AbstractThe synthesis of Leu‐enkephalin selectively17O‐enriched in Gly2and Gly3is reported. The17O‐nmr chemical shifts of [17O‐Gly2, Leu5]‐ and [17O‐Gly3, Leu5]‐enkephalins in H2O are almost identical and independent of the pH. Since hydrogen bonding is the dominant factor governing the chemical shifts of the peptide oxygen, it can be concluded that the hydration state of both oxygens is identical and independent of the pH. The17O chemical shifts of the [17O‐Leu5]‐enkephalin terminal carboxyl group at pH ∼ 1.9 and 5.6 are very different in H2O but very similar in CH3CN/DMSO (4:1) solution. This suggests that the protonation state of the carboxyl group at both pH values in CH3CN/DMSO solution is the same and consequently that Leu‐enkephalin exists in the neutral form at pH ∼ 5.6. In this organic mixed solvent system both Gly2and Gly3oxygen resonances exhibit a significant shift to high frequency by the same extent (Δδ ∼ 30 ppm). It is concluded that both peptide oxygens are not hydrogen bonded to an appreciable extent and that no specific 2 ← 5 hydrogen bonding exists to an appreciable extent. This conclusion is in agreement with the energy of activation for molecular rotation, as determined fromT1measurements, which was found to be almost identical for both [17O‐Gly2, Leu5]‐ and [17O‐Gly3, Leu5]‐enkephal