Helical structures from neutral biopolymers - Physical Gels from Biological and Synthetic Polymers - page 191

0.6 40

B1

0.5 35

A1

30

0.4

0.3 25

B2

0.2 20

A2

0.1 15

Hydrolyzed sample

0.0 10

0 5 000 10 000 15 000 20 000

Time (S)

Helix formation versus time for two molecular masses of beef bone gelatin (A1: 145 700 g mol − 1
andA2:102200gmol − 1 ) and pig skin gelatin (B1: 168 500 g mol − 1 and B2: 74 600 g mol − 1 )
at concentration c = 4.5% w/v. Also shown is a highly hydrolyzed sample (c = 20% w/v,
M w ~11000gmol − 1 ) which has a very poor ability to regenerate helices. The thermal protocol
is also displayed. Adapted from Joly-Duhamel et al.( 2002a ) © 2002 American Chemical
Society.

Figure 7.5

native collagen. Various protocols for decreasing temperature can be used: either con-
tinuous cooling or heating at

fixed rates or rapid temperature decrease similar to a
quenching, followed by long-time annealing at constant temperature, or any combination
that would be required to form a gel. Various parameters which affect helix formation are
well established:

* Gelatin concentration: for identical protocols, the higher the concentration, the larger
the helix fraction. This suggests that nucleation is favoured in concentrated solutions,
but further growth may be slowed in a densely entangled system. The ultimate ratio is
60

70%, at low temperatures and long annealing times.
* Molecular mass: this is a very important parameter. The

-

first extracts are those which
exhibit the most rapid rates of growth. Figure 7.5 shows that extracts with different
molecular masses from mammalian gelatins have marked differences in the renatura-
tion kinetics at identical concentrations (4.5% w/v) and thermal treatments. Very low
molecular mass samples, so-called hydrolyzed samples (M w < 15 000 g mol − 1 ), cannot
recover the helical conformation even at high concentration.
* Source of gelatin sample: as already stated, gelatins from sources such as

fish skin
extracts have different collagen composition and their collagen denatures at lower
temperatures than mammalian gelatins. Therefore their renaturation temperature is
shifted to signi cantly lower values. A comparison of the kinetics of helix renaturation

is shown in Figure 7.6 .
* pH in the range 4 to 10, or salt (NaCl) at concentrations below 0.1 M, do not play an
important role in helix formation for more concentrated gels (greater than a few wt%).
However, in the very dilute range, effects of pH become important. Non-electrolytes