Conformation of a 16-residue zervamicin IIA analog peptide containing three different structural features: 3(10)-helix, alpha-helix, and beta-bend ribbon

Boc-Trp-Ile-Ala-Aib-Ile-Val-Aib-Leu-Aib-Pro-Ala-Aib-Pro-Aib-Pro-Phe-OMe (where Boc is t-butoxycarbonyl and Aib is alpha-aminoisobutyric acid), a synthetic apolar analog of the membrane-active fungal peptide antibiotic zervamycin IIA, crystallizes in space group P1 with Z = 1 and cell parameters a = 9.086 +/- 0.002 A, b = 10.410 +/- 0.002 A, c = 28.188 +/- 0.004 A, alpha = 86.13 +/- 0.01 degrees, beta = 87.90 +/- 0.01 degrees, and gamma = 89.27 +/- 0.01 degrees; overall agreement factor R = 7.3% for 7180 data (F0 greater than 3 sigma) and 0.91-A resolution. The peptide backbone makes a continuous spiral that begins as a 3(10)-helix at the N-terminus, changes to an alpha-helix for two turns, and ends in a spiral of three beta-bends in a ribbon. Each of the beta-bends contains a proline residue at one of the corners. The torsion angles phi i range from -51 degrees to -91 degrees (average value -64 degrees), and the torsion angles psi i range from -1 degree to -46 degrees (average value -31 degrees). There are 10 intramolecular NH...OC hydrogen bonds in the helix and two direct head-to-tail hydrogen bonds between successive molecules. Two H2O and two CH3OH solvent molecules fill additional space with appropriate hydrogen bonding in the head-to-tail region, and two additional H2O molecules form hydrogen bonds with carbonyl oxygens near the curve in the helix at Pro-10. Since there is only one peptide molecule per cell in space group P1, the molecules repeat only by translation, and consequently the helices pack parallel to each other.