Tunable crystallization, degradation, and self-assembly of recombinant protein block copolymers

Abstract

The self-assembly morphology, crystallization kinetics and thermal degradation process of biopolymers with well-defined amino acid sequences was investigated, using a specially created family of silk-based block copolymers patterned after Nephila Clavipes spider dragline silk. The block copolymers are of the form HBAm (m = 1, 2, 3, 6) and HABn (n = 2, 3) with gradient hydrophobic/hydrophilic ratio, where B is a hydrophilic block, A is a hydrophobic block, and H is a histidine-tag. Spider silk films were prepared either from solutions of hexafluoroisopropanol (HFIP), deionized water or methanol (MeOH) for comparison. Secondary structure and crystallinity of the films were monitored by Fourier transform infrared spectroscopy during heating from 30 to 340 °C. Thermal properties were determined by differential scanning calorimetry. Using scanning electron microscopies, micelles were observed in thermally treated films. Fibrillar networks and hollow vesicles were observed in methanol-cast samples, while no micro-structures were formed in HFIP-cast films, indicating that morphology and crystallinity can be tuned by thermal and solvent treatments.Results indicate when we increase the number of repeating unit of A-block in the protein, sample films crystallize more easily. When the volume fraction of A-block increases, the final crystallinity upon thermal treatment increases. Because beta sheets act as cross-linkers between protein chains, the sample films become more thermally stable. Moreover, when samples crystallize, the secondary structure of A-block and B-block is altered in a manner that facilitates formation of bilayer structures which fold into vesicles and tubes during drying.