Evidence for helicity in insect diuretic peptide hormones: computational analysis, spectroscopic studies, and biological assays

The conformation of four insect diuretic hormones has been analyzed computa tionally using secondary structure prediction routines and comparison with structures in the Brookhaven Protein Databank. Based on this analysis, a co mmon seven-residue peptide fragment (DVLRQRL) had a high probability of for ming an a-helix. Circular dichroism (CD) studies found that addition of tri fluoroethanol (TFE) to an aqueous solution of the seven-residue fragment in duces a change from random coil to helix. Subsequent NMR studies in water-T FE (1:1) produced nOe values and (3)J(alpha NH) coupling constants confirmi ng a helical conformation: (3)J(alpha NH) coupling constants for the first five residues (D1 to Q5) were all less than or equal to 6.0 Hz and two medi um-range nOe values (d(alpha N) ((i.i+3))) were observed between V2 and Q5, and R4 and L7. The longer fragments PLDVLRQRL in water-TFE and Lom-DH 1-26 in water alone, both containing the DVLRQRL sequence of the locust (Locust a migratoria) diuretic hormone, maintained the helicity as determined by CD analysis. However, the remaining 20 residues of the locust diuretic hormon e did not maintain the same amount of helicity in water and all of the trun cated fragments were not biologically active.