Chemistry Reference
In-Depth Information
varying from 8 to 20%. Experimental study of glass transition in proteins is all
the more difficult because it occurs in a temperature range where denatura-
tion can occur.
Based on a description of the plasticizing effect of water in terms of
shielding of intra- and intermolecular hydrogen bonds and dipole-dipole
interactions, a method was recently proposed to calculate the glass transition
temperature of biopolymers as a function of water content. The T
g
values for
13 proteins and polysaccharides at different water contents between 5 and
25%, calculated using information about their chemical structure, were found
to differ from experimental values by less than 10-308C (Matveev et al.,
2000). According to these authors, over the three main chemical character-
istics of a protein, i.e. amino acid composition, disulphide cross-links and
molecular weight of the polypeptide, amino acid composition alone is suffi-
cient to calculate T
g
. The T
g
of
-casein, however, was shown to increase (by
some 408C at a water content
20%) after cross-linking with transglutami-
nase (Mizuno et al., 1999).
The glass transition behaviour of fully hydrated proteins is currently a
matter of intensive debate, as it is most important for the understanding of
water-protein interactions. However, the issue is also of interest for the
stability of frozen products. Very different values have been reported for
the glass transition temperature of the maximally freeze-concentrated phase
(T
g
0
) of animal products: on the one hand, in the range -20 to -128C for
various fish and meat products (Levine and Slade, 1989; Brake and Fennema,
1999), and on the other hand, -84 and -858C for beef and egg white, respec-
tively (Simatos et al., 1975), -658C for tuna (Orlien et al., 2003), -908C(T
)
for salmon (Champion, unpublished) and -758C for blood plasma (Simatos
et al., 1975). It may be argued that the low level of these values is due to the
lowering effect of salts and other small molecular weight molecules. It is
consistent, however, with the values reported for fully hydrated BSA and
several other globular proteins, which all fall within the range -67 to -878C
(Chang and Randall, 1992; Inoue and Ishikawa, 2000). The values reported in
this section are the temperature of the middle of the transition considered as
the glass transition.
A dynamic transition is evidenced for fully hydrated proteins at -90 to
-508C from the variation with temperature of the mean-squared displacement
of atoms, as determined by a variety of spectroscopic techniques and dynamic
computer studies (see Gregory, 1995, for review). According to protein
scientists, this low temperature transition and the low water content at higher
temperature appear to be manifestations of a common hydration-dependent
transition, in line with the behaviour expected for a glass transition in which
water acts as a plasticizer (Gregory, 1995). Objections have been raised,
however, that the low temperature transition cannot be what is usually
