Nano Today
ReviewTwo-dimensional peptide based functional nanomaterials
Graphical abstract
Introduction
Two dimensional (2D) nanomaterials are generally associated with inorganic materials such as graphene [[1], [2], [3], [4]], transition metal dichalcogenides (TMD), e.g., MoS2 [[5], [6], [7]], and black phosphorus (BP) [8,9], which have received great attention over the past few years due to their excellent optical, electronic, catalytic [10], and biological properties [11,12]. In recent years, graphene derivatives (e.g. graphene acid) have also been widely used in bio-related applications with good biocompatibility [13]. However, 2D materials are not confined to inorganic structures, instead organic materials form a vast array of important 2D structures, such as 2D polymers (e.g. supramolecular assembly and liquid crystals) [14], lipid bilayers [[15], [16], [17]], DNA self-assembled motifs [18,19] and DNA origami [[20], [21], [22], [23]], as well as protein [24] or polypeptide self-assembled 2D materials [25]. Soft 2D materials are applied in a diversity of applications and many of them possess important biological functions. For instance, lipid bilayers with critical thickness are associated with the elasticity and fluidity of cell membranes [26]. 2D DNA self-assembled motifs and origami structures are utilized as platform for novel biological experiments [27] and for examining the effects of spatial organization [28], electronics on nanodevices [29] and plasmonic circuits through the attachment of nanowires [30], conductive polymers [31] and gold nanostructures [32]. Combined with DNA origami and graphene, graphene pattering (cutting out graphene shapes) can be obtained by the metallized DNA nanolithography for encoding and transferring spatial information on the graphene [33]. 2D protein based-assemblies have been observed and explored in native biological systems, such as the 2D bacterial surface layers [24] (S-layers) that could stabilize membranes, but also be used for structure–function studies on reconstituted integral proteins for biocompatible surfaces preparation, drug targeting and delivery systems as well as for biosensing devices [34]. Although proteins are nature’s most versatile building blocks, compared to similar polymeric molecules such as DNA and RNA, the design of self-assembled proteins into 2D structures is largely inaccessible, plagued by the chemical heterogeneity and multiple interactions between the proteins leading to a lack of control.
Polypeptides are polymers of amino acids linked by peptide bonds. They have the same chemical structure as proteins, but are shorter in length. Oligopeptides, or peptides, are made of small numbers of amino acids, and they are often capable of self-assembling into hierarchical structures with important biological functions. Efforts have been put into achieving ordered nano- and microscale architectures in designed peptide self-assembly, e.g. zero dimensional (0D) nanospheres [[35], [36], [37], [38]], one dimensional (1D) nanofibrils [[39], [40], [41]] and nanotubes [[42], [43], [44]]. Many strategies were utilized to construct the variety of peptide-based nanostructures. For instance, 1D nanofibrils were achieved by self-assembly of amyloid peptides [[45], [46], [47], [48], [49], [50], [51], [52], [53], [54]] or made from ionic self-complementary peptides [37,[55], [56], [57]] and peptide-amphiphiles [40,58,59]. 0D nanospheres were formed by diphenylalanine stacking [36,60,61] or self-assembly of cationic peptides modified with cholesterol as the hydrophobic tail [35]. Furthermore, biodegradable nanocomposites of peptide-assembling fibril/graphene have been constructed for enzyme-sensing with shape-memory properties [62]. The driving force behind the advanced assembled architecture involves weak non-covalent interactions that include hydrogen bonding, and hydrophobic and electrostatic interactions. A number of reviews have introduced and discussed the self-assembled peptide nanomaterial and relative applications mainly associated with 1D or 0D peptide-based nanomaterials [[63], [64], [65], [66]]. However, there has been a lack of review to systematically introduce and present two-dimensional (2D) nanostructure and nanomaterial based peptide self-assembly as well as their relative applications.
Two-dimensional nanostructures and assembled systems are a fundamentally important geometry and link an important physical gap between low dimensional assemblies and three-dimensional structures. In view of the considerable impact on a great many fundamental and applied aspects of biological and material sciences, such as retrovirus carrier, biosensor, the efficient photo-catalyst and the bio-inspired catalytic scaffold, etc, it is therefore significant and valuable to review the recent works related to 2D soft nanomaterials from self-assembled peptides. In recent years, a central goal has been to design novel materials with molecular-level information that can be utilized to direct highly specific intra- and intermolecular interactions promoting self-assembly of thermodynamically stable and structurally defined 2D assemblies of peptides. Moreover, promising 2D nanomaterials based on peptide assembly with excellent functionality can be discovered in several other fields.
