De novo protein design: crystallographic characterization of a synthetic peptide containing independent helical and hairpin domains.

The Meccano (or Lego) set approach to synthetic protein design envisages covalent assembly of prefabricated units of peptide secondary structure. Stereochemical control over peptide folding is achieved by incorporation of conformationally constrained residues like alpha-aminoisobutyric acid (Aib) or DPro that nucleate helical and beta-hairpin structures, respectively. The generation of a synthetic sequence containing both a helix and a hairpin is achieved in the peptide BH17, Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-Gly-Gly-Leu-Phe-Val-DPro-Gly-Leu- Phe-Val-OMe (where Boc is t-butoxycarbonyl), as demonstrated by a crystal structure determination. The achiral -Gly-Gly- linker permits helix termination as a Schellman motif and extension to the strand segment of the hairpin. Structure parameters for C(89)H(143)N(17)O(20) x 2H(2)O are space group P2(1), a = 14.935(7) A, b = 18.949(6) A, c = 19.231(8) A, beta = 101.79(4) degrees, Z = 2, agreement factor R(1) = 8.50% for 4,862 observed reflections > 4 sigma(F), and resolution of approximately 0.98 A.