NMR identification of left-handed polyproline type II helices

NMR characteristics of a model left-handed 3(1)-helical peptide are reported in this study. With temperature and sequence corrections on the predicted random coil (15)N chemical shifts, a significant (15)N chemical shift deviation is observed for the model 3(1) peptide. The (15)N chemical shift differences also correlate well with the molar ellipticities (at 220 nm) of the CD spectra at different temperatures, indicating that the (15)N chemical shift is a sensitive probe for 3(1)-helices. The average (3)J(HNalpha) and (1)J(CalphaHalpha) values of the model peptide are determined to be 6.5 and 142.6 Hz, respectively, which are consistent with the values calculated from the geometry of 3(1)-helices. With careful measurements of amide (15)N chemical shifts and incorporating temperature and sequence effect corrections, the (15)N chemical shifts can be used together with (3)J(HNalpha) and (1)J(CalphaHalpha) to differentiate 3(1)-helices from random coils with high confidence. Based on the observed NMR characteristics, a strategy is developed for probing left-handed 3(1)-helical structures from other secondary structures.