Chemistry Reference
In-Depth Information
motifs to form modules (Chart 10.1c). By choosing the sequence and length of the
b-sheet strand, modules with tunable binding strength can be accessed. Finally, for
biocompatibility and complete reversibility for retraction, mechanically stable small
proteins have also been synthesized in our laboratory as modules to make protein-
based polymers (Chart 10.1d). The thermodynamic reversibility of protein folding
should lead to complete reversibility in retraction. In the next section we will selec-
tively discuss the synthesis, single molecule nanomechanics, and bulk mechanics of
some of our biomimetic systems.
10.3. RESULTS AND DISCUSSION
10.3.1. Synthesis and Studies of 2-Ureido-4-pyrimidone (UPy)
Modular Polymers
We started this research program by making polymers having loops folded by strong
hydrogen bonds (Guan et al. 2004). Based on molecular modeling and single mol-
ecule studies, the six hydrogen bonds between b-strands A
0
and G in the Ig
module of titin play a critical role in its mechanical stability (Marszalek et al.
1999; Lu and Schulten 2000). In our first biomimetic design, a strong quadruple
hydrogen bonding motif, UPy, was employed to direct the formation of loops
along a polymer chain. Sijbesma and coworkers (1997) have shown beautifully
that UPy dimerizes strongly with a dissociation constant (K
d
) of more than 10
28
M
in toluene. Based on the magnitude of the dimerization constant, the free energy
required to break the UPy dimer is more than 11 kcal/mol, which is comparable to
protein unfolding energy and lower than typical covalent bond energies, and therefore
suits our biomimetic study. We chose this system for several reasons: it dimerizes
relatively strongly by precise quadruple H bonding, the breakage energy for the
dimer is lower than that for breaking the macromolecule backbone, and it is relatively
easy to synthesize and to functionalize at the R and R
0
positions.
We chose urethane polymerization for constructing our modular polymers because
polyurethanes (PUs) have excellent biocompatibility and are an important class of
biomaterials (Pinchuk 1994). The synthesis of the UPy containing monomers
began with the alkylation of ethyl acetoacetate with allyl bromide followed by con-
densation with guanidine carbonate to afford an isocytosine, which was further
treated with allyl isocyanate to provide compound 4 (Scheme 10.1). The isocytosine
ring was protected as benzyl ether for improved solubility and for further synthesis of
a control polymer to be used in comparative studies. Hydroboration of the diolefin
3 afforded the protected UPy containing monomer 6. The benzyl protection
was removed by catalytic hydrogenation to give the free UPy monomer 7. Both
the protected UPy monomer 6 and the free UPy monomer 7 will be incorporated
into polymer chains for comparative studies.
Polymerization reactions were carried out via a prepolymer formation followed by
chain extension. One equivalent of poly(tetramethylene oxide) [number-average mol-
ecular weight (M
n
) ΒΌ 1400 g/mol] was injected slowly by a syringe pump into a
