(6bh) Design of Biopolymer Building Blocks for Novel Self-Assembled Materials

Authors: 
Kim, M., Massachusetts Institute of Technology

Bioinspired and biomimetic materials, in particular hydrogels, are currently one of the most interesting areas in academic and industrial research, encompassing both biomedical (drug delivery, tissue engineering) and industrial applications (filters, sensors, and actuators).  To meet these emerging demands, studies have focused on tailoring a material’s mechanical properties (e.g. mechanical strength, extensibility, toughness, injectability, self-healing, and lifetime) and mimicking the physical/chemical properties of natural materials (e.g. extracellular matrix and muscle).  Biopolymers, in particular natural proteins and synthetic polypeptides, have received a great deal of attention as elements in these materials.  Amino acid sequences of polypeptides can be precisely controlled and easily modified through recombinant DNA technology while large batches of high purity and monodisperse polypeptides can be cost-effectively biosynthesized.  More importantly, different sequences can produce unique folded structures that result in distinct biophysical and biochemical properties at the molecular level including mechanical properties, molecular recognition abilities, and intelligent responses to external stimuli.  If these exceptional molecular properties of biopolymers are incorporated into macroscopic materials, multifunctional biopolymer-based materials that exceed those attainable from purely synthetic polymer-based materials can be designed.

Mechanical behaviors of muscle can be potentially mimicked by engineered hyperelastic materials/hydrogels constructed by crosslinking folded protein structures.  Using a custom-designed atomic force microscope (AFM) for single-molecule force spectroscopy, structure-mechanical property relationships of α-helical repeat proteins and globular proteins have been characterized, demonstrating that similarly structured proteins can be used as building blocks for hyperelastic materials due to their reproducible large extensibility and energy dissipation, and fast response to external forces.  In addition, to maintain proper folding and to preserve the desired mechanical properties in macroscopic materials, end-crosslinking of the proteins was investigated.  Simple physical crosslinking was performed by fusing subunits of protein complexes to their termini and it was confirmed to successfully construct the mechanical protein-based supramolecular self-assemblies as designed.  A new technique was also developed to directly quantify the interfacial rupture strength of the subunits in their individual assemblies, permitting the investigation of various protein complexes that can be used as molecular “screws” or “bolts” in novel biopolymeric materials.

Protein (nucleoporin)-based biological hydrogels with selective filtering properties have been mimicked by constructing biosynthetic hydrogels built from newly designed nucleoporin-like polypeptide (NLP) building blocks.  The biological hydrogels in the nuclear pore have been attractive due to their capability of selectively filtering < 0.1% of all proteins in a eukaryotic cell while translocating over 1,000 molecules per second.  Development of similarly behaving biopolymer gels can potentially impact separation technologies and defense applications but is limited by the low synthesis yield of proteins and long gel processing time.  Artificially engineered high-yield NLP building blocks that can mimic the structural properties of nucleoporins have been successfully developed, and rapid gel construction is achieved.  Using a combination of selective permeability fluorescence assays, shear rheology, and Raman spectroscopy, the mechanism of artificially engineered NLP hydrogel function is studied.