The construction of bioactive peptides using β-amino acid-containing sequence patterns is a very promising strategy to obtain analogues that exhibit properties of high interest for medicinal chemistry applications. β-Amino acids have been shown to modulate the conformation, dynamics, and proteolytic susceptibility of native peptides. They can be either combined with α-amino acids by following specific patterns, which results in backbone architectures with well-defined orientations of the side chain functional groups, or assembled in de novo-designed bioactive β- or α,β-peptidic sequences. Such peptides display various biological functions, including antimicrobial activity, inhibition of protein–protein interactions, agonism/antagonism of GPCR ligands, and anti-angiogenic activity.

The native state of a globular protein is essential for its biocatalytic function, but is marginally stable against unfolding. While unfolding equilibria are often reversible, folding intermediates and misfolds can promote irreversible protein aggregation into amorphous precipitates or highly ordered amyloid states. Addition of ionic liquids—low-melting organic salts—offers intriguing prospects for stabilizing native proteins and their enzymatic function against these deactivating reaction channels. The huge number of cations and anions that form ionic liquids allows fine-tuning of their solvent properties, which offers robust and efficient strategies for solvent optimization. Going beyond case-by-case studies, this article aims at discussing principles for a rational design of ionic liquid-based formulations in protein chemistry and biocatalysis.

The first Y(5) receptor-selective analog of neuropeptide Y (NPY), [Ala(31),Aib(32)]NPY, has been developed and biologically characterized. Using competition binding assays on cell lines that express different Y receptors, we determined the affinity of this analog to be 6 nm at the human Y(5) receptor, >500 nm at the Y(1) and Y(2) receptors, and >1000 nm at the Y(4) receptor. Activity studies performed in vitro using a cAMP enzyme immunoassay, and in vivo using food intake studies in rats, showed that the peptide acted as an agonist. Further peptides obtained by the combination of the Ala(31)-Aib(32) motif with chimeric peptides containing segments of NPY and pancreatic polypeptide displayed the same selectivity and even higher affinity (up to 0.2 nm) for the Y(5) receptor. In vivo administration of the new Y(5) receptor-selective agonists significantly stimulated feeding in rats. The NMR solution structures of NPY and [Ala(31),Aib(32)]NPY showed a different conformation in the C-terminal region, where the alpha-helix of NPY was substituted by a more flexible, 3(10)-helical turn structure.

δ-secretase, also known as asparagine endopeptidase (AEP) or legumain, is a lysosomal cysteine protease that cleaves both amyloid precursor protein (APP) and tau, mediating the amyloid-β and tau pathology in Alzheimer’s disease (AD). Here we report the therapeutic effect of an orally bioactive and brain permeable δ-secretase inhibitor in mouse models of AD. We performed a high-throughput screen and identified a non-toxic and selective δ-secretase inhibitor, termed compound 11, that specifically blocks δ-secretase but not other related cysteine proteases. Co-crystal structure analysis revealed a dual active site-directed and allosteric inhibition mode of this compound class. Chronic treatment of tau P301S and 5XFAD transgenic mice with this inhibitor reduces tau and APP cleavage, ameliorates synapse loss and augments long-term potentiation, resulting in protection of memory. Therefore, these findings demonstrate that this δ-secretase inhibitor may be an effective clinical therapeutic agent towards AD.