Influence of Solution Parameters on Phase Diagram of Recombinant Spider Silk-Like Block Copolymers

Abstract

Spider silks form interesting protein-based materials with exceptional mechanical properties from processing in aqueous systems. To more fully understand the role of specific chemical domains in the proteins responsible for these material features, selective regions are utilized to assess structure–function relationships. Toward this goal, the amphiphilic spider silk-like block copolymer HBA6 [hexahistidine tag (H), with hydrophilic (B) and hydrophobic (A) blocks] is studied to generate a partial phase diagram of the morphologies formed under different processing environments. Particles of HBA6 are prepared from a 2 m phosphate buffer at pH 5 and 8, and the resultant phase behavior upon ammonium sulfate (AS) addition is examined at room temperature for protein concentrations from 0.1 to 17 wt%. There is no apparent difference in the sizes of the microspheres formed at pH 8, irrespective of the amount of AS. In contrast, mixed morphologies ranging from unordered structures to clustered sheets to nanofibril-like morphologies are obtained at pH 5 with varying concentrations of AS. A partial ternary phase diagram is reported, representing the different morphologies obtained by changing the concentrations of HBA6, AS, and water. Differences in the morphologies as observed from scanning electron microscopy (SEM) are the result of altered protein-solute, interprotein, and intraprotein interactions caused by changing the pH and AS concentrations. The characteristics of the different morphologies illustrate how material properties based on motifs from spider silks can be tuned towards specific features, an important control point for matching materials to applications.