Cardona, Maria (2018) Responsive Self-Assembly as a Tool for Controlling Chemical Reactivity. [Ph.D. thesis]
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Abstract (italian or english)
Nature uses self-assembly to construct structures and to carry out a variety of functions which are essential to life. The cell membrane is an impressive example of a self-assembled structure, providing a confined space where chemical reactions take place in a microenvironment that is different from the outer aqueous environment. This has proven to be inspirational for chemists, who have harnessed the power of self-assembly processes to construct complex structures which are able to perform advanced functions such as catalysis and sensing. Traditionally, self-assembly processes have been carried out under thermodynamic or kinetic control. Yet, Nature uses dissipative self-assembly to activate functions for a limited period of time. In such systems, a continuous supply of energy is required in order to maintain a functional assembled state. For example, microtubule formation, an essential process for cell division is dependent on chemical energy from GTP to maintain an assembled state. This awareness has led to a recent shift in focus to synthetic self-assembly processes that operate far-from-equilibrium. In a similar way to what occurs in Nature, this provides temporal control over the functions associated to the self-assembled state. In this dissertation self-assembly under dissipative conditions is used as a control element to influence the outcome of chemical reactions.
In the first part, a self-assembled system which operates under dissipative conditions was developed. The surfactant C12TACN•Zn2+ did not aggregate spontaneously up to mM concentrations, but self-assembled into vesicles at low micromolar concentrations in the presence of ATP, which acted as chemical fuel. Assembly formation required the presence of chemical fuel and the enzymatic hydrolysis of ATP by alkaline phosphatase led therefore to spontaneous deaggregation. The addition of ATP in the presence of enzyme therefore led to assembly formation, but only for a limited amount of time. The cycle could be repeated upon the addition of a fresh batch of ATP.
The assembled vesicles were then used as a chemical nanoreactor for the promotion of a hydrazone bond-formation reaction. The reaction between the hydrophobic reagents trans-cinnamaldehyde and 3-hydroxy-2-naphthoic hydrazide was very slow in aqueous buffer and in the unassembled C12TACN•Zn2+ surfactant. However, a strong enhancement in the reaction was observed upon addition of ATP. The vesicles presented a more hydrophobic environment than the surrounding aqueous medium, and this was conducive to the otherwise unfavourable reaction. The observation that rate enhancement is associated to the assembled state implies that temporal control over the assembly obtained by adding ATP can be used a tool to control chemical reactivity.
The responsive self-assembly of the surfactant C12TACN•Zn2+ was then used for the selective formation of hydrazone-bonds. A small reaction mixture was created in which two aldehydes (2-pyridinecarboxylaldehyde and trans-cinnamaldehyde) competed to form a hydrazone with 3-hydroxy-2-naphthoic hydrazide. In the presence of just C12TACN•Zn2+ it was observed that the reaction involving 2-pyridinecarboxylaldehyde proceeded at a higher rate. However, the addition of ATP resulted in vesicle formation, which led to a significant enhancement of the reaction between trans-cinnamaldehyde and 3-hydroxy-2-naphthoic hydrazide. Over time, the product composition changed so that the latter hydrazone was present in greater quantities compared to the first one. Because of time limitations, the reactivity response could not be studied under dissipative conditions yet. However, it is expected that the addition of a temporal dimension to the self-assembly process will lead to unprecedented kinetic profiles.
In a separate study - using monolayer protected gold nanoparticles as a model system - it was investigated whether the TACN•Zn2+ complexes played a role in the activation of ATP for enzymatic cleavage. Participation of the building blocks in the fuel-to-waste conversion process is of key importance for designing dissipative self-assembly schemes. Using 31P NMR studies it was shown that the TACN•Zn2+ complexes in the monolayer, but not isolated ones, accelerated the hydrolysis of ATP by potato apyrase. This implied that the Au NPs played an important role in the dissipation of energy, which brings the system closer to natural ones.
The principles of self-assembly under dissipative conditions were then used to construct another system based on Au NPs containing TACN•Zn2+ head groups and negatively charged cyclodextrin vesicles bearing carboxylate groups. Mixing the systems together led to aggregation as a result of electrostatic interactions. Addition of ATP bearing phosphate groups, however, led to deaggregation, caused by the preferential binding of ATP to the nanoparticles. A transient aggregation process could be installed by adding ATP in the presence of the enzyme potato apyrase. The cycle could be repeated upon the addition of a new batch of ATP. This transient aggregate was then used to obtain temporal control over the nucleophilic aromatic substitution reaction between NBD-chloride and octane-thiol.
The same system could also be made responsive to light by introducing a light-sensitive co-factor. A carboxylate-substituted arylpyrazole was used as guest with a higher affinity in the trans-form compared to the cis-form. This enabled light-controlled switching between the aggregated and de-aggregated state. Further control over the aggregation process could be obtained by including ATP and apyrase in the same system. This allowed the development of a system which was responsive to multiple stimuli.
In this work, we have developed various self-assembled systems that can respond to external stimuli, such as chemical molecules or light. It was shown that this permits switching of the systems between an inactive and active state and, moreover, a transient activation in case the stimulus is spontaneously eliminated. It is shown that assembled states can act as accelerators for a chemical reaction, which does not take place in the unaggregated state. Thus, the stimulus-induced self-assembly process provides a means to control chemical reactivity. This study provides the basis for further developments related to the study of adaptation and the generation of smart and intelligent materials.
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