Due to the vital roles proteins play in life, much effort has been invested in their mimicry by synthetic agents. One approach to this goal is to design unnatural backbone oligomers (“foldamers”) that fold like natural peptides. Despite success in secondary structure mimicry by such species, protein-like tertiary folds remain elusive. A fundamental challenge underlying this task is the design of a sequence of side chains that will specify a complex tertiary folding pattern on an unnatural backbone. We report here a sequence-based approach to convert a natural protein with a compact tertiary fold to an analogue with a backbone composed of ∼20% unnatural building blocks but similar folding behavior as the parent protein.

Thioamides are single atom substitutions of the peptide bond that serve as versatile probes of protein structure.

The thermodynamics of protein folding are dictated by a complex interplay of interatomic interactions and physical forces. A variety of unnatural protein-like oligomers have the capacity to manifest defined folding patterns. While the energetics of folding in natural proteins is well studied, little is known about the forces that govern folding in modified backbones. Here, we explore the thermodynamic consequences of backbone alteration on protein folding, focusing on two types of chemical changes made in different structural contexts of a compact tertiary fold. Our results reveal a surprising favorable impact on folding entropy that accompanies modifications that increase disorder in the ensemble of unfolded states, due to differences in the solvation of natural and unnatural backbones.

A variety of non-biological structural motifs have been incorporated into the backbone of natural protein sequences. In parallel work, diverse unnatural oligomers of de novo design (termed “foldamers”) have been developed that fold in defined ways. In this Perspective article, we survey foundational studies on protein backbone engineering, with a focus on alterations made in the context of complex tertiary folds. We go on to summarize recent work illustrating the potential promise of these methods to provide a general framework for the construction of foldamer mimics of protein tertiary structures.