Biomedical Engineering Reference
In-Depth Information
T / T c
Sol
Gel
1
Biphasic
Volume fraction
Sol - gel state diagram showing typical behaviour for protein gels prepared by heating at pH values
away from pI.
Figure 9.1
the ancient Chinese fashion (Doi, 1993 ; Eiser et al., 2009 ). One critical parameter in
determining whether a transparent gel is formed is the protein isoelectric point pI.
For BSA, pI ~ 5.5 (although some sources give values as low as 4.7), while for
β
-Lg it
is 5.1. Close to the pI, a coagulate is formed rather than a gel. By increasing the charge,
increasing the ionic strength or altering the pH away from pI, increasingly transparent
gels tend to be formed. Conversely, as pI is approached, a gel can still be formed but it
tends to be opaque and to show signi
cant syneresis (propensity for water release), and so
enters a two-phase region. This, qualitatively at least, is consistent with predictions from
the theory of Coniglio et al.( 1982 ). Figure 9.1 , not speci
c to any protein, shows typical
behaviour for gels prepared at pH values away from pI. A clear illustration of these
tendencies is given in the early work of Clark and co-workers (Clark et al., 1981a ). In
Figure 9.2 , TEM images of BSA under various conditions show changes in network
heterogeneity. At or close to pI, a very dense globular structure is seen.
Much work has been performed on the microstructure of
-Lg gels, especially by the
Hermansson group. From this work it was concluded that various gel structures occur,
depending on pH and heating regime. For example, Langton and Hermansson (Langton
and Hermansson, 1992 ) describe what they call
β
'
particulate
'
networks, at pH values in
the range 4
-
6, which are (a)
'
regular
'
in the pH range 5
-
6, with uniform distributions of
particles and pores; (b)
at pH 4.5, where there are coarse, uneven aggregates;
and (c) a mixture of network types at pH 4 with clusters of particles embedded in a
'
irregular
'
ne-
stranded network. The particles in these gels are almost spherical (probably formed by
gelling liquid droplets) and their size depends on pH. Below pH 4 and above pH 6,
ne-
stranded networks (discussed later in this chapter) are formed, with shorter and appa-
rently stiffer linear strands occurring at the lower pH. As the pH is lowered still further,
the width and length of the strands increases, with a similar effect occurring above pH 6.
A recent EM study has indicated that at pH 7 short
5nm
(i.e. close to a single monomer in width) are formed, in agreement with earlier dynamic
light scattering analysis (Grif
'
worm-like
'
rods of width ~3
-
n et al., 1993 ; Kavanagh et al., 2000a , 2000b , 2000c ).
Other microscopy data has suggested, however, that spherical particles of diameter
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