Biomedical Engineering Reference
In-Depth Information
four carbons coplanar-aligned, as well as other forms. Large groups
attached to the cyclohexane ring prefer the equatorial orientation.
However, the 1,3-diaxial interaction between bulky groups in cyclohex-
ane rings can disfavour these conformations. Additional effects, such as
hydrogen bonding in 1,3-diols, can make the prediction of conforma-
tional properties complicated.
Substitution of a methylene moiety with an oxygen atom, i.e. applying
this conformational analysis to pyranoses, does not change the general
conformational properties. However, the heterocycle has non-equivalent
bond lengths and features new stereoelectronic effects. Most prominent
among these effects is the anomeric effect, i.e. the preferred axial orienta-
tion of an anomeric substituent (most often oxygen). Related effects are
the reverse anomeric effect - positively charged anomeric substituents
prefer equatorial orientation - and the exoanomeric effect related to the
lone pair on the exocyclic oxygen. Other bulky substituents, i.e. other
than at C-1, on pyranoid rings still prefer equatorial orientations.
In b-
D
-glucopyranose, all secondary hydroxyls, including 1-OH, are
equatorially oriented.
Hexopyranoses can have two chair conformations,
4
C
1
and
1
C
4
,
depending on whether the 4- or the 1-substituent is above the plane of
the ring (Figure 5.2). Additional important conformational parameters
are given by the j, c glycosidic angles. Furanoses, which have a five-
membered heterocyclic ring, prefer puckered forms in order to avoid
eclipsed
cis
-adjacent hydrogen atoms. The envelope (E) and twist (T)
conformations are preferred, in which there are quasi-equatorial and
quasi-axial orientations of (hydroxyl) substituents.
R
1
O
R
3
N
5
4
5
1
O
O
O
O
H
H
O
φ
ψ
2
3
3
1
4
2
R
2
O
4
C
1
1
C
4
,
angles
ϕ
ψ
Figure 5.2 Left: torsional angles defining the peptide backbone consisting of
a-amino acids. Not shown is the o-angle, which determines the amide bond con-
formation as
trans
(180)or
cis
(0) [6]. Right: the two chair conformations in
pyranoses (
4
C
1
and
1
C
4
), and glycosidic angles [4]
Oligosaccharides can show a staggering structural complexity, as they
can be linear, branched or cyclic. Numerous disaccharides and many
trisaccharides are easily available. Saccharides can be 1,2-, 1,3-, 1,4-,
1,6- but also 1,1-linked, as in trehalose. Combined with the possibility of
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