R. Ravishankar et al., PREFERRED CONFORMATION OF C-LACTOSE AT THE FREE AND PEANUT LECTIN BOUND-STATES, Journal of the American Chemical Society, 120(44), 1998, pp. 11297-11303
A conformational analysis of methyl alpha-C-lactoside (5) is carried o
ut (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 a
pproximation. Applying a modified Karplus equation for the vicinal cou
pling constants observed for the H.a and H.1' protons of 5, the dihedr
al angle (phi = 0.5'-C.1'-C.a-C.4) was deduced to be approximately +28
0 degrees. Nuclear Overhauser effect (NOE) experiments were then condu
cted 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 con
formers A and B, but their relative population may be different. Throu
gh X-ray analysis, it has been definitively established that the confo
rmation (phi = 297(7)degrees and psi = 120(2)degrees) of C-lactose bou
nd 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.