Self-Assembly of Synthetic and Biological Polymeric Systems of ...

Additional Supra-Self-Assembly of HSA J. Phys. Chem. B, Vol. 113, No. 36, 2009 12395
Figure 2. (a-c) ESEM images of different HSA spherulites incubated at pH 7.4 at 65 °C in the absence of added electrolyte along dehydration.
Progressive dehydration allows a better visualization of the spherulites. In part c, some surface cracking can be observed. The scale bars are 200,
100, and 50 µm in parts a, b, and c, respectively. (d) TEM image of HSA fibrils obtained after disruption of grown spherulites by extensive
sonication.
Figure 3. (a) G′ versus time (cure data) for a 0.8 mM HSA solution
heated at 65 °C atpH(9) 7.4, (b) 2.5, and (4) 5.5 in the presence of
100 mM NaCl. (b) Time evolution of storage (G′, O) and loss (G′′, 9)
moduli of a 0.8 mM HSA solution heated at 65 °C and pH 7.4 in the
presence of 100 mM NaCl.
biopolymer solution, with the divergence between G′ and G′′
becoming larger with time and G′′ < G′. Another feature of
these cure curves is the “dip-down” in the modulus at long times,
which may be indicative of slippage of the gel samples. 13
Interestingly, G′, though small initially at both pHs, was larger
than G′′ (the loss modulus) even before the gelling point (see
Figure 3b as an example). This “solid-like” behavior of the
starting protein solutions has been ascribed to colloid-like
Figure 4. Frequency depedence of the storage (G′, closed symbols)
and loss (G′′, open symbols) moduli of a 0.8 mM HSA solution at pH
(b) 7.4, (9) 2.5, and (2) 5.5 in the presence of 100 mM NaCl at 65
°C.
structuring in the pregel solution due to nonclassical DLVO
forces. 50,51 This is indicative of a different self-assembly
mechanism and/or kinetics if compared to what was observed
at pH 5.5, for which G′′ > G′ at short incubation times. At the
latter pH, faster aggregation takes place since enhanced protein/
protein interactions are favored due to the proximity of the
solution pH to the protein isoelectric point. It has been pointed
out that rapid formation of an appreciable amount of intermolecular
-sheets favored by protein/protein interactions 52 involves
very compact structures which precipitate, thereby limiting the
amount of protein available to form protein/water networks
necessary for robust gel formation 53 as in the present case. This
fact is also corroborated comparing the lower G′ values at pH
5.5 to those obtained at pH 7.4 and 2.5.
Figure 4 shows the corresponding frequency spectrum of G′
and G′′ for the gels after completion of the cure experiment.
Samples at pH 2.5 and 7.4 show typical gel spectra. G′ is
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