Characterization of bent helical conformations in polymorphic forms of a designed 18‐residue peptide containing a central gly‐pro segment

An 18‐residue sequence Boc‐Aib‐Val‐Ala‐Leu‐Aib‐Val‐Ala‐Leu‐Gly‐Pro‐Val‐Ala‐Leu‐Aib‐Val‐Ala‐Leu‐Aib‐OMe (UK18) was designed to examine the effect of introducing a Gly‐Pro segment into the middle of a potentially helical peptide. The crystal structures of two polymorphic forms yielded a view of the conformation of three independent molecules. Form 1 (space group P2 1 2 1 2 1, a = 14.620Å; b = 26.506Å, c = 28.858Å, Z = 4) has one molecule in the asymmetric unit, with one cocrystallized water molecule. Form 2 (space group P2 1 2 1 2 1, a = 9.696Å; b = 19.641Å, c = 114.31Å, Z = 8) has two molecules in the asymmetric unit with four cocrystallized water molecules. In Form 1 , residues 1 to 18 adopt ϕ,ψ values that lie in the right‐handed helical (α R ) region of the Ramachandran map. Two residues, Leu (8) (ϕ = −92.0°, ψ = −7.5°) and Leu (17) (ϕ = −94.7°, ψ = −1.7°) adopt conformations that deviate significantly from helical values. In Form 2 , molecule A, residues 2 to 16 lie in the α R region of ϕ,ψ space, with Leu (8) (ϕ = −94.9°, ψ = −2.9°) deviating significantly from helical values. Aib (1) and Aib (18) adopt left‐handed (α L ) helical conformation. Significant distortion is observed at Leu (17) (ϕ = −121.3°, ψ = −31.3°). Molecule B, Form 2 , adopts a right‐handed helix over residues 1 to 17. In all three molecules, a distinct bend in the helix is observed, with the bend angle values varying from 40.8° to 58.9°. © 2011 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 98: 76–86, 2012.