REPETITIVE PROTEIN MATERIALS DESIGN INSPIRED BY SQUID RING TEETH

Open Access
Author:
Jung, Huihun
Graduate Program:
Engineering Science and Mechanics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 03, 2018
Committee Members:
  • Melik C Demirel, Dissertation Advisor
  • Melik C Demirel, Committee Chair
  • Benjamin D Allen, Committee Member
  • Sahin K Ozdemir, Committee Member
  • Seong Han Kim, Outside Member
Keywords:
  • Squid ring teeth
  • Protein materials design
  • Biomimetics
  • Bioelastomer
  • Tandem repeat
  • Protein mechanics
  • Biopolymers
Abstract:
Proteins are biological macromolecules consisting of a primary sequence of numerous amino acids. Structural proteins are the most abundant class of all proteins in nature. They form the protective covering of animal bodies: skin, hair, nails, etc. The primary structures of structural proteins usually consist of repetitions of several amino acid chain segments. These segmented sequences, or motifs, play a key role in building blocks of structural proteins. Scientists in protein material fields have been trying to understand structural proteins as block copolymers. In block copolymers, distinct monomer units are grouped in discrete blocks, and thus they consist of segmented building blocks in their final structures. Design strategies between structural proteins and block copolymers are very similar. Functionalities of both structural proteins and block copolymers are encoded in segmented domains. However, the difference comes from their complexity. Proteins are much more complex systems than block copolymers in their structures. Native structural proteins have repetitive sequences, but these repetitive segments are not perfect-repeat copies. In other words, their sequences are tandem repeats of unit-sequence building blocks with variations in lengths and amino acid compositions. This feature complicates analysis of protein-based materials. In order to understand the behavior of structural proteins, there is a huge demand for materials scientists to develop advanced strategies to build up “perfect-repeat” structural proteins with defined amino acid sequences, to allow better control over material properties and simpler analysis. For this reason, in this dissertation, a new way of designing repetitive proteins will be delineated. A novel protected-digestion of rolling circle amplification (PD-RCA) method will be suggested, and through the PD-RCA, three sets of protein libraries were established. All variants in these libraries were characterized by Fourier transform infrared (FT-IR) spectroscopy, and mechanical analysis (tensile strength in dry protein films and shear modulus in wet protein disks). The mechanical properties of tensile strength and shear modulus can be controlled in tandem-repeat protein designs by varying molecular weights as well as the tie-chain length in repeating units. By designing protein materials with tandem repetition, variation of tie-chain length influences the material properties of a cross-linked network without affecting cross-link coordination number. In other words, longer amorphous chains (tie-chain) in repetitive units decrease the moduli of these materials, but longer amorphous chains do not change the number of strands in each β-sheet. These findings will pave the way to new design rules in protein-materials engineering, especially in repetitive protein designs.