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exhibit a remarkable combination of high strength, toughness, and sometimes
elasticity, three properties that are rarely found in one synthetic polymer (Booth
and Price 1989). Recent advanced structural analysis and single molecule nano-
mechanical studies revealed that the combination of these mechanical properties in
natural materials arises from their unique molecular and nanoscopic structures.
These mechanistic understandings at the molecular level provide inspiration to
materials scientists for designing biomimetic polymers that have a balance of
advanced mechanical properties.
10.1.2. Biomimetic Design of Oligomers with High Order Structures
Whereas it remains a major challenge for chemists to design polymers with high order
structures comparable to their biological counterparts, significant progress has been
made in the last decade in designing short oligomers having well-defined secondary
and even tertiary structures (Cheng et al. 2001; Hill et al. 2001). The most basic rules
of protein folding are now understood to a sufficient extent to allow for the de novo
design and preparation of peptide structures containing a-helices, b-sheets, and
b-turns (Regan and DeGrado 1988; DeGrado et al. 1989; Richardson and
Richardson 1989; Richardson et al. 1992). By placing hydrophilic, hydrophobic,
and turn-forming amino acid residues in a suitable sequence it is possible to generate
structures that fold upon themselves or aggregate in helical or sheetlike conformations
(Richardson and Richardson 1989; Richardson et al. 1992; Xu et al. 2001).
Tremendous efforts have been devoted to the design and synthesis of peptidomi-
metics in which nonpeptide fragments are used as templates to induce structures in
peptides or to mimic elements of protein secondary structures: most notably,
a-helices and b-sheets. Many folded and potentially functional helical oligomers
have been developed, including b-peptides (Seebach and Matthews 1997; Cheng
et al. 2001), g-peptides (Seebach et al. 2001), peptoid oligomers (Horwell et al.
1994), and other foldamers involving unnatural backbones (Gong 2001; Hill et al.
2001; Oh et al. 2001; Orner et al. 2001). A variety of artificial systems consisting
of b-strands linked by unnatural templates have also been developed (Nowick
1999; Stigers et al. 1999; Phillips et al. 2002; Zeng et al. 2002).
The advancement in the understanding of protein structures and in peptidomi-
metics designs has inspired materials chemists to synthesize biomimetic biomaterials
having well-defined secondary and tertiary structures. For example, dendrimers are
designed to mimic the globular architecture of proteins (Tomalia 1996; Moore
1997; Fischer and Vogtle 1999; Newkome et al. 1999; Hawker and Piotti 2000;
Hecht and Frechet 2001; Zimmerman et al. 2002). Supramolecular assembly was ele-
gantly applied to construct polymers having two-dimensional and three-dimensional
(3-D) molecular nano-objects (Ghadiri et al. 1994; Moore 1997; Stupp et al. 1997;
Percec et al. 1998; Zubarev et al. 1999; Boal et al. 2000; Frankamp et al. 2002;
Lu et al. 2003). Biosynthesis through genetic engineering has been utilized to syn-
thesize artificial protein-based polymeric materials with unprecedented precision of
molecular weight and primary sequence (Krejchi et al. 1994; McGrath and Kaplan
1997; Urry et al. 1997; McMillan and Conticello 2000; van Hest and Tirrell 2001).
