Colloidal gels from proteins and peptides - Physical Gels from Biological and Synthetic Polymers - page 282

fibrillar network. MAX8 has one of the valines replaced by a glutamic acid residue, which
allows gel formation both to be faster and to produce a higher modulus G 0 under the same
conditions of solvent and peptide concentration. By changing these parameters, both gels
can be tuned. Using a small (10%) strain gelation could be followed in terms of G 0 and G 00
by oscillatory strain and also by dynamic light scattering methods, and a gelation time
identi Chambon method ( Chapter 3 ). However, both MAX1 and

MAX8 are highly strain sensitive. Under shear

ed using the Winter -

low they have a low viscosity, but can

' heal ' very fast. Rheologically, then, they show more similarity with so-called ' structured

liquids such as xanthan solutions ( Chapter 5 ).

Other peptide systems discussed by Yan and Pochan ( 2010 ) include the decapeptides
of Ramachandran et al.( 2005 ) and the block co-polypeptides of Aulisa et al.( 2009 ).
These show quite similar behaviour, with relatively high G 0 (typically >100 Pa at 1% w/
w) but with a linear viscoelastic limit extending only out to, say, 1% strain. Consistent
with this, it seems all the systems can be readily disrupted by applying a steady shear

'

flow, but then can recover fast, say, in a few minutes. Most experiments seem to be of the
form: (a) measure values of G 0 , (b) shear thoroughly and (c) monitor G 0 recovery as a
function of time (Kohyama and Nishinari, 1993 ). It seems that, so far, no group has
performed rigorous shear overshoot/recovery measurements of the type discussed for
xanthan

' '

in Chapter 5 . As we discuss in Chapter 11 , such fast shear-reversibility is
of great potential value for in vivo delivery of pharmaceutical materials by syringe
injection, and this remains the long-term aim of much of this work.

gels

9.6 Conclusions

Much early work on protein gels was driven by applications in the food, particularly
dairy, industries. This tended to be qualitative in its approach, but work since the 1980s
seems to have improved the quality of this work. The recent huge interest in this area is
two-fold:

first, investigation of the apparent similarities between globular protein gel
formation at low pH and amyloid diseases states; and, second, biomimetic work on
peptide gels, driven originally by the availability of speci cally designed and synthesized

peptides. However, one of the problems with the former work re

ects the very nature of
protein based studies. In many recent papers there seems a disappointing tendency to
neglect necessary sample details in the search for exciting and novel results. This may be
the reason for the lack of agreement between various groups; consequently we do implore
workers in this area to be more speci c about sample treatments, thermal histories etc., so

progress can continue to be made.

References

Adamcik, J., Jung, J.-M., Flakowski, J. et al., 2010. Nat. Nanotechnol. 5 , 423 - 428.

Aggeli, A., Bell, M., Boden, N. et al., 1997a. Nature 386 , 259 - 262.
Aggeli, A., Bell, M., Boden, N. et al., 1997b. J. Mater. Chem. 7 , 1135 - 1145.