Carbohydrates in Peptide and Protein Design - Peptide and Protein Design for Biopharmaceutical Applications - page 179

a- and b-orientations of glycosidic bonds, as well as furanoid and
pyranoid ring forms, this generates a high degree of structural diversity
for a biopolymer. Functional groups in natural carbohydrates include not
only hemiacetal and acetal moieties, primary and secondary alcohols, but
also amino groups, e.g. in glucosamine and amino-glycosides, and car-
boxylic acid groups, e.g. in glucuronic acid. However, these are rare com-
pared to the hydroxyls. Additional modifications of oligosaccharides, such
as acetyl and carbamoyl groups in lipochitin oligosaccharide nodulation
factors, can give other functionalities to oligosaccharides. The glycosidic
linkage can contain O-, S-, N- and C-atoms. Man-made, abiotic carbohy-
drates can, of course, contain any imaginable chemical modification.
It is often stressed that oligosaccharides posses an unsurpassed
positional diversity, based on their hydroxyls, due to the possibility
of positional isomers (e.g. 1,2- vs. 1,3-linkages), equatorial vs. axial
orientation, and the a-vs.b-orientation of the glycosidic linkage (for
a clear presentation of this concept, see [7]). However, this takes a
purely 'digital' view of the information content, based on hydroxyl
functional groups, and one should contrast this with the high
functional group diversity found in amino acid side chains. The func-
tional group diversity in most common oligosaccharides is similar to
that found in the side chains of Ser/Thr, Asn/Gln and Asp/Glu, while
the side chains of e.g. Arg (guanidinium), Cys (thiol), His (imidazole),
Tyr (phenol) and Trp (indole) do not have an equivalent in common
oligosaccharides.

5.3 CARBOHYDRATES IN PEPTIDOMIMETICS

For an extensive overview of peptidomimetics, see Chapter 3. Here, only
a few topics comparing conformational aspects of carbohydrates and
peptides are presented. While oligosaccharides are secondary gene pro-
ducts, proteins are primary products assembled from a-amino acids.
However, a large variety of nonproteinogenic, nonribosomal amino
acids and peptides are also found; they are biosynthesized by enzymes
and thus are also secondary gene products or elements of these. Examples
include b- and g-amino acids as well as a-alkylated amino acids such as
a-aminoisobutyric acid (Aib). Furthermore, post-translational modifica-
tion of ribosomal peptides and proteins, e.g. glycosylation, are also fre-
quently-occurring. Thus, the structural diversity of natural peptides
extends far beyond oligomers of proteinogenic a-amino acids. Two of