Design of peptides: Crystal and molecular structure of a 3 10 ‐helical peptide N‐Boc‐ L ‐Phe‐dehydro‐Phe‐ L ‐Val‐ L ‐Phe‐dehydro‐Phe‐ L ‐Val‐Och 3

Highly specific peptide structures can be designed by inserting dehydro residues into peptide sequences. The peptide N‐Boc‐ L ‐Phe‐dehydro‐Phe‐ L ‐Val‐ L ‐Phe‐dehydro‐Phe‐ L ‐Val‐OCH 3 , synthesized by conventional procedures, crystallizes from methanol‐‐water mixtures at 4°C in the tetragonal space group P4 3 with cell parameters a = b = 13.829 ± 0.003 Å, c = 27.587 ± 0.008 Å, V = 5275.5 ± 0.2 Å 3 , Z = 4, d m = 1.152 ± 0.005 g cm −3 , d cal = 1.150 ± 0.005 g cm −3 . The overall residual factor R = 0.084 for 2342 reflections, with 2θ max = 140° using CuKα radiation. The backbone torsion angles are θ 1 = −171(1)°, ω 0 = 168 (1)°, ϕ 1 = 77 (2)°, ψ 1 = 41 (2)°, ω l = 169 (1)°, ϕ 2 = −46 (2)°, ψ 2 = ‐24 (2)°, ω 2 = 179 (1)°, ϕ 3 = −63 (2)°, ϕ 3 = −19 (2)°, ω 3 = 171 (1)°, ϕ 4 = −67 (2)°, ψ 4 = −8 (1)°, ω 4 = 169 (1)°, ψ 5 = −61 (1)°, ϕ 5 = −26(1)°, ω 5 = 177(1)°, ψ 6 = −122 (1)°, ϕ   6 T= 26 (2)°. The peptide adopts a 3 10 ‐helical conformation with three intramolecular hydrogen bonds ( i + 3 → i ) involving carbonyl oxygen atoms of Phe1, dehydro‐Phe2, Val3, and the NH groups of Phe4, dehydro‐Phe5, and Val6 with distances of 3.01 (1), 2.82 (1), and 3.09 (2) Å, respectively. The structure determination revealed that a hexapeptide with two dehydro‐Phe residues at ( i + 2) and ( i + 5) positions generates a 3 10 ‐helical conformation. The helical peptide molecules are arranged in a major helical arrangement about the 4 3 axis. These helices are packed parallel to the c axis and form several interdigitating hydrogen bonds. The interhelix hydrogen‐bonding regions are separated by van der Waals interactions involving the side chains of Phe, dehydro‐Phe, and Val residues. © 1993 John Wiley & Sons, Inc.