Preferred Conformation of <i>C</i>-Lactose at the Free and Peanut Lectin Bound States

Abstract

A conformational analysis of methyl α-C-lactoside (5) is carried out (Table 1), which suggests the preferred solution conformation of 5 to be described as a mixture of conformers A (φ = 300°; ψ = 60°) and B (φ = 300°; ψ = 300°) to a first approximation. Applying a modified Karplus equation for the vicinal coupling constants observed for the H.a and H.1‘ protons of 5, the dihedral angle (φ = O.5‘−C.1‘−C.a−C.4) was deduced to be approximately +280°. Nuclear Overhauser effect (NOE) experiments were then conducted on 5-D2 and its parent methyl α-O-lactoside in a 7:3 mixture of pyridine-d5 and methanol-d4, qualitatively demonstrating that the conformational characteristics of both methyl α-C- and α-O-lactosides at the free states are represented as a mixture of two conformers A and B, but their relative population may be different. Through X-ray analysis, it has been definitively established that the conformation (φ = 297(7)° and ψ = 120(2)°) of C-lactose bound to peanut lectin is practically identical to the conformation (φ = 291(6)° and ψ = 118(9)°) of its parent O-lactose bound to the same protein.