Solutions structure of α2D, a nativelike de novo designed protein

De novo protein design provides an attractive means for testing and refining the principles governing the stability and tertiary structure of proteins. We describe the NMR solution structure of a 35-residue peptide designed to form a helix-loop-helix that dimerizes into a four-helix bundle. Structures were calculated on the basis of 834 NMR-derived restraints including 140 long-range NOEs. With 24 restraints per residue, the structure is well determined (0.28 Angstrom RMSD for backbone residues 3-33) and includes many features of the design yet adopts a novel topology that was unexpected. The forces that caused this peptide to adopt this unique fold are discussed.