Chemistry Reference
In-Depth Information
proteins existing in both soft and hard tissues (Kellermayer et al. 1997; Rief, Gautel,
et al. 1997; Marszalek et al. 1999; Li et al. 2000) and composite materials such as
seashells (Smith et al. 1999) and bone (Thompson et al. 2001) that can combine
important mechanical properties including strength, toughness, and elasticity.
Recent single molecule and nanomechanical studies of these natural materials
began to reveal the molecular origins for the combination of these mechanical prop-
erties. These mechanistic understandings at the molecular level provide inspiration to
materials scientists for designing biomimetic polymers that have a balance of
advanced mechanical properties (Winningham and Sogah 1997; Rathore and
Sogah 2001; Lee, Dellatore, et al. 2007; Lee, Lee, et al. 2007; Westwood et al. 2007).
One common strategy employed in natural material design is the programming of
secondary molecular forces into strong covalent polymers to further enhance the
physical performance. A key mechanism used in natural materials is to program
weak molecular forces, intermolecularly or intramolecularly, that serve as reversible
sacrificial bonds to dissipate energy while gaining further extension. This sacrificial
bonding concept has generated significant interest in the biophysics and materials
science communities (Smith et al. 1999; Fantner et al. 2006). My group has been
using titin as the model for developing biomimetic polymers consisting of a linear
array of modules folded by sacrificial weak hydrogen bonds (Guan et al. 2003,
2004; Roland and Guan 2004; Guan 2007; Guzman et al. 2007; Kushner et al.
2007). Titin uses intramolecular weak forces to fold a long chain polymer into a
bead-on-string architecture for combining mechanical strength, toughness, and elas-
ticity into one system. In the first generation of biomimetic design, we synthesized
polymers having loops held by UPy quadruple H bonding units and successfully con-
ducted both single molecule force-extension and bulk stress-strain studies for this
modular polymer (Guan et al. 2004). AFM single chain stretching results show
that the mechanical properties of individual chains can be precisely measured at a
single molecule level. The bulk tensile testing tests show that the modular polymer
indeed possesses a combination of high tensile strength and high toughness.
In our second-generation biomimetic design, we successfully synthesized a series
of peptidomimetic b-sheet modules (Roland and Guan 2004). Both computational
SMD simulation and experimental single molecule force microscopy were used to
understand the relationship between weak molecular interactions and mechanical
stability. The DCL modules exhibit significantly improved regularity in sequential
unfolding events as revealed by AFM single molecule force microscopy. It was
further demonstrated that other factors in addition to thermodynamic stability, such
as the trajectory of the pulling force relative to the duplex axis and possible kinetic
folding intermediates, play an important role in the mechanical stability of the supra-
molecular assemblies (Guzman et al. 2007).
Finally, we successfully demonstrated one particular application of our biomimetic
modular concept in improving bulk polymer properties. Through introduction of a
small amount of reversibly unfolding modular cross-linker into 3-D network poly-
mers, the mechanical properties of the network were drastically enhanced (Kushner
et al. 2007). Most strikingly, at increasing cross-linking density both the modulus
and tensile strength were significantly improved without sacrificing the extensibility.
