The solid-state conformation of the cyclic decapeptide phakellistatin 8, cyclo[Pro(1)-Pro(2)-Ile(3)-Phe(4)-Val(5)-Leu(6)-Pro(7)-Pro(8)-Tyr(9)-Ile(10)] has been determined by X-ray methods. [Crystal data : orthorhombic; P2(1)2(1)2; a 20.294(2), b = 24.141(6), c = 13.903(3) Angstrom. Least-squares refinement of 10061 reflections (I > 2 sigma(I)) led to residuals R-1 = 0.0651 (Sheldrick wR(2) = 0.1749).] The cyclic decapeptide includes (1) a 5-->1 transannular ct-turn type PI-bond, encompassing the Pro(1), Pro(2), and Ile(3) residues and involving the Phe(4) amide hydrogen and the Ile(10) carbonyl, (2) an intramolecular 3-->1 type VIa gamma-turn type of II-bond, encompassing the Phe(4) residue and involving the Val(5) amide hydrogen and the Ile(3) carbonyl, and (3) an intramolecular 4-->1 type VIa beta-turn H-bond, encompassing the Pro(7) and Pros residues and involving the Tyr(9) amide hydrogen and the Leu(6) carbonyl. All backbone dihedral angles fall within normal, low-energy regions except for two amino acid residues, Phe(4) (Phi, Psi = 71 degrees, -41 degrees) and Tyr(9) (phi, Psi = 75 degrees, 31 degrees), 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 : Pro(1) C-s-C(gamma)exo, Pro(2) C-2-C(beta)exo(C(gamma)endo), Pro(7) C-s-C(gamma)endo and Pro(8) C-2-C(beta)exo(C(gamma)endo). 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 alpha-tunas 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.