DFT application in conformational determination of cellobiose

The potential energy of β-cellobiose in water, as a function of the H1'C1'O'C4 (ϕ) and C1'O'C4H4 (ψ) dihedral angles, is investigated using DFT methods and three energy minima (syn-ϕ/syn-ψ, anti-ϕ/syn-ψ, and syn-ϕ/anti-ψ) are identified. It is found that the hydrogen bonding and the H...H repulsion are vital for the energy variation of cellobiose. The geometry structure varies significantly when cellobiose is surrounded by 52 water molecules. NMR chemical shift calculations suggest that the C4 (C1') chemical shifts (δ) are sensitive to the conformational changes, with the values of 80.4 ppm (103.3 ppm) at syn-ϕ/syn-ψ, 70.0 ppm (98.9 ppm) at anti-ϕ/syn-ψ, and 75.6 ppm (95.3 ppm) at syn-ϕ/anti-ψ. The chemical shifts detection of characteristic positions is helpful for the determination of conformations, especially for those with equivalent energies. The chemical shielding tensor analysis shows that for C1', the directions and amplitudes of two shielding components (σ(22) and σ(33)) vary distinctly with ϕ and ψ whereas for C4, σ(33) of C4 remains unchanged, but σ(22) varies by 13 (26) ppm from syn-ϕ/syn-ψ to anti-ϕ/syn-ψ (syn-ϕ/anti-ψ). The distinctive C4 and C1' chemical shifts at different conformations suggest that they can be useful in (ϕ, ψ) dihedral angle determinations.