Effects of short elastin-like peptides on filamentous particles and their transition behavior.

While elastin-like polypeptides and peptides (ELPs) have been used for various stimulus-responsive applications, details of their switching remain unclear. We therefore constructed a novel series of filamentous phage particles displaying a high surface density of short ELPs. The surface display of ELPs did not disrupt either particle shape or dimensions, and the resulting ELP-phage particles were colloidally stable over several weeks. However, in spite of a saturating surface density, macroscopic aggregation of ELP-phages cannot be triggered in water. To investigate the underlying mechanisms we examined conformational changes in the secondary structure of the phage proteins by circular dichroism and tryptophan fluorescence, which indicate partial protein unfolding in ELP-phage particles. To gain further insight into the ELP itself, analogous "free" ELP peptides were synthesized and characterized. Circular dichroism of these peptides shows the onset of β-type conformations with increasing temperature, consistent with the accepted view of the microscopic transition that underlies the inverse phase behavior of ELPs. Increased guest residue hydrophobicity was found to depress the microscopic transition temperature of the peptides, also consistent with a previously proposed intramolecular hydrogen-bonding mechanism. Importantly, our results indicate that although the nanoscale presentation state can suppress macroscopic aggregation of ELPs, microscopic transitions of the ELP can still occur. Given the growing use of ELPs within supra-molecular scaffolds, such effects are important design considerations for future applications.