Abstract
MDP, Murabutide and the MDP[D-Ala) analogue have been studied dissolved in dimethylsulfoxyde by means of 1H-n.m.r., including the 2D.2Q 1H- 1H inadequate and NOESY (Nuclear Overhauser Effect Spectroscopy) experiments. The results confirm the “S” shaped conformation constituted by two adjacent β turns in MDP. The first turn is a tight “type II β turn” and takes place around the MurNac (N-acetylmuramic acid) moiety of MDP and Murabutide and is stabilized by a C 10 hydrogen bonding between the Ala NH (1 + 3) and the acetamido C = 0 (1). The second turn is centered around L-Ala-D-iGln and requires the α-carboxamide group of D-iGln for its stabilization and thus does not occur in Murabutide. The L-Ala → D-Ala Inversion in MDP[D-Ala] produces substantial conformational modifications leading to a cyclic structure with the iGln α-carboxamide protons and the acetamido methyl group in close spatial proximity. The dissociation of the adjuvant, antiinfection and pyrogenic activities in the 3 compounds is well depicted by these variations of conformations.
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