Cellobiose conformers

Table V. Cellobiose conformers in PEFACl. Units as in Table IV...

Fig. 6.—Allowed Rotational Conformation, Enclosed by Solid Line, of the Glucosidic Bond of 1,6-Anhydro- -cellobiose Hexaacetate (3), Calculated as a Function of the H-l,H-4 and H-l.H-5 Interatomic Distances. (Reproduced from Ref. 49.)...

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 4.2 Conformation map of cellobiose. Enclosed area defines allowed conformations in which there are no major conformational restrictions arising from interactions between non-bonded atoms.

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).

Figure 2. A contour diagram of the conformational energy of p-cellobiose computed from eqn. (6) holfing constant all variables except < ), v see ref. 5 for details. The rigid glucose residue geometry was taken from ref. 23, and the valence angle p at 04 was chosen as 116 in accordance with the results of pertinent crystal structure determinations. Contours are drawn at 2,4, 6, 8,10,25, and 50 kcal/mol above the absolute minimum located near ( ), v = -20 , -30 higher energy contours are omitted.

Figure 6. Relaxed or adiabatic conformational energy surface for p-cellobiose as computed by French Contours are drawn at 2,4,6, 8, and 10 kcal/mol above the minimum near < ), = 20 , -60 .

A short presentation of the Consistent Force Field is given, with emphasis on parametrization and optimization of energy function parameters. For best possible calculation of structure, potential energy functions with parameter values optimized on both structural and other properties must be used. Results from optimization with the Consistent Force Field on alkanes and ethers are applied to glucose, gentiobiose, maltose and cellobiose. Comparison is made with earlier and with parallel work. The meaning and use of conformational maps is discussed shortly. 

French has recently presented comparisons of rigid and relaxed conformational maps for cellobiose and maltose obtained with the MMP2(1985), which includes ano-meric effects. The fully relaxed maps show interesting details. 

For cellobiose, the situation is slightly different, as seen in Table V and Figure 4. The most obvious difference is that only five minima are found with MMP2 and PEFACl. In PEFACl, essentially two conformers are populated, and almost equally so they span the diffraction results, as summarized by French (19). 

Figure 4. Conformational Map of Cellobiose. + PEF300, X PEF400, o PEFACl, D MMP2...

Conformational Anafysis of a Disaccharide (Cellobiose) mth the Molecular Mechanics Program (MM2)... 

The effects of propagated distortions of the residue are shown in Figure 4, a CA map without contouring that was prepared with the standard driver. The gtgtRR starting model of cellobiose had an energy of 31.4 kcal/mol (its conformation was < ) = 20, - -60). ... 

Figure 5. a) The starting model of cellobiose (gtgtRR) after rigid rotation to -80, 0 -100. b) The result of attenpted optimization by MM2, c) The same linkage conformation, but the structure was taken from the study that produced the map in Figure 4. 

Figure 9. Potential energy surface for cellobiose at 400 K. The trajectory of conformational changes during a portion of the simulation are shown on the left. Energy contours in the vicinity of minima 1-3 are shown on the right. Barrier heights 5.3 Kcal/mol between minima 1 and 2,1.3 Kcal/mol between 2 and 3. (MM2(85) functions).

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