Preferred conformation of C-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.