Computation of the electronic and spectroscopic properties of carbohydrates using novel density functional and vibrational self-consistent field methods

A novel density functional method is presented for the calculation of electronic and thermodynamical properties of oligosaccharides. This method, termed K2-BVWN, offers two advantages; it scales as N-3, where N is the number of basis functions, and there are only two adjustable parameters. The current density functional method is tested in terms of reproducing high level gas phase ab initio calculations in eleven low energy conformers of D-glucopyranose including exo-anomeric and different hydroxymethyl orientations (G(-), G(+), and T). The K2-BVWN method is also tested in terms of reproducing the spec troscopic features of D-glucopyranose and D-mannopyranose (alpha/beta) as compared with both a vibrational self-consistent field calculation (VSCF) as well as experimental infrared spectroscopy. The VSCF calculations offer the advantage that it is possible to include higher order mode coupling and anharmonic effects directly into the calculation of the vibrational frequencies. In general, the K2-BVWN method reproduces the ab initio energetic trends of the different conformers of D-glucose. While the absolute energies are not the same between the ab initio and the K2-BVWN method, both methods do predict a preference for the alpha-anomer in the gas phase (0.4 kcal/mol ab initio, 0.0 - 0.5 kcal/mol K2-BVWN). The K2-BVWN method was able to reproduce the experimental and VSCF calculated spectrum of both D-glucopyranose and D-mannopyranose in the frequency range between 1500 - 800 cm(-1). Because the current density functional method is both relatively quick and accurate, it represents a significant advancement in the development of oligosaccharide force fields.