β‐Turn conformations in crystal structures of model peptides containing α,α‐Di‐n‐propylglycine and α,α‐Di‐n‐butylglycine

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Abstract
The crystal state conformations of three peptides containing the α,α‐dialkylated residues. α,α‐di‐n‐propylglycine (Dpg) and α,α‐di‐n‐butylglycine (Dbg), have been established by x‐ray diffraction. Boc‐Ala‐Dpg‐Alu‐OMe (I) and Boc‐Ala‐Dbg‐Ala‐OMe (III) adopt distorted type II β‐turn conformations with Ala (1) and Dpg/Dbg (2) as the corner residues. In both peptides the conformational angles at the Dxg residue (I: ϕ = 66.2°, ψ = 19.3°; III: ϕ = 66.5°. ψ = 21.1°) deviate appreciably from ideal values for the i + 2 residue in a type II β‐turn. In both peptides the observed (N…O) distances between the Boc CO and Ala (3) NH groups are far too long (1: 3.44 Å: III: 3.63 Å) for an intramolecular 4 → 1 hydrogen bond. Boc‐Ala‐Dpg‐Ata‐NHMe (II) crystallizes with two independent molecules in the asymmetric unit. Both molecules HA and HB adopt consecutive β‐turn (type III‐III in HA and type III‐I in IIB) or incipient 310‐helical structures, stabilized by two intramolecular 4 → 1 hydrogen bonds. In all four molecules the bond angle N‐Cα‐C′ (τ) at the Dxg residues are ≥ 110°. The observation of conformational angles in the helical region of ϕ,ψ space at these residues is consistent with theoretical predictions. © 1995 John Wiley & Sons, Inc.

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