NMR spectroscopic evidence that helodermin, unlike other members of the secretin/VIP family of peptides, is substantially structured in water

The structure in water and additionally in 50% trifluoroethanol (TFE) solution of helodermin, an amidated peptide consisting of 35 amino acids, was elucidated by 2D H-1 NMR spectroscopy initially from H alpha chemical shifts and qualitative NOE data. Detailed structures were calculated from the quantitative NOE data which were used as distance restraints in molecular dynamics and energy minimization calculations. Regions of stable secondary structure were defined from the resulting final peptide conformations using a new fitting program that takes into account the summed RMS differences between all structures for short segments of 2-5 residues in length. This procedure allows a reasonably objective method of defining the edges of stable structure. In contrast to other members of the secretin/VIP family of peptides, helodermin shows a defined secondary structure in water alone and possesses an alpha-helix from Glu-9 to Leu-23 that was further stabilized and slightly extended (Phe-6 to Ala-24) on addition of TFE. The N- and C-termini were unstructured in both solutions. Such features, in particular the observation of a linear helix 18 +/- 2 residues in length, are common to other members of the family and become more pronounced in hydrophobic environments. The data provide further circumstantial evidence that an alpha-helix conformation is necessary for receptor binding. The prolonged physiological action of helodermin, compared to its C-terminal deletion analogues and VIP, is at least in part due to the unusual stable secondary structure.