Dissipative Self-Assembled Vesicular Nanoreactor

In nature, self-assembly has been exploited in many ways to create molecular systems for biological processes. This idea has inspired many chemists in recent decade to use self-assembly to access various function of molecular level. However, there is a major difference between the synthetic and natural self-assembled systems. The synthetic self-assembly produces the most thermodynamically stable product. On the other hand, many natural self-assembly processes are energetically uphill and require a continuous consumption of energy to maintain its structure; this is referred as dissipative self assembly.

The formation and dissociation of actin filament, one of the example dissipative self-assembly.
(J. Baum, A. T. Papenfuss, B. Baum, T. P. Speed, and A. F. Cowman, Nature Rev. Microbiol., 2006, 4, 621-628)

Recently, researchers from University of Padova successfully found a novel strategy for the dissipative vesicular structure that are stable to maintain its shape in the present of chemical fuel.

In this system, C₁₆TACN.Zn(II) surfactant was used with ATP as stabilisator of the vesicle. Initially, the aggregation of C₁₆TACN.Zn(II) surfactant in the absence of ATP was studied and 1,6-diphenyl-1,3,5-hexatriene (DPH) was added as fluorescent probe to determine its critical aggregation concentration (CAC). Besides that, dynamic light scattering (DLS) measurements showed that the aggregate has hydrodynamic diameter around 6 nm which suggesting that C₁₆TACN.Zn(II) surfactant aggregates into micelles.

(a) Schematic representation of the dissipative self-assembly vesicles and (b) hydrodynamic diameter of the aggregates.

Interestingly, in the presence of ATP a different behaviour where an intermediate phase was observed and this indicates that the concentration of ATP presence determines the number of aggregation. In this process, the aggregate has larger hydrodynamic diameter, around 60 nm which indicate vesicular structure, and the sphericla structure was confirmed by transmission electron microscope (TEM) and cryo-TEM. It is noteworthy that the vesicles are formed under saturation conditions and that ATP and C₁₆TACN.Zn(II) surfactant are present in ratio of around 1:3.

The key feature this dissipative self-assembly is the introduction of an additional process where the the chemical energy stored in ATP and eliminate stabilising interactions between ATP and the surfactant. In this case. potato apyrase was used to hydrolyse ATP into adenosine 5'-monophosphate (AMP) and orthophosphate and a gradual decrease in signal was observe when potato apyrase enzyme is present. This indicate the decay process of the vesicle and the hydrolysis products, AMP and orthophosphate, do not stabilise the vesicle.

After successfully developing a novel way to form this out-of-equilibrium state, the next step would how this system can be used. It is well known that the non-polar environment of aggregates such as micelles or vesicles can be used as a medium for chemical reactions that would not happen in water. To investigate this property, the nucleophilic aromatic substitution between 4-chloro-7-nitrobenzofurazan (NBD-Cl) and 1-octanethiol (C₈-SH) was used; this reaction yielded 9 % in aqueous medium.

(a) Scheme of the aromatic nucleophilic substitution reaction carried out in the vesicular system, (b) representation of the kinetic processes, (c) yield (%) of the NBD-SC₈ as a function of time in different concentration of ATP and (d) in different concentration of potato apyrase enzyme at a fixed ATP concentration (5.5 μM). Both (c) and (d) were done in the presence of C₁₆TACN.Zn(II) (30 μM), NBD-Cl (1 μM), and S₈-OH (1.4 μM).

Kinetic measurement followed by ultra performance liquid chromatography (UPLC) was used tom monitor the reaction and it showed a gradual increase in the concentration of product and reached constant values. From this experiment, it is clearly shown that different ATP concentrations affect the overall yield of the reaction. The data suggest that the amount of ATP dictates the number of vesicles formed and thereby determines how much of the hydrophobic reactants are solubilised in the hydrophobic bilayer. From this results, it is interesting how ATP indirectly control how much product is formed. In dissipative state, i.e. when enzyme is present, the reaction stops earlier as the enzyme concentration increases resulting a lower yield. It is noteworthy that in this system product inhibition was observed, it explains why the yields level off, so the vesicles have to be regarded as reactors, not catalyst.

In conclusion, this study has developed a new strategy on the transient stabilisation of vesicles via non-covalent interactions between surfactants and ATP, which acts as chemical fuel. This system also shows the ability to sustain chemical reaction in the presence of chemical fuel and indirectly controls the reaction. Furthermore, this study has set a new direction towards a self-assembly system which is no longer under thermodynamic control with life-like properties.

Reference
S. Maiti, I. Fortunati, C. Ferrante, P. Scrimin, and L. J. Prins, Nature Chem., 2016, 8, 725-731.