N-ACETYL-L-PHENYLALANYL-L-ALANINAMIDE

The conformation of the peptide chain in N-acetyl-L-phenylalanyl-L-ala ninamide (NAFAA), C14H19N3O3, is rather extended and falls in the E re gion of the phi, psi map, according to the classification of Zimmerman , Pottle, Nemethy & Scheraga [Macromolecules (1977), 10, 1-9]. The val ues of phi, psi torsion angles are like those of an antiparallel beta pleated sheet. Side-chain conformation of Phe residue is defined by ch i(1) = -174.5(4)degrees and chi(2) = 105.5(6)degrees and comes within the B-2 class of the observed statistical distribution for the aromati c residues in peptides [Cody, Duax & Hauptman, (1973). Int. J. Peptide Protein Res. 5, 297-308]. Crystal packing is ruled by four intermolec ular hydrogen bonds that involve all the donor groups, the acetyl O at om acting as a double acceptor. In the crystal there are ribbons of mo lecules translated along the a axis of the P2(1) space group and joine d through three hydrogen bonds. The fourth hydrogen bond interconnects screw-related ribbons. The phenylalanyl rings stack parallel to the a direction with interplanar distances of 3.334 (7) Angstrom.