Design Strategies for the Sequence-Based Mimicry of Side-Chain Display in Protein β‑Sheets by α/β-Peptides

The sophistication of folding patterns and functions
displayed
by unnatural-backbone oligomers has increased tremendously in recent
years. Design strategies for the mimicry of tertiary structures seem
within reach; however, a general method for the mimicry of sheet segments
in the context of a folded protein is an unmet need preventing realization
of this goal. Previous work has shown that 1→1 α→β-residue
substitutions at cross-strand positions in a hairpin-forming α-peptide
sequence can generate an α/β-peptide analogue that folds
in aqueous conditions but with a change in side-chain display relative
to the natural sequence; this change would prevent application of
single β-residue substitutions in a larger protein. Here, we
evaluate four different substitution strategies based on replacement
of αα dipeptide segments for the ability to retain both
sheet folding encoded by a parent α-peptide sequence as well
as nativelike side-chain display in the vicinity of the β-residue
insertion point. High-resolution structure determination and thermodynamic
analysis of folding by multidimensional NMR suggest that three of
the four designs examined are applicable to larger proteins.