Preferred conformation of C-lactose at the free and peanut lectin bound states

A conformational analysis of methyl alpha-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 (phi = 300 degrees; psi = 60 degrees) and B (phi = 300 degrees; psi = 300 degrees) 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 (phi = 0.5'-C.1'-C.a-C.4) was deduced to be approximately +280 degrees. Nuclear Overhauser effect (NOE) experiments were then conducted on 5-D-2 and its parent methyl alpha-O-lactoside in a 7:3 mixture of pyridine-d(5) and methanol-d(4), qualitatively demonstrating that the conformational characteristics of both methyl alpha-C- and alpha-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 (phi = 297(7)degrees and psi = 120(2)degrees) of C-lactose bound to peanut lectin is practically identical to the conformation (phi = 291(6)degrees and psi = 118(9)degrees) of its parent O-lactose bound to the same protein.