Edited by Gang Wei and Sangamesh G. Kumbar
Foreword
I am delighted to write the foreword for Artificial Protein and Peptide Nanofibers edited by Drs. Wei and Kumbar. This book presents a fine summary of the present status of designer peptides and proteins, their structures and properties, and their associated nanofibers for biomedical applications. This book will be extremely useful as a reference source for all those working in the areas of proteins/peptides, polymer chemistry/physics, biomaterials, tissue engineering, drug delivery, and regenerative medicine. Proteins and peptides of both synthetic and natural origin have been widely used as biomaterials for a variety of biomedical applications and have greatly impacted the advancement of modern medicine. These polymeric materials have the ability to self-assemble as fiber matrices that closely mimic the native architecture of the extracellular matrix. Synthetic peptides are designed to mimic biological patterns either to study biological systems or to influence tissue healing responses. They offer controllable physicochemical properties needed for certain biomedical applications including sensors, imaging, drug delivery, targeted therapies, tissue engineering, and electronic devices. Particular emphasis is made here on the procedures to synthesize protein and peptide nanofibers via molecular self-assembly, supramolecular chemistry, electrospinning, templates, and enzymatic synthesis. New proteins/peptides, as well as modifications to existing peptides, are continually being developed and applied to meet new challenges of biomedical applications. Nanofiber matrices in the form of implants, coatings, scaffolds, and drug delivery devices have been created to manipulate cell-material interactions and promote tissue regeneration, as well as for biosensing, imaging, and diagnostics. This intensive literature review on these topics is covered in 18 chapters written by experts in their fields from different parts of the world who present excellent overviews that will be useful for a wide audience. In my opinion, this book will be an ideal resource for those working in materials science, polymer science, chemical engineering, nanotechnology, and biomedicine. I believe this textbook will be a part of homes, libraries, and classrooms throughout the world.
Supramolecular self-assembly: A 1 facile way to fabricate protein and peptide nanomaterials
Luyang Zhaoa and Xuehai Yana,b
aState Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China, bCenter for Mesoscience, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China
1.1 Introduction
Supramolecular self-assembly, which has its roots in biology, plays a vital role in the construction of natural materials with various fascinating properties and functions. It bridges the starting building blocks and the final complex systems through a bottom- up approach with delicately tuned intermolecular interactions [1–3]. As typical examples, light-harvesting pigments in the chlorosome self-assemble into large supramolecular systems, which efficiently transfers energy to the reaction center by pigment-pigment interactions [4]. In the calicivirus, the contiguous shell is constructed by 180 copies of a protein molecule via finely tuned hydrophobic interactions [5]. These nature-inspired or nature-derived self-assembly protocols are highly promising for the future development of functional supramolecular materials. However, natural materials have hierarchical architecture and complicated functions, while artificial materials are comparatively simple in supramolecular structures with confined functionalities [6]. How to construct desired supramolecular structures with abundant functions using simple self-assembly units, thus, remains one of the great challenges.
Proteins and peptides are a class of amino acids-composed natural compounds with definite molecular structures and unique functions in biological processes. Proteins possess sophisticated stereo-structures, acting as ideal building blocks for organisms with a broad range of functions. In comparison, holding the abilities of easy manipulation and synthesis, peptides with specifically designed structures show superior self-assembly properties [7–12]. The self-assembling peptides as building blocks enable diverse supramolecular structures such as nanotubes [13, 14], nanofibrils [15, 16], nanobelts [17], and hydrogels [18–20], which show great functionalities for a broad range of applications. In addition to their individual self-assembly, proteins and peptides may induce the assembly of functional species as templates, resulting in not only homogeneous nanostructures (nanotubes, nanofibrils, etc.), but even more complexed hierarchical structures [21–23]. The unique advantage of protein/peptide-involved supramolecular assembly lies in their expanded and improved properties that their individual components do not possess [24–26]. Consequently, proteins and peptides are ideal species for constructing functional nanomaterials.
