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

The conformation of four insect diuretic hormones has been analyzed computationally using secondary structure prediction routines and comparison with structures in the Brookhaven Protein Databank. Based on this analysis, a common seven-residue peptide fragment (DVLRQRL) had a high probability of forming an alpha-helix. Circular dichroism (CD) studies found that addition of trifluoroethanol (TFE) to an aqueous solution of the seven-residue fragment induces a change from random coil to helix. Subsequent NMR studies in water-TFE (1:1) produced nOe values and 3JalphaNH coupling constants confirming a helical conformation: 3JalphaNH coupling constants for the first five residues (D1 to Q5) were all < or = 6.0 Hz and two medium-range nOe values (dalphaN (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 (Locusta migratoria) diuretic hormone, maintained the helicity as determined by CD analysis. However, the remaining 20 residues of the locust diuretic hormone did not maintain the same amount of helicity in water and all of the truncated fragments were not biologically active.