A systematic study of fundamentals in α-helical coiled coil mimicry by alternating sequences of β- and γ-amino acids.

Aimed at understanding the crucially important structural features for the integrity of α-helical mimicry by βγ-sequences, an α-amino acid sequence in a native peptide was substituted by differently arranged βγ-sequences. The self- and hetero-assembly of a series of αβγ-chimeric sequences based on a 33-residue GCN4-derived peptide was investigated by means of molecular dynamics, circular dichroism, and a disulfide exchange assay. Despite the native-like behavior of βγ alternating sequences such as retention of α-helix dipole and the formation of 13-membered α-helix turns, the αβγ-chimeras with different βγ substitution patterns do not equally mimic the structural behavior of the native parent peptide in solution. The preservation of the key residue contacts such as van der Waals interactions and intrahelical H-bonding, which can be met only by particular substitution patterns, thermodynamically favor the adoption of coiled coil folding motif. In this study, we show how successfully the destabilizing structural consequences of α → βγ modification can be harnessed by reducing the solvent-exposed hydrophobic surface area and placing of suitably long and bulky helix-forming side chains at the hydrophobic core. The pairing of αβγ-chimeric sequences with the native wild-type are thermodynamically allowed in the case of ideal arrangement of β- and γ-residues. This indicates a similarity in local side chain packing of β- and γ-amino acids at the helical interface of αβγ-chimeras and the native α-peptide. Consequently, the backbone extended residues are able to participate in classical "knob-into-hole" packing with native α-peptide.