Synthesis, and Crystal and Molecular Structure of the $ 3_{10}-Helical \aplha,\beta $-dehydro pentapeptide Boc-Leu-Phe-Ala- \bigtriangleup Phe-Leu-Ome

\alpha, \beta -Dehydro amino acid residues are known to constrain the peptide backbone to the \beta-bend conformation. A pentapeptide containing only one \alpha, \beta dehydrophenylalanine (\bigtriangleup Phe) residue has been synthesized and crystallized, and its solid state conformation has been determined. The pentapeptide Boc-Leu-Phe-Ala- \bigtriangleup Phe-Leu-OMe $(C_{39}H_{55}N_5O_8, M_w = 721.9)$ was crystallized from aqueous methanol. Monoclinic space group was P21, a = 10.290(2)A, b = 17.149(2)A, c = 12.179(2) A, \beta = 96.64(1)A with two molecules in the unit cell. The x-ray $(Mo K_\alpha, \lambda = 0.7107A)$ intensity data were collected using a CAD4 diffractometer. The crystal structure was determined by direct methods and refined using least-squares technique. R = 4.4% and $R_w$ = 5.4% for 4403 reflections having |F0| \geq 3 \sigma (|F0|). All the peptide links are trans and the pentapeptide molecule assumes $3_{10}$ -helical conformation. The mean \phi , \psi values, averaged over the first four residues, are -64.4, -22.4 respectively. There are three 4 \longrightarrow 1 intramolecular hydrogen bonds, characteristic of $3_{10}$,-helix. In the crystal, the peptide helices interact through two head-to-tail. N-H-O intermolecular hydrogen bonds. The peptide molecules related by $2_1$, screw symmetry form a skewed assembly of helices