Incorporation of (2S,3S)-4,4,4-trifluorothreonine (F-3-Thr) instead of Thr in the enkephalin-derived hexapeptide led to the apparent conformational alteration due to the strong electron-withdrawing effect of the trifluoromethyl group by comparison with the original compound on the basis of their various NMR measurements. (C) 2007 Elsevier Ltd. All rights reserved.