Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
whole distribution of molecular weights. The molecular weight heterogeneity of
polysaccharides can be described by several types of average molecular weight. The
two most common methods in use for averaging are the number-average, M n (which
weighs the polymer molecules according to the number of molecules having a
specific molecular weight) and the weight-average, M w (which weighs the polymer
molecules according to the weight of molecules having a specific molecular weight).
In a population of molecules where N i is the number of molecules and w i the weight
of molecules having a specific molecular weight M i, these two averages are defined
as:
(14.1)
(14.2)
Static light scattering has been used to determine M w of chitosan. 9,10 However,
osmometry is also a convenient method to determine M n for chitosans. 11 The latter
method is much less influenced by the presence of aggregates. 11,12
14.2.4 MOLECULAR MASS DISTRIBUTION
M
w
M
n
Â
Âi
NM i i
i
=
N
Â
Âi
wM i i
i
= =
w
i
The fraction M w/M n is called the polydispersity index (PI). In a polydisperse molecule
population, the relation M w > M n is always valid, whereas in a monodisperse molecule
population M w = M n. Processes occurring during the production process or in the
raw material prior to extraction may affect molecular weight distribution. Because
of these processes, molecular weight distribution of chitin in its native form may
differ from the distribution observed in the extracted chitin solutions. For a randomly
degraded polymer, M w = 2M n. A PI of less than 2.0 may suggest that some fractionation
has occurred during the production process. Certain procedures can also cause
loss of the high-molecular-weight fraction or the low-molecular-weight tails of the
distribution. A PI of more than 2.0 indicates a wider distribution and may suggest
nonrandom degradation or the presence of aggregates.
By combining classical GPC (gel permeation chromatography) with a lightscattering
detector, the molecular weight distribution can be determined for a sample.
It is important that the GPC column separates the molecules over the entire molecular
weight distribution, which may be a problem for high-molecular-weight samples.
Ottøy et al. 13 have reported on analytical and semipreparative GPC of chitosans.
Reversible interactions between chitosans and different column packings were found
to occur, which strongly influenced the relationship between elution volume and
molecular weight. Thus, care should be taken in the analysis of molecular weight
distribution of chitosans by GPC systems calibrated with, for example, pullulan
standards.
i
Âi
Âi
NM
2
i i
NM
i i