In this chapter, we focus on self-assembling protein- and peptide-modulated nanomaterials in functional applications, mainly including biological materials, biomimetic photo-catalytic systems for energy use, and semiconductive materials. In addition, the supramolecular assembly mechanisms are clarified at first for their substantial importance in bridging the structures and functions between starting building blocks and supramolecular products.
1.2 Protein and peptide assembly mechanism
Elucidating the protein and peptide assembly mechanism, especially the complicated noncovalent intermolecular interactions, is highly important because they determine the final assembly structure, morphology, stability, and functionalities [27, 28]. The mechanism complexity lies on two aspects. Firstly, the decisive noncovalent interactions include electrostatic interaction, hydrogen-bonding, π-π interaction, van der Waals interaction, and hydrophobic interaction. They are not independent, but have latent influences with each other and usually show synergistic or competitive effects in the self-assembly [29–31]. Secondly, the self-assembled architectures are not only dependent on the chemical structure of the starting building block, but on various thermodynamic or kinetic conditions during the assembly process, such as environmental temperature, pH, ionic strength, reaction rate, external electric or magnetic field, and the existence of nucleation seed [6, 32–38]. This means one set of starting building blocks can form different nanostructures with distinctive properties [39–42].
1.2.1 Protein self-assembly
The inherent complex structure of the protein itself results in its intermolecular interactions encompassing all possible forms as listed above. These weak interactions may distribute on different sites of the protein molecule surface depending on the surficial functional groups. As a consequence, the self-assembly of protein is rather a synergistic effect driven by several noncovalent interactions than merely one dominant interaction.
Therefore, it is not suitable to classify the protein self-assembly by the dominant intermolecular interactions. Instead, the various protein self-assembly processes may be distinguished by different experimental strategies, because protein self-assembly is severely dependent on the environmental conditions [43–45]. These experimental strategies include host-guest interaction, ligand-receptor interaction, template-assisted assembly, covalent-conjugation, and coordination-driven assembly. While there have been several review works elucidating the mechanism of protein assembly [43, 46], here only a few typical strategies are represented to show the unique features of protein self-assembly.
One of the typical and efficient biological self-assembly strategies is the symmetric fusion-based self-assembly. Inspired by many proteins that can self-assemble into larger highly symmetrical architectures [47, 48], the symmetry-based protein self-assembly is available by creating fusions of known protein capsids that already have intrinsic subunit-subunit interaction interfaces [49]. Yeates and coworkers reported that dimeric M1 matrix protein and trimeric bromoperoxidase as two natural oligomeric domains could combine together by genetic fusion in a predetermined orientation with a semirigid helical linker. The protein building blocks self-assembled into a tetrahedral cage containing 12 subunits and holding specific symmetries of 3-fold and 2-fold rotational axis from each oligomeric species [50]. In order to acquire a more delicate self-assembled architecture that could be resolved at atomic level, a series of mutation was further conducted to avoid potential flaw that might cause heterogeneity. Guided by computational design, triple mutant was resulted, and the intermolecular interactions and steric hindrance were modified, leading to a successful crystallization of a homogeneous 12-subunit 16-nm protein cage [51]. In the symmetric fusion-based self-assembly, the driving force of the assembly lies on the strong tendency of each assembly unit to recognize other units, which could be either inherent or externally modified, and an axis that renders constructing a specific geometric pattern.
In comparison, chemical assembly strategies of protein self-assembly are featured by the induction of relatively concrete noncovalent interactions as driving forces [52]. The assembly, especially, can be controlled by the strength, directionality, and selectivity of metal-ligand interactions, where the coordination between metal ion and electronegative ligand is stronger than all noncovalent interactions and has explicit directionality. Tezcan and coworkers reported that a 4-helix bundle heme protein, Cytochrome cb562 (cyt cb562), could be constructed to a 16-helix architecture through Zn2 + mediation [53]. The C2-symmetrical interface of cb562 provided a structural point on which two bis-histidine moieties were incorporated to produce the variant (MBPC-1) for selectively binding metal ions. The key role of metal ion coordination for protein self-assembly was revealed by sedimentation velocity and crystallography analysis, where Zn(II) ions stabilized the tetrameric assembly with two pair of V-shaped MBPC-1 wedged into one another (Fig. 1.1).