Conformational analysis of [Cpp1, Sar7, Arg8] vasopressin by 1H-NMR spectroscopy and molecular mechanics calculations.

A combined 1H-NMR and molecular mechanics study of [Cpp1, Sar7]AVP was performed in order to select the most probable conformations in DMSO solutions. The NMR constraints obtained were employed in the selection of starting conformations of the cyclic moiety of the analog. In particular, the diminished accessibility of the Asn5 NH proton to solvent and the close contact between Cpp1 and Cys6 C alpha H protons suggests a beta-turn conformation at the Phe3-Gln4 residues. Energy minimization was carried out both in the ECEPP/2 (rigid-valence geometry) and in the AMBER (flexible-valence geometry) force fields. Comparison of the experimental and calculated values of NMR characteristics has revealed that conformations containing type I, II, and III beta-turns at the Phe3-Gln4 residues are in reasonable agreement with the experimental data, with a dynamic equilibrium between the beta I (beta III) and beta II type structures of the cyclic part being the most probable. All of these conformations prefer the negative chirality of the disulfide bridge (theta 3 approximately -90 degrees). Five representative conformations were chosen for the acyclic tail: one with a beta I, one with a beta II'-turn at the Sar7-Arg8 residues, two extended-type conformations, and a conformation with a gamma-turn at Sar7. Because only high-energy extended conformations were in agreement with NMR data, it was concluded that the acyclic tail has considerable conformational flexibility in solution. The conformations obtained are discussed in terms of the structure-function relationship of the neurohypophyseal hormone analogs.