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However, every third residue is α-1,6-linked, providing a particularly flexible
structure with a persistence length of only 1.3 nm (discussed further in Section
2). Note that pullulan has no charged groups.
CH2
CH2OH
O
CH2OH
O
OH
O
OH
HO
OH
O
OH
O
OH
O
OH
CH2
O
OH
HO
O
OH
Figure 2.6.1. Repeating unit of pullulan: Maltotriose units (Three Dglucose residues linked α-1,4 as in amylose) linked by a flexible α-1,6
linkage.
Pullulan is available as nearly uniform fractions (extensively fractionated) in a
wide range of molecular weights (M = 5.000 – 850.000) and therefore much
used as standards and reference materials in molecular weight
determinations.
2.7. DEXTRANS
2.7.1.
Introduction
Dextrans are known in the polysaccharide field mainly because they are
commercially available as standards with narrow molecular weight
distributions over a wide range of molecular weights (< 1000 Da to over 106
Da). They are therefore used as standards or calibration substances.
Dextrans are also important in other areas of medicine and biotechnology.
They are known as ‘plasma expanders’ because of their non-ionic and
chemically inert properties. Another area is the development of dextran-based
gel filtration particles, which are chemically cross-linked dextrans of various
porosities and particle sizes (Sephadex).
According to Wikipedia (as of Oct. 2012), ‘.. dextran is synthesized from
sucrose by certain lactic-acid bacteria, the best-known being Leuconostoc
mesenteroides and Streptococcus mutans. Dental plaque is rich in dextrans.
Dextran is also formed by the lactic acid bacterium Lactobacillus brevis to
create the crystals of tibicos, a water kefir fermented beverage which
supposedly has some health benefits. Dextran was first discovered by Louis
Pasteur as a microbial product in wine…’
2.7.2.
Chemicalstructure
Dextrans are based on α-1,6-linked glucan backbones (Figure 2.7.1)
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