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MOLECULAR self-assembly, ubiquitous in nature, is emerging as a force to be

reckoned with not only in biology. Put simply, the process takes a disorganised system of components and, using the components’ inherent properties and interactions between them, forms an organised structure – all by itself. A better understanding of how these mechanisms work in nature has sparked exciting developments in other areas such as chemical synthesis, engineering,

Drugs, diagnosis and electronics. Molecular self-assembly is not just for biologists, says Tan Huey Ling

Molecules and the art of self-assembly

nanotechnology, polymer science, and materials science.

a ‘growing’ interestScientists across many disciplines have become interested in molecular self-assembly because it is key to understanding biology and a variety of diseases at the molecular level. In the last few years, advances in the use of peptides (the ‘building blocks’ of proteins) have enabled the production of biological materials for a wide range of applications, including novel supra-molecular structures

These systems are attractive because they can build uniform, functional units or arrays which can be exploited at meso- and macroscopic scale for both lifescience and non-lifescience applications (such as building nanowires and high-energy-density batteries).

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and scaffolding for tissue repair. Today, the study of biological self-assembly systems is a rapidly-growing field that truly crosses the boundaries of existing disciplines. Self-assembling systems range from bi- and tri-block copolymers to complex DNA structures as well as simple and complex proteins and peptides. These systems are attractive because they can build uniform, functional units which can be used at meso- and macroscopic scale for both lifescience and non-lifescience applications such as building nanowires and high-energy-density batteries.

oligopeptides – the new ‘Lego’?A new class of oligopeptide-based biological materials was accidentally

discovered by Shuguang Zhang of Massachusetts

Institute of Technology (MIT). Oligopeptides

(peptides containing a small number of amino acids) are short, simple to design, extremely versatile, and

easy to synthesise. They have provided

insight into the chemical and structural

principles of peptide self-assembly.

There are three types of self-assembling peptide systems.• Type I peptides form β–sheet structures (a secondary ‘pleated’ structure of protein) in aqueous solution. Put simply, you could visualise these as ‘Lego bricks’, with pegs and holes, that can only be assembled into particular structures. They consist of alternating hydrophilic and hydrophobic amino acid residues, which allow them to form complementary ionic pairs within each chain and/or between different chains. While they have some of the common features of uncharged peptides, such as hydrophobicity and hydrogen bonding, they also have unique charge properties that can control how they aggregate. Adding monovalent alkaline cations or introducing the peptide solutions into physiological media causes these oligopeptides to spontaneously assemble into macroscopic structures which can be fabricated into various geometric shapes. Such structures could be used to make new biologically-compatible scaffolds for controlled drug release, tissue repair, and tissue engineering.• Type II peptides can act as ‘molecular switches’ because they are made to self-assemble and disassemble by varying conditions such as pH, temperature, or crystal lattice packing. They’ve been studied extensively because it’s hoped that better understanding of the interactions between these proteins will help us understand the mechanisms and causes of some protein conformational diseases, including scrapie, Huntington’s, Parkinson’s, and Alzheimer’s. • Type III peptides, like molecular ‘hook-and-loop’ fasteners, self-assemble onto surfaces (rather than with each other) to form monolayers. These oligopeptides are useful as they allow a variety of other molecules or specific cells to attach to their end functional groups, and can thus be used in testing and detection. The chemical groups on the peptide can also react with a surface anchor to form covalent bonds. As such, biological surface engineering

Figure 1 (right): At the University of Michigan, scientists made biodegradable polymers that could self-assemble into hollow, nanofibre spheres. These spheres can be filled with cells and injected into wounds to form a support structure for the growing cells. Once the cells are held in place, the spheres will dissolved harmlessly.

University of Michigan

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is an emerging technology that will provide new methods to study cell-to-cell communication and cell behaviour for tissue repair, immunity, and normal tissue homeostasis.

new ways of working for drugsMany proteins and peptides are being developed for use as drugs, but they can’t be administered orally, because they would not survive the journey through the digestive tract. This means they have to be injected, which limits their use. Macromolecular self-assembly could provide a solution by engineering delivery vehicles that make the drug both more effective and easier to take. It’s possible to create a self-assembling material for encapsulation and drug delivery that has all the advantages of the conventional cross-linked materials normally used for controlled release. These materials can be designed to self-assemble spontaneously (or in response to an external stimulus) and be tailored to have specific properties set by the monomer they are made of (see Figure 1).

In some cases it’s also possible to form the drug’s active pharmaceutical ingredient when self-assembly is triggered, which allows precise control of the concentration of the active pharmaceutical ingredient within the carrier – and this can be a real advantage.

There are benefits to be had for drugs delivered via blood vessels too – the delivery vehicle can increase the drug’s circulation half-life (a measure of how long it stays in the body), and in some cases, target it at a desired tissue. Better delivery and targeting could lead to reductions in dose concentrations and frequency of administration, which in turn could reduce side effects.

To date, molecular self-assembly has mostly been used to create drug delivery vehicles – chiefly micelles and vesicles made from lipids and polymers. However, we can also use self-assembly to build other structures such as tubules, fibrils, or complex systems such as microspheres and molecular hydrogels (see Figure 2 for examples of some common self-assembling monomers).

Figure 2: common self assembling monomersCommon self-assembling monomers include lipids, block copolymers, peptides and proteins. Intermolecular interactions that drive and define self-assembly include hydrophobic association and the formation of polar interactions, respectively. The resultant structures formed through self-assembly are shown. The hydrophilic portions are coloured blue and hydrophobic portions orange.

