Evaluation of NMR diffusion measurements for the conformational analysis of flexible peptides

The use of diffusion coefficients measured with pulsed-field gradient NMR spectroscopy for the determination of the relative population of conformers in solutions of the human Growth Hormone peptide fragment, hGH(9-19), has been studied in aqueous and in trifluoroethanol (TFE)/ water solutions. The peptide is a good model compound for this study because it adopts a predominantly random coil conformation in aqueous solution and is helical in TFE. The results of the diffusion measurements suggest that the peptide exhibits predominantly random coil structures in aqueous solution and adopts a more helical conformation in solutions containing increasing mole fractions of TFE, consistent with the qualitative findings of the standard CD and NMR experiments to probe peptide conformation. These results indicate that diffusion coefficients measured with NMR can provide additional information about temperature- and solvent-induced changes in the extent of the helical conformer for hGH(9-19) in aqueous solution and in solutions containing various mole fraction of TFE, respectively.