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Hybrid polymers containing peptide blocks were also reported to mimic the b-sheet
structures in natural silk (Winningham and Sogah 1997; Qu et al. 2000; Rathore and
Sogah 2001).
Despite this important progres, a huge gap still exists between synthetic and
natural polymers (such as proteins) in terms of high order structures. Whereas
advanced synthetic methodologies now allow us to make synthetic polymers with
covalent bonds as strong as natural polymers, we are still at a very primitive stage
in programming secondary molecular forces into polymers to control their organiz-
ation into high order structures. Can we follow Nature's strategies by introducing
secondary molecular forces into covalent polymers to achieve advanced properties
that are so far beyond our reach? In addition, through molecular and supramolecular
designs coupled with advanced property studies at various length scales (single mol-
ecule, nano- and microscopic, and bulk), can we connect the molecular structure of a
polymer to its bulk properties so that we can start to rationally design new polymeric
materials with desired properties?
10.2. BIOMIMETIC CONCEPT OF MODULAR POLYMER DESIGN
10.2.1. Titin as Model for Modular Polymer Designs
With these fundamental questions and goals in mind, my group uses exceptionally
well-designed natural polymers as models for our biomimetic polymer designs.
One specific model we have conducted extensive biomimetic studies on is titin, a
giant protein (3000 kDa, 1 mm long) of the muscle sarcomere. Titin is composed
of 300 modules in two motif types, immunoglobulin (Ig) and fibronectin type III
domains (Fig. 10.1; Wang 1996; Maruyama 1997). Whereas actin and myosin are
motor proteins responsible for muscle contraction, titin contributes to muscle's mech-
anical strength, toughness, and elasticity (Erickson 1994; Labeit and Kolmerer 1995;
Granzier et al. 1996). Single molecule studies have shown that titin exhibits a remark-
able combination of high mechanical strength, fracture toughness, and elasticity
(Kellermayer et al. 1997; Rief et al. 1997a; Tskhovrebova et al. 1997; Marszalek
et al. 1999; Li et al. 2000; Oberhauser et al. 2001). Further studies have revealed
that titin's combination of these properties arises from its unique modular domain
structures (Kellermayer et al. 1997; Rief, Gautel, et al. 1997; Tskhovrebova et al.
1997; Marszalek et al. 1999; Smith et al. 1999; Clausen-Schaumann et al. 2000;
Li et al. 2000; Oesterhelt et al. 2000; Oberhauser et al. 2001; Li et al. 2002;
Marszalek et al. 2002). Sequential unfolding of the domains results in the sawtooth
Figure 10.1 Modular multidomain structure of titin protein.
