Raman Optical Activity of Biological Molecules - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 162

spectra of monosaccharides display bands that are sensitive to sugar ring
conformation, the relative disposition of hydroxyl groups around the ring, the
absolute configuration and orientation (axial or equatorial) of groups attached
to the anomeric carbon and the exocyclic CH 2 OH conformation. ROA spectra
of di- and oligosaccharides also display bands that are sensitive to the confor-
mation of the C-O-C glycosidic linkage, while the spectra of polysaccharides
provide information as to whether the structure is disordered or has extended
order.
As an example, we present in Fig. 7.7 the ROA spectrum of AGP [49], a
much-studied glycoprotein whose exact biological function is unknown [50].
So far no X-ray crystal or NMR structure has been reported for AGP, but
spectroscopic studies in combination with homology modelling of the polypep-
tide component have provided insight into the structure [51]. The polypep-
tide moiety is based on a single chain composed of 183 amino acids with two
disulphide bridges. The carbohydrate content makes up 45% of the molecular
weight of AGP and contains five or six highly sialyated oligosaccharide chains,
N-glycosylated to asparagine residues. The polypeptide chain is thought to
adopt a lipocalin-type fold based on an eight-stranded antiparallel -sheet

closed back on itself to form a continuously hydrogen-bonded barrel with
strands linked by

β

β -hairpins and a long loop [51]. The lipocalin fold is also

found in -lactoglobulin, whose ROA spectrum is shown in Fig. 7.5b. Those

ROA bands for AGP thought to originate from the carbohydrate component
are marked with an asterisk in Fig. 7.7, and many of the unmarked ROA
bands of AGP are similar to those observed for

β

β -lactoglobulin, indicating

that their folds are similar.

Fig. 7.7. Backscattered SCP Raman ( I R + I L )andROA( I R − I L ) spectra of bovine
α 1 -acid glycoprotein (AGP). The AGP ROA bands thought to originate mainly in
the carbohydrate component are marked with an asterisk