Combining Catalytic Microparticles with Droplets Formed by Phase Coexistence: Adsorption and Activity of Natural Clays at the Aqueous/Aqueous Interface

Abstract

Natural clay particles have been hypothesized as catalysts on the early Earth, potentially facilitating the formation of early organic (bio) molecules. Association of clay particles with droplets formed by liquidliquid phase separation could provide a physical mechanism for compartmentalization of inorganic catalysts in primitive protocells. Here we explore the distribution of natural clay mineral particles in poly(ethylene glycol) (PEG)/dextran (Dx) aqueous two-phase systems (ATPS). We compared the three main types of natural clay: kaolinite, montmorillonite and illite, all of which are aluminosilicates of similar composition and surface charge. The three clay types differ in particle size, crystal structure, and their accumulation at the ATPS interface and ability to stabilize droplets against coalescence. Illite and kaolinite accumulated at the aqueous/aqueous interface, stabilizing droplets against coalescence but not preventing their eventual sedimentation due to the mass of adsorbed particles. The ability of each clay-containing ATPS to catalyze reaction of o-phenylenediamine with peroxide to form 2,3-diaminophenazone was evaluated. We observed modest rate increases for this reaction in the presence of clay-containing ATPS over clay in buffer alone, with illite outperforming the other clays. These findings are encouraging because they support the potential of combining catalytic mineral particles with aqueous microcompartments to form primitive microreactors.Clay minerals, which are composed of aluminosilicates with layered structures, are major components of soils and sedimentary rocks and among the most abundant minerals at the surface of the Earth 1 . Clays have found a wide variety of applications since ancient times including ceramics 2 , electrochemistry 3 , organoclay/polymer nanocomposites 4 and as catalysts in chemical reactions [5][6][7] . The availability of clay minerals and their potential for catalytic activity led to the proposal of these materials as inorganic catalysts for chemical evolution of biomolecules on the early Earth 8 . Since then clay particles have been demonstrated as catalysts for polymerization of activated nucleotides and amino acids 9 , lipid self-organization 10-12 and many other possible prebiotic reactions 11,13,14 .An important step in the transition from nonliving towards living matter is thought to be compartmentalization of molecules and of reactions 15,16 . Candidates for early-Earth compartments include crevices or pores in rocks 17 , self-assemblies of lipids or simpler amphiphiles to form vesicles [18][19][20][21][22][23] , and aqueous droplets formed by liquid-liquid phase separation 15,21,24 . Combining catalytic mineral surfaces with compartments such as amphiphile vesicles or droplets formed by aqueous/aqueous phase coexistence is therefore appealing as primitive protocell models. Szostak and coworkers reported that clay minerals accelerate the spontaneous conversion of fatty acid micelles into vesicles and some clay might get encapsulated within the vesicle...

Modulating the Partitioning of Microparticles in a Polyethylene Glycol (PEG)-Dextran (DEX) Aqueous Biphasic System by Surface Modification

“…Jauregi et al [11] could separate ampicillin and phenylglycine crystal mixture by selective interfacial partitioning in a water/alkanol biphasic system. Recently, Cakmak and Keating [12] reported that natural clays, which work as reaction catalysts, partitioned differently by their kind in PEG/DEX ATPS and showed slight increase of reaction rate in ATPS compared to that in buffer. Also, Deng et al [13] reported that PEG/DEX ATPS could separate Merrifield resins and N-methylimidazolium grafted Merrifield resins by controlling the ATPS compositions, graft ratio and different anions of resins.…”

“…We chose FT-IR microscopy [5] mainly because of its simplicity and minimal amount of sample needed, compared to other known methods. Among them, zeta potential measurement [12,17], vibrating sample magnetometry (VSM) [18], X-ray photoelectron spectroscopy (XPS) [17,19], and thermogravimetry [15] can be considered as other tools for confirmation of surface modification, which may provide more quantitative information on the degree of modification.…”

Dynamic Synthetic Cells Based on Liquid–Liquid Phase Separation

“…This has led severalg roups to describe such droplets as water-in-water (w/w) emulsions. Spherical colloidsw ith diameters typicallyg reater than 100 nm, including nanoparticles, [85,86] lipid vesicles [87,88] and protein clusters, [89] as well as high-aspect-ratio colloids, such as clays, [90] nanorods [91] or protein fibrils, [92] adsorb more effectively to interfaces with lows urface tension [92,93] and have therefore been used to stabiliseP EG/dextran aqueous two-phase systems (Figure 2D,E ), as well as complex coacervate droplets. Spherical colloidsw ith diameters typicallyg reater than 100 nm, including nanoparticles, [85,86] lipid vesicles [87,88] and protein clusters, [89] as well as high-aspect-ratio colloids, such as clays, [90] nanorods [91] or protein fibrils, [92] adsorb more effectively to interfaces with lows urface tension [92,93] and have therefore been used to stabiliseP EG/dextran aqueous two-phase systems (Figure 2D,E ), as well as complex coacervate droplets.…”

“…[83] Although conceptually trivial,t he stabilisation of w/w emulsions by surface-actives peciesi st echnically challenging, due to the very low interfacial tensionsi nvolved (surfacetensions cales as thermal energy over the square of molecule size, [84] thus resulting in low values for the macromolecules involved in LLPS). Spherical colloidsw ith diameters typicallyg reater than 100 nm, including nanoparticles, [85,86] lipid vesicles [87,88] and protein clusters, [89] as well as high-aspect-ratio colloids, such as clays, [90] nanorods [91] or protein fibrils, [92] adsorb more effectively to interfaces with lows urface tension [92,93] and have therefore been used to stabiliseP EG/dextran aqueous two-phase systems (Figure 2D,E ), as well as complex coacervate droplets. [94,95] Membraneso btained after the interfacial assembly of such large colloidal objects are usually highly permeable to molecular and macromolecules, due to the large size of the pores created.…”

Recent Advances in Origins of Life Research by Biophysicists in Japan

“…However, in spite of all of this focus on lipid vesicles, primitive compartments need not necessarily be lipid bilayer vesicles, as there are many other model systems that could have afforded essentially the same properties as lipid vesicles, including amphiphilic peptide vesicles [104], cationic organic amphiphile membrane vesicles [87], inorganic chemical cells [105], oil-in-water microdroplets [69], coacervate [106][107][108] (Figure 4) and aqueous two-phase system [109,110] droplets, and even membraneless polyester microdroplets [111], each of which is important for not only biophysicists, but also materials scientists. We look forward to the further collaboration between these two fields in Japan and abroad not only for further elucidation of the assembly and dynamics of primitive compartments in the context of OoL research, but also perhaps for the development of new therapeutic tools or new discoveries about modern biological systems as well [46].…”