QM/MM distortion energies in di‐ and oligosaccharides complexed with proteins*

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Abstract
To investigate whether linkages between monosaccharide residues are unusually distorted by their interactions with proteins, ϕ and ψ values for fragments of cellulose and starch were taken from the Protein Data Bank. These experimental conformations were then plotted on energy surfaces that were calculated with a hybrid of HF/6‐31G* and MM3(96) energies. Energy values corresponding to each crystallographic conformation were then pooled. Nearly 70% of the 210 structures had energies of 1 kcal mol−1 or less. A cumulative frequency analysis showed that most points fell on a curve that had an exponential decrease in the number of observed structures as the energy increased. This is analogous to a Boltzmann distribution but at higher temperature. This analysis showed that more than 90% of the linkages were not unusually distorted, and the distribution was similar to that found for small‐molecule crystals of carbohydrates. However, above 2 kcal mol−1, the observed points deviated from the curve. Most of these high‐energy observations were from linkages being broken by enzymatic attack, but others were not, and some scissile linkages were not unusually distorted. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 416–425, 2001*

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