Crystal and Molecular Structure of Benzyloxycarbonyl-a- Aminoiso butyryl-L-Prolyl Type I11 P-Turn* Methylamide: The Observation of an X2-Pro3

The crystal and molecular structure of N-benzyloxycarbonyl-a-aminoisobutyryl-L-prolyl methylamide, the amino terminal dipeptide fragment of alamethicin, has been determined using direct methods. The compound crystallizes in the orthorhombic system with the space group P212~21. Cell dimensions are a = 7.705 A, b = 11.365 A, and c = 21.904 A. The structure has been refined using conventional procedures to a final R factor of 0.054. The molecular structure possesses a 4- 1 intramolecular N-H---0 hydrogen bond formed be-tween the CO group of the urethane moiety and the NH group of the methylamide function. The peptide backbone adopts the type 111 P-turn conformation, with 4 2 =-51.0°, + * =-39.7", &j =-65.0', $3 =-25.4'. An unusual feature is the occurrence of the proline residue a t position 3 of the P-turn. The observed structure supports the view that Aib residues initiate the formation of type 111 @-turn conformations. The pyrrolidine ring is puckered in Cy-exo fashion