Cellobiose hydrogen bonds

Cellulose consists of several thousand o-glucose units linked by l- 4-/3-glyco-side bonds like those in cellobiose. Different cellulose molecules then interact to form a large aggregate structure held together by hydrogen bonds. 

The torsion angles predicted by conformational analysis agree closely with those of crystalline cellobiose as measured by X-ray diffraction, the conformation of which is restricted by two chain-stabilising intramolecular hydrogen bonds between 0(3 )-H and 0(5) and also between 0(2 )-H and 0(6) (Figure 4.3). These are also found in cellulose and they assist in maintaining the highly extended conformation which allows it to function as a structural polymer. 

Figure 1. Conformations of cellobiose with inter-residue intramolecular hydrogen bonding. (a,b) conformations with two inter-residue bonds, (c) hydrogen bonding observed in the crystal structure (18).

Our initial, small models of an isolated cellulose chain ranged from the dimer (cellobiose) to the octamer. The dynamics of these fragments have thus far been simulated only in vacuum, using different potential energy functions such as those of MM2(85) (9) and AMBER (10), with and without contributions from electrostatic terms and hydrogen bonds, etc. (The program DISCOVER, customized for carbohydrates and for operation on the Alliant FX/80 computer, has been used (12).) Generally, the time span for the simulations has been of the order of several hundred picoseconds to 1 nanosecond. 

TGA analysis shows that polymer degradation starts at about 235°C which corresponds to the temperature of decomposition of the cellobiose monomer (m.p. 239°C with decom.). Torsion Braid analysis and differential scanning calorimetry measurements show that this polymer is very rigid and does not exhibit any transition in the range of -100 to +250 C, e.g. the polymer decomposition occurs below any transition temperature. This result is expected since both of the monomers, cellobiose and MDI, have rigid molecules and because cellobiose units of the polymer form intermolecular hydrogen bondings. Cellobiose polyurethanes based on aliphatic diisocyanates, e.g. HMDI, are expected to be more flexible. 

The difference in frequency between the parallel type bonds in cellulose II and III and those of cellulose I is appreciable and suggests that the molecular form of the cellobiose residue is different in cellulose I from the other two modifications. However, since the frequency of a hydrogen-bonded OH group is very sensitive to the 0 0 distance, this difference in molecular configuration may not be very great. 

It may be concluded that the low field envelope in the C4 carbon peak can be assigned to cellobiose units having fewer intramolecular hydrogen bonds (i.e., the region around such cellobiose units in cellulose molecules has a very disordered conformation). We cannot simply conclude that in the absence of any intramolecular hydrogen bonds, the peak would narrow significantly, since cellobiose units in cellulose molecules in the solid state can take on a large number of conformations. 

Fie. 9.1a, b. Inter-residue intramolecular hydrogen bonding in a cellobiose (CELLOB02), b me-thyl-/ -maltopyranoside monohydrate (MALTOS11) (one is direct, one is through a water molecule)... 

The hydrogen bonding in cellobiose and methyl cellobioside as models for that in the celluloses. In the absence of crystal structure analyses of higher oligomers of 1 - 4-linked /5-glucopyranose, cellobiose is frequently used as a model for interpreting the X-ray fiber diffraction patterns of the celluloses, especially since the cellobiose unit is considered to be the repeating unit of the polysaccharide chain. 

The favored models are anti-parallel or parallel-up. In both models, one chain has two intramolecular inter-residue hydrogen bonds as in cellulose I, while the other chain has only the 0(3)H- 0(5 ) observed in cellobiose. All hydroxyls are utilized. The parallel-down models appear to be inconsistent with this criterion however, this does not necessarily exclude them from consideration [515]. 

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