DISTINCT CONFORMATIONAL PREFERENCES OF 3 CYCLIC BETA-CASOMORPHIN-5 ANALOGS DETERMINED USING NMR-SPECTROSCOPY AND THEORETICAL-ANALYSIS

The conformational properties of three cyclic beta-casomorphin analogs based on the general formula H-Tyr-c[-D-Orn-2-Nal-D-Pro-Xaa-] (2-Nal = 2-naphthylalanine; Xaa = D-Ala, Sar or NMe-Ala) in DMSO solution wer e investigated using NMR spectroscopy in conjunction with molecular mo deling techniques. The D-Ala(5)- and Sar(5)-analogs (compounds 1 and 2 , respectively) are potent mixed mu-agonist/delta-antagonists with hig h mu- and delta-opioid receptor affinities, whereas the NMe-Ala(5)-ana log (compound 3) is a potent mu-agonist and a weak partial delta-agoni st. Distinct conformational differences emerged for the three compound s studied. Flexibility in the bare ring structures was found to increa se in the order 3<2<1. The increased structural rigidity of 3 may be r esponsible for its low delta-receptor affinity as compared to the two other analogs. A low fractional population of conformers containing tw o cis peptide bonds was found for compound 2 but not for analog 1 or 3 . Initial evidence for this observation was obtained from NMR differen tial line-broadening experiments and later confirmed by molecular mech anics simulations. Comparison of the temperature dependence of amide p roton chemical shifts acquired for the three cyclic analogs indicate a large degree of intramolecular hydrogen bonding for 1 but not for the other two peptides. (C) Munksgaard 1996.