Biomedical Engineering Reference
In-Depth Information
4.3
Dynamic Reassembly of Self-Assembling Peptides
The self-assembling process is reversible and dynamic (Fig.
10
). Since these peptides
are short and simple, numerous individual peptides can be readily self-organized
through the weak interactions including hydrogen bonds, ionic bonds, hydrophobic
and van der Waals interactions as well as water-mediated hydrogen bond forma-
tions. Once self-assembled, nanofi bers can be broken mechanically with sonication
(Yokoi et al.
2005
) and can undergo dynamic self-reassembly repeatedly, in a pro-
cess that resembles a self-healing process (Fig.
10
). Since the driving energy of their
assembly in water is governed by variety of interactions, this phenomenon can be
further exploited for production and fabrication of many variants of such
materials.
Unlike processed polymer microfi bers in which the fragments of polymers can-
not readily undergo reassembly without addition of catalysts or through material
processing, the supramolecular self-assembly and reassembly event we uncovered
here is likely to be wide spread in many unrelated fi brous biological materials where
there are numerous weak interactions involved. Self-assembly and reassembly are a
very important property for fabricating novel materials, and it is necessary to fully
understand its detailed process in order to design better biological materials. We
unequivocally demonstrated the reassembly process since we used the same peptide
solutions from a single experimental test tube throughout the four repeated experi-
mental cycles. This remarkable and rapid, initiated within minutes and fully accom-
plished within ~1-2 with hours, reassembly is interesting because there may be a
little nucleation for regrowth of the nanofi ber from the addition of monomers that
could only be produced during sonication. It is plausible that a large population of
the sonicated nanofi ber fragments contains many overlap cohesive ends due to
un-disrupted alanine hydrophobic side that may quickly fi nd each other (Fig.
9d
).
The situation is analogous and commonly found in sonicated and enzymatic digested
DNA fragments.
4.3.1
Kinetics of Nanofi ber Reassembly and a Plausible Reassembly Process
The reassembly kinetics is a function of time. Perhaps, similar to DNA reassembly,
the reassembly of peptides largely depends on the concentrations of the short com-
plementary fragments. In this case, the fragments are the sonicated peptide nanofi -
bers with possible presence of sonicated monomers.
In order to understand the dynamic reassembly, we proposed a plausible sliding
diffusion molecular model to interpret these observations of reassembly of the self-
assembling RADA16-I peptides (Fig.
11
). Unlike the left-handed helical structures
observed in KFE8 (Marini et al.
2002
), a different self-assembling peptide, no heli-
cal structures were observed for RADA16-I using AFM and transmission electron
microscopy (TEM) (Holmes et al.
2000
; Gelain et al.
2006
) .
