期刊
CELLULOSE
卷 18, 期 4, 页码 897-909出版社
SPRINGER
DOI: 10.1007/s10570-011-9539-6
关键词
Hydroxyl orientation; Crystal structure; Quantum mechanics; Clockwise
资金
- US Department of Agriculture, Agricultural Research Service
- New Hungary Development Plan [TAMOP-4.2.1/B-09/1/KMR-2010-0002]
Theoretical and experimental gas-phase studies of carbohydrates show that their hydroxyl groups are located in homodromic partial rings that resemble cooperative hydrogen bonds, albeit with long H center dot center dot center dot O distances and small O-H center dot center dot center dot O angles. On the other hand, anecdotal experience with disaccharide crystal structures suggested that these clockwise 'c' or counter-clockwise (reverse 'r') sequences are not prevalent in the crystalline state. The situation was clarified with quantum mechanics calculations in vacuum and in continuum solvation, as well as Atoms-In-Molecules analyses. From the experimental side, the Cambridge Structural Database was searched. Geometric criteria for these sequences were developed. A criterion based on 120 degrees ranges of hydroxyl orientations accepted 4% of sequences as having 'c,c' or 'r,r' orientations instead of the 7% expected based on chance. Criteria based on an O-H center dot center dot center dot O angle > 90 degrees and a 90 degrees lower limit of the absolute value of the H-O-C center dot center dot center dot H improper torsion accepted 7.0% of the 358 sequences as 'c,c' or 'r,r'. Highly variable orientation of the hydroxyl groups in crystals was seen to depend mostly on strong inter-residue or intermolecular hydrogen bonds. That lack of specific orientation in general for the crystal structures was supported by the solvated calculations that showed very little variation in the energy when one of the hydroxyl groups in 1,2-dihydroxycyclohexane was rotated. The vacuum calculations found the energy to vary with rotation by more than 4 kcal/mol, confirming the gas-phase experiments and calculations on more complicated molecules. Molecules examined in some detail include scyllo inositol and native and methylated cellobiose.
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