'Reverse' Hofmeister effects on the sol-gel transition rates for an α-helical peptide-PEG bioconjugate.

We examine the dynamics of the sol-gel transition for end-functionalized linear- and 4-arm-peptides bioconjugated to poly-ethylene glycol (PEG) in aqueous environments with increasingly chaotropic (Cl- < Br- < I-) anions. A 23-amino acid peptide sequence is rationally designed to self-assemble upon folding into the ordered α-helical conformation due to the hydrophobic effect. We use Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR) to quantify the ensemble average reversible secondary structure transitions as a function of electrolyte concentration and specific ion effects along the Hofmeister series. Subsequently, microrheology is used to quantify the kinetics of the gelation process, as it relates to folding and specific ion interactions. Our key findings were non-intuitive. We observe the faster evolution of the gel transitions in systems with more chaotropic anions. For our peptides in aqueous solution, "water-structuring" ions yield faster assembly behavior with a viscoelastic exponent, n, closer to unity representing self-assemblies that are Rouse-like. In contrast, ions that are "water-breaking" resulted in smaller viscoelastic exponents where self-assembly dynamics result in a viscoelastic exponent that suggests polymer entanglements.

Protein Engineered Triblock Polymers Composed of Two SADs: Enhanced Mechanical Properties and Binding Abilities.

Recombinant methods have been used to engineer artificial protein triblock polymers composed of two different self-assembling domains (SADs) bearing one elastin (E) flanked by two cartilage oligomeric matrix protein coiled-coil (C) domains to generate CEC. To understand how the two C domains improve small molecule recognition and the mechanical integrity of CEC, we have constructed CL44AECL44A, which bears an impaired CL44A domain that is unstructured as a negative control. The CEC triblock polymer demonstrates increased small molecule binding and ideal elastic behavior for hydrogel formation. The negative control CL44AECL44A does not exhibit binding to small molecule and is inelastic at lower temperatures, affirming the favorable role of C domain and its helical conformation. While both CEC and CL44AECL44A assemble into micelles, CEC is more densely packed with C domains on the surface enabling the development of networks leading to hydrogel formation. Such protein engineered triblock copolymers capable of forming robust hydrogels hold tremendous promise for biomedical applications in drug delivery and tissue engineering.