In this review, a variety of strategies for constructing 2D nanoscale assemblies with specific interactions of polypeptides in a well-predictable manner are presented. First, we introduce the molecular basis of peptide based nanostructures and nanomaterials by considering the structure of amino acids and the secondary structures of peptides, which form the essence and basic building blocks for construction of any peptide assembled nanostructure. Sequentially, the controlled fabrications of two-dimensional assemblies on the nanoscale are presented. It is summarized how different 2D nanoscale assemblies can be achieved by the self-assembly of peptides with distinct secondary structure, illustrated by several instances from α-helix, triple-helix, β-strand, etc. In the following section, peptide nanofibril arrays and other assemblies are introduced, which can further assemble into macrofilms considered as a 2D material based peptide assembly on the macroscale. These bottom-up strategies can arrange polypeptides and proteins into well-ordered 2D structures, which can have wide-ranging applications, such as membrane-based separations with specific mechanical properties, the control of surface properties in nanodevice and nanosensor fabrication, and retroviral transduction in biology, etc. We present the possible strategies for constructing the promising 2D soft nanomaterials by peptide self-assembly and discuss the mechanism of molecular interaction of peptides as the key to the creation of 2D peptide assembly. This will help shed light on the development of biomaterials and the future applications in nanotechnology and relative fields of 2D soft nanomaterials.
Section snippets
Molecular basis of peptide based 2D nanomaterials
Peptides are composed of amino acids [67] that can self-assemble into supramolecular structures with a variety of shapes and dimensions. Structural insight in the diversity of nanostructures originates from the basic primary structure and secondary structure of peptides. The primary structure is the sequence of peptides (Scheme 1a), where the amino acids can be classified according to their different properties, e.g.: aliphatic or aromatic, neutral, negative or positive charge etc, which can
2D nanomaterial self-assembled from peptides with distinct secondary structures
In the last section we introduced the secondary structures of peptide and protein, which are essential for peptide self-assembly into the variety of nanostructures and nanomaterials. In this section, we will first introduce the characterization of 2D peptide based nanostructure and then present strategies for constructing 2D nanostructures and nanomaterials based on the different secondary structure of the peptides.
2D material on the macroscale built from peptide assembly
In the last section, we introduced ways to construct 2D architectures by peptide self-assembly with the variety of secondary structures of peptides. Herein, we will introduce strategies to make macrofilms by hierarchical self-assembly of proteins. The hierarchical self-assembly is considered to be a powerful tool for creating the nanostructured materials. In this strategy, the macrofilm is generally considered to be constructed in a two-step process, i) peptides with β-sheet conformation
Applications of 2D nanostructural materials fabricated from self-assembled polypeptides
Peptides and proteins can be self-assembled into 2D nanostructure and nanomaterial such as nanosheet and film-like structure via a variety of strategies. These bottom-up strategies can arrange polypeptides and proteins into well-ordered 2D structures, which can have wide-ranging applications, not only due to shape effects, such as membrane-based separations with specific mechanical properties, but also due to the way specific amino acids can be integrated in the designed peptides. In the
Conclusion and perspective
The cases presented in this review address the role of self-assembled polypeptides in building up 2D nanostructures and 2D nanomaterials. The molecular structure of the peptides was investigated and discussed with the goal of constructing peptide nanostructure and nanomaterial. The role of the secondary structure of peptides such as α-helix, triple helix, β-sheet, etc, was highlighted and have been summarized in Table 1. Several strategies based on distinct secondary structures of peptides were
Acknowledgements
This research was supported by grants from the Danish National Research Foundation, AUFF-NOVA project from Aarhus Universitets Forsknings fond, EU H2020RISE 2016-MNR4SCell project, and the National Natural Science Foundation of China (No. 51503087, 21573097).
Dr. Lei Liu is a Professor and deputy director at Institute for Advanced Material, School of Material Science and Engineering, Jiangsu University, China. He received his Master’s degree and Ph.D in National Center for Nanoscience and Technology, China in 2007 and 2010. He worked in iNANO, Aarhus University, Denmark as a postdoc from 2010-2013. His research interest is peptide self-assembly, peptide based nanomaterial and their relative biomedical applications.
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Dr. Lei Liu is a Professor and deputy director at Institute for Advanced Material, School of Material Science and Engineering, Jiangsu University, China. He received his Master’s degree and Ph.D in National Center for Nanoscience and Technology, China in 2007 and 2010. He worked in iNANO, Aarhus University, Denmark as a postdoc from 2010-2013. His research interest is peptide self-assembly, peptide based nanomaterial and their relative biomedical applications.
Lasse Hyldgaard Klausen received his Ph.D. degree in nanoscience from Aarhus University, Denmark in 2016. His research interests are focused on the nano-bio interface, especially through the use of novel scanning probe microscopy techniques for providing a fundamental understanding of physicochemical interactions. He is currently a postdoc at the Department of Chemistry, Stanford University, USA.
Mingdong Dong received his Ph.D from Aarhus University, Denmark in 2006, and performed his postdoctoral research in Harvard University. Then he started to work as assistant professor and associate professor in the Interdisciplinary Nanoscience Center, Aarhus University. Currently, he is the group leader of the Bio-SPM group. His research is focused on both the implementation and further development of novel scanning probe microscope techniques for studying new functional materials.