Helix termination and chain reversal: crystal and molecular structure of the alpha, beta-dehydrooctapeptide...

The crystal structure of the dehydro octapeptide Boc-Val-DeltaPhe-Phe-Ala-Leu-Ala-DeltaPhe-Leu-OH has been determined to atomic resolution by X-ray crystallographic methods. The crystals grown by slow evaporation of peptide solution in methanol/water are orthorhombic, space group P2(1)2(1)2(1). The unit cell parameters are a= 8.404(3), b= 25.598(2) and c= 27.946(3) Angstrom, Z=4. The agreement factor is R = 7.58% for 3636 reflections having (vertical bar Fo vertical bar) > or = 3sigma (vertical bar Fo vertical bar). The peptide molecule is characterised by a 3(10)-helix at the N-terminus and a pi-turn at the C-terminus. This conformation is exactly similar to the helix termination features observed in proteins. The pi-turn conformation observed in the octapeptide is in good agreement with the conformational features of pi-turns seen in some proteins. The alphaL-position in the pi-turn of the octapeptide is occupied by DeltaPhe7, which shows that even bulky residues can be accommodated in this position of the pi-turns. In proteins, it is generally seen that alphaL-position is occupied by glycine residue. No intermolecular head-to-tail hydrogen bonds are observed in solid state structure of the octapeptide. A water molecule located in the unit cell of the peptide molecule is mainly used to hold the peptide molecule together in the crystal. The conformation observed for the octapeptide might be useful to understand the helix termination and chain reversal in proteins and to construct helix terminators for denovo protein design.