Peptide and protein based materials in 2010: from design and structure to function and application

Why this themed issue?

The past 10 years have seen enormous progress in the design and engineering of protein- and peptide-based (polypeptide-based) functional nanomaterials. As a result, some of these are now temptingly close to applications in energy, nanotechnology and biomedicine. Given this progress and the promise of applications with real societal benefit now on the horizon, it is perfect timing to publish a themed issue on the topic of Peptide and protein based materials in 2010 covering the forefront research on design, synthesis, engineering, characterisation and functionalization of materials wholly or partially composed of peptide or protein components.

Current and next-generation polypeptide-based nanomaterials

Although challenges remain, scientists are now able to design polypeptide-based nanostructures de novo, or engineer naturally derived systems both rationally and with confidence. These studies employ a variety of strategies, and cover a wide range of synthetic through biological materials. Our contributors have done a superb job in covering these many and varied examples, and in conveying the key contemporary issues in the field.

For example, regarding materials adapted, borrowed or inspired from biology (e.g. collagens) Hartgerink (DOI: 10.1039/b919455j) covers model collagens; Weiss (DOI: 10.1039/b919452p) discusses elastins; Zhang & Lu (DOI: 10.1039/b915923c) describe β-sheet-based systems and various approaches to peptide amphiphiles; in another two reviews, Klok (DOI: 10.1039/b914339b) covers α-helix/coiled-coil systems, and Woolfson and Mahmoud (DOI: 10.1039/c0cs00032a) touch on this subject. Whilst still bio-inspired, others describe systems based on entirely new strategies not found in nature, such as the Xu (DOI: 10.1039/b919450a) and Ulijn (DOI: 10.1039/c0cs00035c) reviews of materials based on combining short peptides with aromatic conjugates; and Kros's (DOI: 10.1039/b919446k) review on aliphatic- and polymer-peptide conjugates.

Getting functional

While the quest for sequence-to-structure relationships, or rules, for de novo design continues, the main focus is now towards the development of the next generation, functional polypeptide-based structures. General routes or strategies to decorating, or functionalizing, biomaterials are outlined by Woolfson and Mahmoud (DOI: 10.1039/c0cs00032a). Specific examples of such functionality include responsiveness to applied stimuli, as reviewed by Löwik and van Hest (DOI: 10.1039/b914342b), which is relevant for example to drug delivery. By incorporating functionality into these systems, an effective interface can be created between these soft nanostructures or biological systems to allow for peptide systems that interact with, adapt to, or direct biological processes as relevant to next generation molecular biomaterials as discussed by Collier (DOI: 10.1039/b914337h), and soft nanotechnology covered by Matsui (DOI: 10.1039/b917574c). As described by Ulijn (DOI: 10.1039/c0cs00035c), another level of complexity in this area is to combine self-assembly processes with catalysis, to create responsive molecular networks, which may lay the basis for achieving motility and mechanical force in response to external stimuli.

In addition, hybrid structures, which may incorporate synthetic organic and hard inorganic components are increasingly being developed for example in the context of nanoparticle-based sensing, where the unique size dependant properties of nanoparticles are exploited, see the review by Stevens (DOI: 10.1039/b919461b). Effective production of these hybrid structures will rely on a better fundamental understanding of peptide/nanomaterial interactions as discussed by Naik (DOI: 10.1039/b918035b). Again this area is covered in general terms by Woolfson and Mahmoud (DOI: 10.1039/c0cs00032a).

Understanding what we create

Both at the basic level and as these systems become more sophisticated, analysis and characterisation is challenging but critical if we are to fully understand and exploit what we have created. Characterisation must span the atomic, molecular, supramolecular and bulk regimes. This requires the applications of methods such as X-ray diffraction as covered by Serpell (DOI: 10.1039/b919453n), which is often performed on dried samples; for linear dichroism, see the Rodger (DOI: 10.1039/b912917k) review, which provides opportunities to figure out the precise orientation of functional groups within structures in solution phase; and, as pointed out by Pochan (DOI: 10.1039/b919449p), rheology is important for analysing the mechanical properties of gel phases, especially now that it is becoming increasingly clear that the route of self-assembly plays a major role in dictating properties of the resulting materials.

Supramolecular assembly, and playing catch-up with the DNA community

An exciting aspect of next generation nanostructures is the exploration of supramolecular assembly and function; by the latter, we mean functionality that results from supramolecular assembly, and is not found in the building blocks themselves. This type of functionality may include catalytic activity, as described in the Xu (DOI: 10.1039/b919450a) review, which may lead to new cost effective enzyme mimics and supramolecular (opto-)electronic functions; and maybe of use in interfacing biological with electronic systems, relevant to future sensing or bio-energy devices as outlined by Matsui (DOI: 10.1039/b917574c).

The peptide community often, and quite understandably, regards with envy the progress made by the DNA nanotechnologists in this area of self-assembly. The multi-component assemblies, nanoscale devices, assembly lines and even DNA-based computers that are being considered and delivered by this route are extremely impressive, even awe inspiring. There is good and encouraging evidence that polypeptide-based materials are on the right path to competing in some of these areas, however. Of course, these communities should use the right tool for the job, and tackle target designs using the appropriate approach, be it using DNA- polypeptide-based tectons or both.

Ultimately, for certain applications, polypeptide-based materials and machines may prove more versatile than their DNA counterparts, as peptides and proteins are Nature's functional workhorses; and polypeptide-based materials and devices can, in principle, be fully encoded at the genetic level, opening up exciting applications in biomedicine and bio-inspired energy-related technologies. Indeed, it is evident from this themed issue that peptides are not exclusively seen as useful structural components, but include their use as actuators, surfactants, selective ligands for molecular recognition, nucleation sites for biomineralisation, etc. Ultimately these functionalities can be fully encoded at the genetic level, adding significantly to the toolbox of the synthetic biologist.

The future: towards real-life applications

This themed issue illustrates that the current state of research in polypeptide-based self-assembling systems is buoyant, and its future is bright. Now that the design rules and engineering rules for these are becoming increasingly understood, it is perhaps time to take on new challenges. These will include the further development of multi-component systems, and understanding the dynamics and complexity that come with them. As the field moves increasingly towards these aspects and onto multicomponent functionalised systems, applications in increasingly wide ranging areas will be possible, such as active biomaterials, biosensors and lab-on-a chip devices, bio-inspired light harvesting devices, biomolecular motors and so on.

And finally…

We are delighted with this themed issue, and hope you enjoy reading it and using it. Of course it would not be possible without the enthusiasm, commitment, hard work and skill of our contributors. We are grateful to them all for making this a very special issue. They are world-leading scientists from physics, chemistry, biology, materials science and engineering, and we believe that they have covered the area with authority and clarity. We may, of course, have missed some particular areas, if so we take personal responsibility and apologise. Nonetheless, we hope that this themed issue provides a complete overview of the state of this interdisciplinary field in 2010, and will be used as a source of reference and inspiration for the growing community of peptide materials scientists and engineers. This is an exciting time for polypeptide-based nanomaterials, and we are excited to see what the next decade will bring.

Footnote

† Part of the peptide- and protein-based materials themed issue.

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