Carbohydrates in Peptide and Protein Design - Peptide and Protein Design for Biopharmaceutical Applications - page 177

criteria but by a set of interconnecting resemblances among the members
of the family [3].
The most prominent carbohydrates are the two trioses (glyceraldehyde),
four tetraoses, eight pentoses and sixteen hexoses (for a general overview
of carbohydrate chemistry, see [4]). They are distributed in enantiomeric
(mirror image) pairs, in which the highest-numbered (furthest from the
aldehyde/ketone) asymmetric centre defines whether the pair is of the D -or
L -form. Pentoses and hexoses can exist as five- and six-membered cyclized
forms, i.e. as furanoses ( f ) and pyranoses ( p ), respectively. Substituents on
these rings can normally either be equatorially or axially oriented.
Aldoses contain an aldehyde functionality, and ketoses (uloses) incorpo-
rate a keto functionality, most often at C-2, as in fructose. While the
aldose open-chain form contains an aldehyde moiety, the ring-closed
form contains a hemiacetal or acetal moiety. The hemiacetal hydroxyl
at C-1 (anomeric centre in aldoses) can be in an a-orb-orientation. A
prominent nomenclature example is a- D -Glc p : the ' p ' indicates a six-
membered pyranoid ring, while 'a' indicates an axial orientation for the
anomeric hydroxyl [5].
There are three different received ways to depict cyclic monosaccharides
as pyranoses. Shown in Figure 5.1 is a- D -Glc p in (a) the Haworth repre-
sentation, (b) the six-membered puckered ring (e.g. chair-like) and (c,d) the
representation in which the ring is coplanar with the paper. While the two
former are used very widely in carbohydrate chemistry, the coplanar
representation is used more in e.g. natural product chemistry. Although
they contain the same information, the two former more readily visualize
the 3D structure of carbohydrates.
At a first approximation, the conformational properties of pyranoses
can be exemplified by cyclohexane [4]. Cyclohexane exists in two major

OH

OH

HO

OH

HO OH

O

O

O

OH HO

HO

HO OH

OH OH

OH

HO O

HO

OH

HO OH

OH

A B C D

Figure 5.1 Representations of a- D -Glc p

conformational classes: a rigid chair form and a flexible form. The chair
form is free from torsional strain and van der Waals interaction, while the
flexible forms suffer from both and thus are energetically less favoured.
The group of flexible forms includes boat and skew forms, each having