31 / 92
- 30 -
Polymers and Self Assembly: From Biology to Nanomaterials
Wednesday Speaker Abstracts
Racemic Hydrogels from Enantiomeric Peptides: Predictions from Linus Pauling
Joel Schneider
.
National Cancer Institute-NIH, Frederick, USA.
We have reported that hydrogel materials can be prepared from self-assembling beta-hairpin
peptides. For example, the 20-residue peptide MAX1 rapidly self-assembles into a hydrogel
network of monomorphic fibrils whose molecular structure was recently determined by solid
state NMR. The enantiomer of MAX1, namely DMAX1 assembles affording a hydrogel of
identical crosslink density, mesh size, and mechanical rigidity to the MAX1 gel. Surprisingly,
the gelation of a 1 wt % equimolar solution of peptide enantiomers occurs more rapidly resulting
in a racemic hydrogel network whose mechanical rigidity is over four-fold greater than gels
prepared from either pure enantiomer. Keeping in mind that the total amount of peptide in the
racemic gel is equal to that of either pure enantiomeric gel, this observation is truly unexpected
and suggests that biomolecular chirality, at the level of the monomer, is directly influencing the
mechanical properties of the self-assembled hydrogel. We interrogated the self-assembly process
and resulting fibrillar and network morphologies of the racemic gel employing CD spectroscopy,
isotope-edited FTIR, transmission electron microscopy labeling experiments, small angle
neutron scattering, diffusing wave spectroscopy, solid state NMR and molecular modeling to
uncover the molecular basis for this behavior. We show that the enhancement in hydrogel
rigidity does not result from an increase in network crosslink density, as one might predict.
Instead, the racemic gel is more mechanically rigid because each fibril in its network is, itself,
more rigid. In light of the NMR structure of pure MAX1 fibrils, the mechanism of enantiomeric
assembly and their molecular arrangement in the solid state will be presented. The mode of
molecular assembly uncovered in our studies was predicted by Linus Pauling in 1953 in the
course of deriving models of the pleated beta-sheet, a fold ubiquitous in protein structure.