light-responsive self-assembly towards supramolecular machines
Supaporn Kwangmettatam is a PhD student in the research group Biomolecular Nanotechnology. Her supervisor is prof.dr. N.H. Katsonis from the faculty of Science and Technology.
Incorporating artificial molecular motors and switches into self-assembled systems allows transferring and amplifying the forces that these molecules generate, across increasing length scales. Interfacing the mechanics of such molecules with larger self-assembled architectures represents a viable strategy to fight against overwhelming Brownian storm and viscosity of liquid environment. Thus the focus of my research was on developing controllable light-responsive supramolecular systems by integrating molecular photoswitches into self-assembled architectures. Photo-switching was selected as an ideal strategy to alternate between assembling and non-assembling isomers reversibly, repeatedly, with an exquisite control over the timescale of the process, without accumulating chemical waste and in combination with spontaneous reversed switching. More specifically, the photo-switching was used to mediate the transformation of building blocks, from a form that is not compatible with self-assembly and thus suppresses self-assembly, into a photo-generated isomer that participates into the self-assembly because the interactions it promotes are compatible with the desired architecture. The artificial molecular photoswitches involved in this work were azobenzenes and spiropyrans. Upon irradiation with light, azobenzenes offer geometrical changes between cis and trans isomers, whereas spiropyrans provide large differences in their charged character.
Chapter 1 provides a general introduction to this thesis.
Chapter 2 reviews recent progress in the design and synthesis of supramolecular tubular systems. Our motivation to work with tubular systems lies in the versatility displayed by the supramolecular tubes that operate in the cell. Cellular microtubules, “the muscles of the cell”, are indeed supramolecular machines that produce directional forces through a continuous influx of energy: they pull chromosomes apart through catastrophic disassembly, and shape shift cells as they grow using chemical energy. Microtubules are characterized by a persistence length of over five microns – in other words, a microtubule is rigid over cellular dimensions, which mostly derives from its large cross section. The propensity to produce forces efficiently and in the right temperature regime, with control over directionality, is thus inherently encoded into the tubular structure: they are in essence supramolecular cylinders that are hollow and stiff, and that are assembled from molecules that undergo chemically fueled conformational switching.
In Chapter 3, we show how strain builds up in wholly artificial tubules, upon trans-to-cis photo-switching. The light-fueled accumulation of the strain eventually leads to the catastrophic burst of the tubules, in a mechanism that is reminiscent of the disassembly mechanism of cellular microtubules. From an energetic point of few, such processes will likely by key to the development of artificial de-polymerization machines.
In Chapter 4, we demonstrate a strategy that allows connecting the tubules discussed in chapter 3, with either surfaces, or with each other. In this strategy, biotinylated building blocks act as interfacing agents that incorporate into the original tubules without causing structural deformations. The mixed tubules, composed of both the biotinylated and the original building blocks, show a propensity to bundle when the biotin moiety binds with streptavidin, a protein that can be either added in solution, or used to functionalize surfaces. Ultimately, we anticipate that the strategy outlined in this chapter will allow i) amplifying the action of individual tubules into bundles and ii) transferring the forces generated by the growing/disassembling tubules to other objects.
In chapters 5 and 6, the dynamic molecule mediating the conversion of light into mechanical operation is a spiropyran. In contrast to azobenzenes, spiropyrans do not undergo large geometrical changes upon irradiation with light, however, they switch from a non-charged form (the spiropyran) to a charged zwitterionic form (the merocyanine), which paves the way towards alternative strategies to achieve dynamic molecular control over self-assembled architectures.
In Chapter 5, we have synthesized a spiropyran-based amphiphile that self-assembles into vesicles, in water. Upon irradiation with light, merocyanine converts into the spiropyran, which leads to the transient and reversible expansion of the vesicles. The original mechanism for this breathing motion involves self-limiting growth of the merocyanine-containing building blocks. In future work, we envision that mixing the switchable building blocks reported here, with a range of flexible amphiphilic building blocks, can lead to the formation of phase-segregated areas in hybrid vesicles. In these artificial formations, the presence of localized deformations and of differential surface stiffness is likely to mediate complex shape transformations, under irradiation with light. Overall, in this chapter, we demonstrate that dynamic aggregates based on spiropyrans hold potential as adaptive compartmentalized nanosystems.
While chapter 5 focuses on vesicular dynamic architectures, in Chapter 6, we present efforts towards combining tubes with spiropyran switches, because they would promote tubular growth upon irradiation with light, when the polar protonated merocyanine converts into the spiropyran. In order to optimize the likeliness of p-p stacking in the spiropyran form, the planarity of the core building blocks was enhanced by adding two benzene rings into the molecular design. While supramolecular aggregates were indeed formed upon irradiation with light, we could not find evidence supporting the light-fuelled formation of tubes.
Overall, we have introduced a framework for the development of light-controllable nanosystems in water, with a dynamic behavior ranging from assembly, to disassembly and transient growth. Moreover, we have taken the formative steps to harness the mechanically-purposeful operation of dynamic supramolecular tubules, as to generate measurable forces from the nanoscale, and ultimately establish operational principles for chemo-mechanical transduction in supramolecular systems. The inspiring versatility displayed cellular microtubules sets the stage for the potential of this research.
