(713d) Effects of Surface-Displayed Stimulus-Responsive Polypeptides

The steric or electrostatic stabilization of colloidal particles is often necessary to prevent unwanted aggregation. This control is often achieved through surface chemistry methods, yet at the nanoscale, interfacial properties can dominate the bulk behavior. Using virus particles as tunable colloids introduces flexibility into the creation of new functional materials. Our strategy has been to alter distinct surface features on filamentous bacteriophage particles with elastin-like polypeptides (ELP) by recombinant DNA methods. The resulting particles may be thought of as having ?grafted? layer of ELP (at maximum surface coverage), due to the nature of the recombinant protein construction. Various applications exploiting the stimulus-responsive nature of ELPs have been reported, and here we discuss our findings in the context of controlling particle aggregation.

For the lengths and compositions studied, modified particles do not aggregate from solution at low ionic strength, as monitored by DLS. However, the hydrodynamic size of the particles depends on the ELP details, suggesting conformational differences among the ?grafted,? or end-tethered, ELPs. Second virial coefficients reveal vanishingly low values ? and hence the low driving force for aggregation. The results are consistent with the classical picture of critical solution phenomena at low concentrations, where to drive phase separation, solvent quality must be increasingly poor. We have altered the solvent quality by using different Hofmeister anions (and salt concentrations) and confirmed depression of the transition temperature. Thus, under appropriate conditions, grafted ELPs can precipitate particles from solution that are significantly larger than the polypeptides themselves. Extrapolation of these data to the limit of vanishing salt predicts transition temperatures much higher than ambient for all the systems studied.

To achieve a more practical temperature range for the ELP transition, we are now expanding our repertoire of ELPs to include a more hydrophobic guest residue, tryptophan. Preliminary circular dichroism studies indicate structural changes in the grafted ELPs, even in the absence of a macroscopic transition, consistent with literature evidence. The reversible nature of the ELP transition will provide an opportunity for combining targeted binding and stimulus responsive technologies onto a single platform ? that is, particles capable of conditionally binding to target moieties.