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

12396 J. Phys. Chem. B, Vol. 113, No. 36, 2009 Juárez et al.
Figure 5. (a and b) ESEM and (c) AFM images showing progressive coarsening of fibril strands of HSA gels after incubation at 65 °C atpH7.4
in the presence of 20, 100, and 250 mM NaCl, respectively (scale bars in parts a and b are 10 and 5 µm, respectively; AFM image size is 5 µm
× 5 µm). (d) TEM and (e) AFM images of fibrils from coarsed HSA gels after disruption by dilution and sonication at physiological pH (AFM
image size is 5 µm × 5 µm). (f and h) AFM and (g) ESEM images of particulate gels of HSA at pH 5.5 after incubation at 65 °C in the presence
of 20 (f and g) and 150 mM NaCl (h) (scale bar in part g is 2 µm; AFM image size is 5 µm × 5 µm).
essentially frequency-independent, so we can assert that the
storage moduli will have a finite value at very long times (low
frequencies); i.e., it confirms the solid-like properties consistent
with the development of a network, as previously commented.
On the other hand, G′ values at physiological pH are larger than
those at acidic pH, which denotes the formation of a stiffer gel.
This is also supported by the essentially frequency-independent
behavior of G′′ at pH 7.4. In contrast, under acidic conditions,
a slight minimum in G′′ can be observed, which becomes much
more intense at pH 5.5. This minimum at pH 5.5 can be assigned
to molecular packing, 54 and is in agreement with the lower G′
values, the frequency-dependent behavior, and the particulate
structure of the gels formed at this pH, as seen below.
Figure 5 shows ESEM and AFM images of the gels formed
at pH 7.4, 5.5, and 2.5 at different ionic strengths. For
physiological conditions after incubation at 65 °C in the presence
of 20 mM NaCl, a fibrillar gel formed by fine strands can be
observed (Figure 5a). These strands are formed by interconnected
amyloid-like fibrils as denoted by the cross- pattern
derived from XRD analysis (see Figure S1 in the Supporting
Information), and in agreement with the rheology data. Moreover,
upon strong dilution and sonication the strands dissociate
into fibrils which showed a predominant curly morphology, as
observed previously for HSA 42 and other proteins such as
2 microglobulin, 55 R-crystallin, 56 or bovine serum albumin
(BSA). 57 Nevertheless, some mature straight fibrils and ribbonlike
structures could also be observed (see Figure S2 in the
Supporting Information).
When the ionic strength is increased at pH 7.4 up to 250
mM NaCl, a coarsening of the gel is progressively observed,
180