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Explanation on Ligand Exchange Mode


Shodex SUGAR series allow the separation of monosaccharides and disaccharides using aqueous eluent under ligand exchange and GFC modes. Basically, saccharides are eluted in order of decreasing molecular weight by GFC mode. The series provides base line stability and linearity over a wide range when quantifying samples, thus permitting the detection of refractive indexes with high sensitivity.

Ligand exchange mode is a separation mode, which employs ion interaction between metal ions (M+) which are the counter ions of the packing material, and hydroxyl (OH) groups of the saccharides. Saccharides take chair form structure which is energetically stable. Hydroxyl groups are bonded with carbon molecules of chair form and there are two bond types, one is axial(Ax) and another is equatorial(Eq). In the figure above, two cases are illustrated.
(a) Triplet: Three hydroxyl groups having Ax-Eq-Ax structure form a complex with the counter ion. In this case, saccharides are strongly retained by the counter ion.
(b) Pair: Two hydroxyl groups having Ax-Eq structure form a complex with the counter ion. In this case, saccharides are rather strongly reteined by the counter ion but not so strongly as the case of triplet.
In the table below, the numbers of pair and triplet in monosaccharides are shown.

Depending on not only the structure of saccharides but also the type of counter ion, the intensity of interactions applied to the saccharides varies. And, the order of intensity is as follows:

Ag+ < Li+ < Na+ < Zn2+ < Ca2+ < Ba2+ < Pb2+

SUGAR SP0810 (Pb2+) is effective in separating monosaccharides from each other as it is a column in which ligand exchange mode functions most strongly. SUGAR SP0810 (Pb2+) and SUGAR SC1011 (Ca2+), which have divalent metal ions attached to the packing material, can separate saccharides from sugar alcohol as these columns retain sugar alcohol strongly.

The counter-ion of SUGAR KS-801 packing material is Na+. This column elutes saccharides basically in order of decreasing molecular weights as GFC is the dominant mode of this column. However, the column also can separate monosaccharides from each other to some extent as ligand exchange mode contributes to the separation.

In case of Shodex SUGAR columns, GFC mode, ligand exchange mode and adsorption & partition mode are combined in balance to enable good separation of saccharides.


Number of ax-eq-ax triplets and ax-eq pairs in monosaccharides having apyranose structure

Pyranose alpha-
anomer
beta-
anomer
Ketohexose D-Psicose 1xt 1xt
D-Tagotose 1xp 1xt
D-Fructose 1xp 2xp
D-Sorbose 1xp
Aldopentose D-Ribose 1xt(3xp) 2xp
D-Arabinose 1xp 2xp
D-Lyxose 1xp 2xp
D-Xylose 1xp
Pyranose alpha-
anomer
beta-
anomer
Aldohexose D-Talose 1xt(2xp) 1xt(3xp)
D-Allose 1xt(3xp) 2xp
D-Glucose 2xp 1xp
D-Galactose 2xp 1xp
D-Mannose 1xp 2xp
D-Altrose 1xp 2xp
D-Idose 1 to 0xp 1xp
D-Glucose 1xp
L-Fucose 2xp 1xp
L-Rhamnose 1xp 2xp

t:tripret(ax-eq-ax), p:pair(ax-eq)
Ketohexose : R.W.Goulding, J.Chromatogr., 1975, vol 103, p229-239.

Aldose: S.Honda, J.Chromatogr., 1984, vol.291, p317-325