Methods
Input Structures. Model disaccharide and glycopeptide structures were built with
Sybyl 6.5 (Tripos, Inc.). In order to generate the chemical shift surfaces as a function
of the glycosidic bond conformation, a grid with 18 _ 18 20o intervals in the 0 _ 340o
range for f and y was constructed, to give a total of 324 input structures for each
model disaccharide. For each structure in the grid, the f and y dihedrals were held
constant and the rest of the molecule fully optimized (AM1 semiempirical, Spartan
5.01), resulting in an adiabatic energy surface for each model molecule. The resulting
structures were employed in 13C chemical shift estimations.
Isotropic 13C Chemical Shift Calculation. The GIAO (Gauge Independent Atomic
Orbital) method of Pulay and coworkers was used, as implemented in Gaussian98.12
Due to the computational expense of calculations at the 6-311G** theory level, shift
surfaces were computed using the 3-21G basis set, and scaled to results from
reference 6-311G** level calculations for selected 13C saccharide resonances. In
order to obtain the scaling factor, we first performed GIAO 13C calculations using the
3-21G and 6-311G** basis sets on fully optimized (AM1 semiempirical) models of the
eight disaccharide for which surfaces were to be derived. The computed 13C shifts
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