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Solution phase synthesis of a constrained Leu(5)-enkephalin amide analogue 10 is reported, in which the cyclic ferrocenyl containing subunit 7 was introduced as a mimetic of the tetrapeptide Tyr-Gly-Gly-Phe unit. Temperature dependence of the chemical shift of the amide protons of 10 indicated a hydrogen bond between the same amino acid residues as observed for the natural Leu-enkepalin in the sin
