Crystal and molecular structure of benzyloxycarbonyl-&#945;-aminoisobutyryl-L-prolyl methylamide: the observation of an X<SUB>2</SUB>-Pro<SUB>3</SUB> Type III &#946;-Turn

The crystal and molecular structure of N-benzyloxycarbonyl-&#945;-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 P2<SUB>1</SUB>2<SUB>1</SUB>2<SUB>1</SUB>. Cell dimensions are a = 7.705 &#197;, b = 11.365 &#197;, and c = 21.904 &#197;. The structure has been refined using conventional procedures to a final R factor of 0.054. The molecular structure possesses a 4 &#8594;1 intramolecular N-H - O hydrogen bond formed between the CO group of the urethane moiety and the NH group of the methylamide function. The peptide backbone adopts the type III &#946;-turn conformation, with &#966;<SUB>2</SUB> = -51.0&#176;, &#968;<SUB>2</SUB> = -39.7&#176;, &#966;<SUB>3</SUB> = -65.0&#176;, &#968;<SUB>3</SUB> = -25.4&#176;. An unusual feature is the occurrence of the proline residue at position 3 of the -turn. The observed structure supports the view that Aib residues initiate the formation of type III &#946;-turn conformations. The pyrrolidine ring is puckered in C<SUP>Y</SUP>-exo fashion