Team says approach provides insights into complexities of hierarchical assembly in nature.

Scientists have developed a physical process that allows a harmless virus to mimic the way single macromolecules such as collagen self-assemble into diverse and complex functional chiral structures. Researchers at the University of Berkley, California and the Lawrence Berkeley National Laboratory found that the bacteriophage M13 can be used as a self-templating building block that is capable of self-assembling into structures with complex hierarchical organization.

The approach essentially involves dipping a glass support into a solution of virus particles and then slowly drawing it out again. The virus particles adhere to and self-assemble on a glass support, but how they fit themselves together depends on parameters such as virus concentration in solution and the speed at which the glass is pulled out.

Reporting their results in Nature, Seung-Wuk Lee, Ph.D., and colleagues, say that by controlling these two variables they were able to generate three types of hierarchically organized thin-film assemblies including highly complex  ‘ramen-noodle-like’ structures that could bend light in a manner not before observed in nature or in other engineered materials.

Moreover, by engineering the virus particles to express peptides that mediate the growth of soft and hard tissues, the viral films could be used as scaffolds for tissue growth, suggesting a very real potential for applications in regenerative biomedicine. The Berkeley team’s work is described in a paper titled “Biomimetic self-templating supramolecular structures.”

While the helical structure of the collagen macromolecule is thought to be crucial in the ability of its fibers to form disparate structures under defined physical cues, studying the mechanics of collagen assembly has been hampered by an inability to engineer its chemical and physical properties.

Dr. Lee et al. turned their attention instead to investigating whether the E. coli bacteriophage M13 could be prompted to self-assemble into disparate structures dependent on just the alteration of physical cues. M13 was selected as a building block for self-templating because the virus has a helical, nanofibrous shape akin to collagen and is also able to display multiple functional motifs.

Their approach to generating hierarchically structured virus involved allowing the virus to self-assemble into a thin film on a glass slide. The process essentially involved dipping a glass substrate into a bath of virus in saline and then pulling it out again at precisely defined speeds. Varying the speed at which the support was pulled out and changing the virus concentration allowed the researchers to control the liquid’s viscosity, surface tension, and rate of evaporation during the film growth process.

The interplay between all these parameters determined how the virus particles assembled. At low concentrations the phage particles formed nemetically ordered bundles of fibres. Periodic changes in particle flux to the meniscus due to stick-slip motion meant that these were arranged on the glass support as regularly spaced, alternating ridge (stick region) and groove (slip region) stripes lying perpendicular to the pulling direction.

As the virus solution concentration and pulling speed were increased, the self-assembly became more complex, resulting in the bundles within each ridge first taking on a helical structure and then as the concentration was increased further, a flat, helical ribbon-like structure. At the highest virus concentration the deposited films formed periodic structures with even more levels of hierarchical order and formed what the researchers describe as ‘ramen-noodle-like’ structures, which they called smectic helicoidal nanofilaments (SHNs).

All the self-templated structures were tunable by varying parameters that affected the kinetics and thermodynamics of assembly such as phage concentration, pulling speed, ionic concentration, phage surface chemistry, and substrate surface properties. For example, the researchers note, the width, height, and interspacing of the stick-slip patterns increased with increasing phage concentration, whereas increasing the pulling speed caused the height and interspacing to decrease.

Of particular interest was the finding that the self-templated structures exhibited very different polarizing and other optical properties. Depending on the hierarchical organization of the filaments, some displayed iridescence, and others were colored, which could be attributed to the same type of light scattering that results in the bright skin coloration seen in some mammals such as blue-faced monkeys.

Because hierarchically organized extracellular matrices act as templates that provide the physical and biochemical cues for controlling the formation of soft and hard tissues, the researchers looked to see whether their virus-based materials could have similar properties. They first engineered the phage particles to display peptides that promote cell adhesion and bone mineral formation and then generated fabricated films having alternating SHN and nematic hierarchical structures as tissue-guiding matrices.

They found that cells cultured on the films recognized the underlying microstructures and their bodies and internal actin filament networks parallel to the long axes of the phage fibres on nematic regions but perpendicular to them on SHN regions.

When SHN films were then treated with a solution containing calcium and phosphate ions, they acted as templates for the biomineralization of calcium phosphate, resulting in the production of tooth-enamel-like organic-inorganic composites. “Thus we recapitulated aspects of tissue formation by using self-templated materials composed of genetically engineered phages displaying specific functional motifs,” the authors state.

They claim their work has demonstrated that self-templating assembly strategies that occur in nature can be mimicked using a synthetic system to control the growth of phage-based films. “Our system provides insight into how environmental factors control the kinetics during the conversion of helical macromolecules into higher-ordered structures in nature and how these same factors can be used to control assembly in the laboratory,” the team adds. “Further development and tuning of synthetic self-templating systems hold promise for the development of advanced structural, optical, and biomedical materials.”

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