Detection of nascent polyproline II helices in solution by NMR in synthetic insect kinin neuropeptide mimics containing the X-Pro-Pro-X motif

The conformations of three synthetic peptide analogs containing the dPro-dP ro-dXaa motif (dXaa=dThr, dGlu, dAsn) in aqueous solution were studied by a combination of NMR and molecular modeling simulations. The three compounds were identified from a random D-amino acid tripeptide library on the basis of their ability to either mimic or block the diuretic activity of neurope ptides of the insect kinin family. TOCSY and ROESY correlations, as well as abnormal secondary chemical shifts for protons on the D-proline residues w ere employed to obtain conformational ensembles consistent with the experim ental NMR data for the three analogs using an in vacuo simulated annealing protocol. Similar secondary structures were found for the three molecules a fter refinement, in agreement with the similarities observed between their NMR spectra. Unrestrained molecular dynamics simulations with explicit wate r representation indicate that the structural motifs found in vacuo are sta ble in aqueous solution. The three analogs can be considered initiators of right-handed poly D-proline II helices, mirror images of the poly L-proline II left-handed helical motifs normally found in proline-rich proteins. The role of these secondary folds on binding of the analogs to the kinin recep tors is discussed.