BETA-TURN CONFORMATIONS IN CRYSTAL-STRUCTURES OF MODEL PEPTIDES CONTAINING ALPHA,ALPHA-DI-N-PROPYLGLYCINE AND ALPHA,ALPHA-DI-N-BUTYLGLYCINE

The crystal state conformations of three peptides containing the alpha , alpha-dialkylated residues, alpha,alpha-di-n-propylglycine (Dpg) and alpha,alpha-di-n-butylglycine (Dbg), have been established by x-ray d iffraction. Boc-Ala-Dpg-Ala-OMe (I) and Boc-Ala-Dbg-Ala-OMe (III) adop t distorted type II beta-turn conformations with Ala (1) and Dpg/Dbg ( 2) as the corner residues. In both peptides the conformational angles at the Dxg residue (I: phi = 66.2 degrees, psi = 19.3 degrees; III: ph i = 66.5 degrees, psi = 21.1 degrees) deviate appreciably from ideal v alues for the i + 2 residue in a type II beta-turn. In both peptides t he observed (N...O) distances between the Boc CO and Ala(3) NH groups are far too long (I: 3.44 Angstrom; III: 3.63 Angstrom) for an intramo lecular 4 --> 1 hydrogen bond. Boc-Ala-Dpg-Ala-NHMe (II) crystallizes with two independent molecules in the asymmetric unit. Both molecules IIA and IIB adopt consecutive beta-turn (type III-III in IIA and type III-I in IIB) or incipient 3(10)-helical structures, stabilized by two intramolecular 4 --> 1 hydrogen bonds. In all four molecules the bond angle N-C-alpha-C' (tau) at the Dxg residues are greater than or equa l to 110 degrees. The observation of conformational angles in the heli cal region of phi,psi space at these residues is consistent with theor etical predictions. (C) 1995 John Wiley & Sons, Inc.