthesis of chiral α-hydrazino acid building blocks and their incorporation into peptidomimetics using solid-phase methods. Using the N-amino group as a nucleophilic handle, we prepared a series of covalently constrained dipeptide surrogates and evaluated their effect on β-hairpin stability with a model system of folding. We then developed an electrophilic aminating protocol to expand building block scope, and demonstrate that acyclic N-amino peptides

(NAPs) exhibit enhanced β-sheet propensity using two separate β-hairpin model systems. We explored additional N-heteroatom substituted peptides by describing a method to prepare N-hydroxy peptides (NHPs) and show they can be incorporated into β-hairpin folds without energetic penalty. Lastly, we applied our minimalist approach to β-strand peptidomimetics to the development of ligands targeting β-rich amyloid structure. We describe the design and synthesis of a NAP-based library derived from the aggregation-prone ²⁷⁵VQIINK²⁸⁰ and ³⁰⁶VQIVYK³¹¹ hexapeptide sequences in the tau protein. Biophysical and cellular transmission assay data reveal that NAPs are capable of blocking tau fibrilization and seeding in a sequence-specific manner. Data derived from model N-amino peptides suggest that extended conformations are stabilized through cooperative steric, electrostatic, and H-bonding interactions from the NH₂ substituent.

Overall, we demonstrate for the first time that N-amination represents an important step towards a unified peptidomimetic approach towards the stabilization of β-sheet/strand structures. We anticipate the conformational and non-aggregating characteristics of NAPs will be an effective tool in the design of soluble sequence-specific ligands of amyloidogenic proteins.