Solution structure of an analogue of vasoactive intestinal peptide as determined by two-dimensional NMR and circular dichroism spectroscopies and constrained molecular dynamics.

Structures have been determined for a potent analogue (VIP') of vasoactive intestinal peptide (VIP) in methanol/water solutions. In CD studies, both VIP and VIP' were helical in methanol/water, with the percentage of {alpha}-helix increasing with percentage methanol. The pH had little effect on the structure. Complete {sup 1}H NMR assignments were made for VIP' in 25% methanol at pH 4 and 6 and in 50% methanol at pH 6, using two-dimensional COSY, NOESY, and relay-COSY experiments. There were no widespread changes in chemical shifts between the samples at pH 4 and 6; however, widespread changes were observed between the samples in 25% and 50% methanol. Complete sets of NOEs were obtained for VIP' in 25% methanol, pH 4, and in 50% methanol, pH 6. These NOEs were converted into distance constraints and applied in molecular dynamics and energy minimization calculations using the program CHARMM. A set of low-energy structures was obtained for VIP' in each solvent system. In 25% methanol, VIP' has two helical segments at residues 9-17 and 23-28. The remainder of the structure is not well determined. In 50% methanol, residues 8-26 form a regular, well-defined {alpha}-helix and residues 5-8 form a type III {beta}-turn. The remaining residuesmore » are not ordered. These structural assessments agree with the CD data. In the lowest energy structure in 50% methanol, the side chains of Asp{sup 3}, Phe{sup 6}, Thr{sup 7}, and Tyr{sup 10} are clustered together--these residues are conserved throughout the family of peptide hormones homologous to VIP.« less