Crystal Conformation of the Cyclic Decapeptide Phakellistatin 8:  Comparison with Antamanide

The solid-state conformation of the cyclic decapeptide phakellistatin 8, cyclo[Pro1-Pro2-Ile3-Phe4-Val5-Leu6-Pro7-Pro8-Tyr9-Ile10] has been determined by X-ray methods. [Crystal data: orthorhombic; P21212; a = 20.294(2), b = 24.141(6), c = 13.903(3) Å. Least-squares refinement of 10061 reflections (I >2σ(I)) led to residuals R1 = 0.0651 (Sheldrick wR2 = 0.1749).] The cyclic decapeptide includes (1) a 5→1 transannular α-turn type H-bond, encompassing the Pro1, Pro2, and Ile3 residues and involving the Phe4 amide hydrogen and the Ile10 carbonyl, (2) an intramolecular 3→1 type VIa γ-turn type of H-bond, encompassing the Phe4 residue and involving the Val5 amide hydrogen and the Ile3 carbonyl, and (3) an intramolecular 4→1 type VIa β-turn H-bond, encompassing the Pro7 and Pro8 residues and involving the Tyr9 amide hydrogen and the Leu6 carbonyl. All backbone dihedral angles fall within normal, low-energy regions except for two amino acid residues, Phe4 (Φ, Ψ = 71°, −41°) and Tyr9 (Φ, Ψ = 75°, 31°), the side chains of which are found to fold back over the peptide backbone. Conformations of the four proline residues for phakellistatin 8 can be classified as follows: Pro1 Cs-Cγexo, Pro2 C2-Cβexo(Cγendo), Pro7 Cs-Cγendo and Pro8 C2-Cβexo(Cγendo). Examination of phakellistatin 8 and the decapeptide antamanide show the two compounds are quite similar, both in overall backbone conformation, proline ring conformations, and the presence of nearly identical α-turns involving one of the Pro-Pro pairs. Both molecules are highly hydrated when crystallized from aqueous solvents and both exhibit channel formation in the solid state.