To investigate the structural function of the C-terminal amide group of endomorphin-2 (EM2, H-Tyr-Pro-Phe-Phe-NH2), an endogenous l-opioid receptor ligand, the solution conformations of EM2 and its C-terminal free acid (EM2OH, H-Tyr-Pro-Phe-Phe-OH) in TFE (trifluoroethanol), water (pH and ), and aqueous DPC (dodecylphosphocholine) micelles (pH and ) were investigated by the combination of 2D 1 H-NMR meas-urement and molecular modelling calculation.