36
1
General Principles
⊡ Table 5
Steric interactions for methyl β-D-galactopyranosyl-(1→3)-β-D-glucopyranoside and its C-disaccharide analogue
Conformer
Interaction
Value for C-glycoside (kJ mol −1 )
ValueforO-glycoside (kJ mol −1 )
g+
O - 2 C 1
10.4
10.4
H-3O-5
1.9
1.9
C-2H-1
3.8
3.8
O-2 Hb
1.9
O-4Ha
1.9
C-1Hb
3.8
Total
23.7
16.1
g−
C-4O-5
10.4
10.4
O-4C-1
10.4
10.4
O-2 Hb
1.9
O-2Hb
1.9
Total
24.6
20.8
a
C-4H-1
3.8
3.8
C-2O-5
10.4
10.4
C-1 H-2
3.8
3.8
C-1 H-4
3.8
3.8
O-2Ha
1.9
O-2 Hb
1.9
O-4Hb
1.9
Total
27.5
21.8
1.4.2 Conformations of Trisaccharides and Oligosaccharides
No new factors arise when the conformations of trisaccharides or oligosaccharides are considered but intermonosaccharide interactions can become more important. For reviews,
see [350,351,352]. However, discussion of these conformations using the terms employed
with disaccharides is much more complicated because of the multiple maps and because
of the possibility of coordinated motion. An alternative description uses a generalized order
parameter, S 2 , which is related to the spatial restriction of internal reorientation, with values ranging from 0 to 1 [353,354,355,356,357]. The dynamics of isotropically tumbling
molecules are described in terms of overall and internal correlation times. The generalized
order parameter [353] has been extended to include two distinct correlation times [358]. When
the oligosaccharide is relatively rigid, S 2 for all segments is close to 1. Terminal monosaccharides in linear oligosaccharides often have lower values of S 2 than the central regions
consistent with independent motion [359,360], as do hydroxymethyl groups [361].