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This is an Open Access article under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Enkephalin-related peptide 64 and the fluorinated analogue 65. The electron-withdrawing trifluoromethyl group of 65 disrupts a key hydrogen bond, leading to a different conformation as determined by NOESY experiments.