Monomers

Lipid

Block copolymer

Peptide/protein

Helix

SheetHydrophobic interactions Fibrillar networks

Lamellar sheets

Vesicle

Micellar disk

Spherical micelle

Wormlike micelle

Vesicular tube

Electrostatic interactions

Hydrogen bonding

Molecular interactions Self-assembled structures

H

H3N

H3C

H3C

H3C

H3C

H3C

H3CCH3

CH3

CH3

O

O- +

NOH

H

H

H

H

H

Self-assembly offers endless opportunities and has a significant future in a long list of potential applications: micro/optoelectronics, catalysis, energy/magnetic storage, biotechnology, and novel materials which adapt according to their environment, to name a few.

Source: M

onica C. B

ranco and Joel P. S

chneider, U

niversity of Delw

are, New

ark.

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testing testingSelf-assembly is also proving its worth as a practical tool in diagnostics, where biological materials are detected and quantified using techniques such as immunological recognition assays, enzymatic reactions, and DNA- or RNA-based technology. Diagnostic immunoassays include products such as home pregnancy testing kits that contain antibodies which detect minute traces of the human chorionic gonadotropin (hCG) hormone. Other products include diagnostic kits for HIV or hepatitis virus infections. There are high hopes that using nano-scale assemblies and fabrication could significantly improve the sensitivity as well as the specificity of the diagnosis process. Through better understanding and by miniaturising the detectors and molecular markers in such kits, it should be possible to make them more sensitive and efficient. This means that tests for key biological parameters such as glucose levels could be done on smaller blood samples, for example.

bugs to electronicsAnother and somewhat surprising key research direction involves using peptide and hybrid–peptide (a peptide-inorganic material macromolecule) building blocks to make metallic nanowires. This would make it much easier to synthesise and modify large quantities of these simple building blocks. We already know that various types of peptide nanotubes can form 1D metal assemblies (eg glycylglycine bolaamphiphile peptide nanotubes and peptide-amphiphile nanofibres). These fibres are formed by self-assembly of hydrophilic peptides that are joined to a hydrophobic aliphatic tail. More complex assemblies such as bacteriophages and viruses can also be used as an organic–inorganic ‘template’. These viruses, self-assembled at the nanoscale, are very effective as seamless templates for making various inorganic materials. Filamentous bacteriophages are particularly effective, as they contain various protein motifs (including single-chain antibodies) on their surface, a technique known as ‘phage display’. This technique, which is widely used for selecting various peptide-binding motifs (cataloguing signals, that direct the recombinant proteins to incorporate on the bacterial cell surface), has been used to select peptide motifs that can bind various inorganic metallic and semiconductive nano-particles. These phages can then be aligned to form macroscopic metal or semiconductive wires. Such wires were recently used in a demonstration to produce electrodes for thin lithium-ion batteries. By binding gold

Figure 3: Northwestern University researchers recently made a breakthrough when they demonstrated the ability to cause nanorods made from gold and polymers to self-assemble into complex shapes, including this sphere.

Figure 4: Scientists at HP Labs have produced a conductive wire 10 atoms wide by vapourising erbium onto a silicon surface. Such ‘grown’ wires could become the basis of the crossbar architecture that HP has advocated as an alternative method of making semiconductors.

Chad

Mirkin, N

orthwestern U

niversityC

had M

irkin, Northw

estern University

to the viruses and then reducing the cobalt ions, researchers at Northwestern University in the US created composite wires that contained both cobalt oxide and gold. Because these wires have very good specific capacity, they make superb electrodes for very energy-dense batteries (see Figure 3).

s-layers to semi-conductorsNano-structures of ordered ‘s-layers’ (bacterial surface layer proteins) could be used in nanolithography, a technique used to make semiconductor integrated circuits (see Figure 4). Purified s-layer building blocks spontaneously reassemble into well-ordered 2D crystals under in vitro conditions. This property has been used to show that it’s possible to recrystallise s-layer sub-units on various substrates that are suitable for nanofabrication, such as silicon or silicone oxide wafers. Bio-mimetic surfaces built with s-layers are stable even when exposed to strong solvents or extreme temperatures. However, a much more controlled and specific way of making highly ordered nano-patterned affinity matrices is to use genetic construction methods to tune the functional and structural features of s-layer fusion proteins. Diagnostic tools, vaccines, or biocompatible surfaces, as well as specific bio-mineralisation strategies have all been developed in this way.

high hopes, big challengesSelf-assembly offers endless opportunities and has a significant future in a long list of potential applications such as micro/optoelectronics, catalysis, energy/magnetic storage, biotechnology, and novel materials which adapt according to their environment, to name a few. Not least of these is the prospect of self-assembling ‘nano-machines’. While these are nothing out of the ordinary in nature (eg functional molecular components of living cells) the challenge is to mimic the idea and use it in practicable technology. An obvious, life-changing example would be nano-machines which could seek out and destroy cancerous cells in the body, or detect defects and blockages in organs or blood vessels. While recent studies have brought a degree of understanding of peptide self-assembly at the molecular level, our biggest challenge now lies in figuring out how we mimic this process systematically and consistently. When we can do this, the possibilities are endless. tce

Tan Huey Ling (hueyling@salam.uitm.edu.my) is an academic researcher in the chemical engineering faculty at Malaysia’s Universiti Teknologi MARA