De novo design and structural characterization of an alpha-helical hairpin peptide: a model system for the study of protein folding intermediates.

The de novo design and structural characterization of an alpha-helical hairpin peptide (alpha-helix/turn/alpha-helix, alpha t alpha) are reported. The peptide is intended to provide a model system for the study of the interactions of secondary structural elements during protein folding. Both the diffusion-collision and framework models of protein folding envision that the earliest intermediates in protein folding are transient secondary structures or microdomains which interact and become mutually stabilizing. Design principles for the alpha t alpha peptide were drawn from the large body of work on the structure of peptides in solution. Computer modeling was not used in the design process. Study of alpha t alpha by circular dichroism and two-dimensional nuclear magnetic resonance indicates that the designed peptide is monomeric, helical, and stable in aqueous solution at room temperature. Analysis of two-dimensional nuclear magnetic resonance experiments indicates that the two helices and the turn form in the intended positions and that the helices associate in the designed orientation. Development of alpha t alpha represents an advance in protein design in that both the secondary structural elements and designed tertiary interactions have been realized and can be detected in solution by nuclear magnetic resonance. The resulting model system resembles a protein folding intermediate and will allow the study of interacting helices in a context that approximates an early stage in protein folding.