Synthesis and conformational analysis of a series of galactosyl enkephalin analogues showing high analgesic activity

Abstract

Two galactosyl derivatives of [DMet2,Pro5] enkephalin-amide (compound 1), namely [DMet2,Pro5] enkephalin [N1.5-beta-D-galactopyranosyl] amide (compound 2) and O1.5-(beta-D-galactopyranosyl) [DMet2,Hyp5] enkephalin-amide (compound 3) have been synthesized. Such glycosylpeptides have been shown to be extremely potent analgesic agonists. The conformational analysis of these three compounds in DMSO-d6 solution has been carried out using two-dimensional NMR methods. Both the parent compound (1) and the beta N-galactosyl derivative (2) show similar NMR parameters which are consistent with fairly rigid beta-strands at both the N-terminus and C-terminus, connected by a glycine residue that displays a mixture between multiple conformational states. Thus, although the beta N-galactosyl derivative (2) has been shown to be significantly more potent than the parent compound (1) in the tail immersion and paw pressure tests of analgesia, no correlation can be established between the conformation of (1) and (2) in DMSO and the difference in analgesic activity. In contrast, important conformational differences with respect to (1) and (2) have been detected in the beta O-galactosyl derivative (3). In this case, only one of the likely conformations for (1) and (2) are consistent with the experimental data. These data show that the position of the galactose residue in compound (3) causes Gly3 to loose flexibility leading to a more rigid folded conformation. Such a change in conformation could be related to the difference in analgesic activity between (2) and (3).