Spontaneous formation of autocatalytic sets with self-replicating inorganic metal oxide clusters Haralampos N. Mirasa, Cole Mathisa, Weimin Xuana, De-Liang Longa, Robert Powa, and Leroy Cronina,1 aSchool of Chemistry, University of Glasgow, G12 8QQ Glasgow, United Kingdom Edited by Thomas E. Mallouk, The Pennsylvania State University, University Park, PA, and approved March 25, 2020 (received for review December 10, 2019) Here we show how a simple inorganic salt can spontaneously form of any added ligands or reducing agents (14). When a reducing autocatalytic sets of replicating inorganic molecules that work via agent is used, it is possible to produce a family of reduced clusters ≡ 3– molecular recognition based on the {PMo12} [PMo12O40] Keggin where the nuclearity can increase up to a maximum of 368 and ≡ 22– ion, and {Mo36} [H3Mo57M6(NO)6O183(H2O)18] cluster. These diverse structures such as nanosized spheres {Mo132}, wheels 16– small clusters are able to catalyze their own formation via an autocat- {Mo154}, capped wheels {Mo248} ≡ [H16Mo248O720(H2O)128] ,and alytic network, which subsequently template the assembly of gigantic 48– “lemon”-shaped {Mo368} ≡ [H16Mo368O1032(H2O)240(SO4)48] ≡ 14– molybdenum-blue wheel {Mo154} [Mo154O462H14(H2O)70] ,{Mo132} clusters (Fig. 1). Even with a reducing agent less than 10 specific ≡ VI V 42– [Mo 72Mo 60O372(CH3COO)30(H2O)72] ball-shaped species contain- classes of large reduced molybdate clusters are known (Fig. 1) ing 154 and 132 molybdenum atoms, and a {PMo12}⊂{Mo124Ce4} ≡ VI V VI V 5– despite an incalculable number of possibilities, and the reactions [H16Mo 100Mo 24Ce4O376(H2O)56 (PMo 10Mo 2O40)(C6H12N2O4S2)4] that form the clusters are fast. Here we show that this system is an nanostructure. Kinetic investigations revealed key traits of autocata- example of a spontaneously arising self-reproducing inorganic au- lytic systems including molecular recognition and kinetic saturation. A tocatalytic set, and that the autocatalytic nature of this process stochastic model confirms the presence of an autocatalytic network facilitates the formation of the well-known class of gigantic involving molecular recognition and assembly processes, where the larger clusters are the only products stabilized by the cycle, isolated molybdenum-blue clusters. This is interesting because the forma- due to a critical transition in the network. tion mechanism of the class of inorganic gigantic protein sized molecules known as molybdenum blues has been a mystery, since autocatalysis | autocatalytic sets | self-assembly | molecular replication | the statistical formation of a plethora of clusters should prevent the “ ” CHEMISTRY origin of life formation of magic numbers of well-defined nanomolecules (15). Results and Discussion iological self-replication is driven by complex machinery re- To investigate the kinetics of the formation of the species we Bquiring large amounts of sequence information too complex – monitored the reactions using stopped flow with an ultraviolet to have formed spontaneously (1 4). One route for the emer- – A gence of self-replicators is via autocatalytic sets (5–8), which are (UV)-vis based detection system (Fig. 2 ). Here, the contents of collections of units that act cooperatively to replicate. Experi- syringes containing the reagents for cluster assembly are injected mentally autocatalytic sets have been based on RNA, or pep- into an observation cell driven by a piston operating at 8 bar. tides, and require sequence information, that is unlikely to emerge spontaneously itself (3, 4). Similarly, the design of di- Significance rected molecular networks composed of molecules not used in biology gives insights into how complex self-organized systems Self-replication is an important property of life, yet no one build themselves (9), but again these systems are too complex to knows how it arose, and the machinery found in modern cells form randomly. The identification of examples of molecular- is far too complex to have formed by chance. One suggestion is level organization of autocatalysis outside of biology, would that simple networks may become able to cooperate and hence give insights into how the universal "lifelike" chemistry can be replicate together forming autocatalytic sets, but no simple (10), as well as allow the development of new approaches to the systems have been found. Here we present an inorganic au- catalytic production and self-assembly of functional molecular tocatalytic, based on molybdenum blue, that is formed spon- nanomaterials (11, 12). Our previous investigations suggested taneously when a simple inorganic salt of sodium molybdate is that molybdenum blue chemical systems are governed by a reduced under acidic conditions. This study demonstrates how spontaneously organized cooperative network of fast reactions. autocatalytic sets, based on simple inorganic salts, can lead to Specifically we had discovered an intermediate structure in the spontaneous emergence of self-replicating systems and which a central {Mo36} cluster appears to template the assembly solves the mystery of how gigantic molecular nanostructures VI V 14– of the surrounding {Mo150} ≡ [Mo 130Mo 20O442(OH)10(H2O)60] of molybdenum blue can form in the first place. wheel (13). As a result we hypothesized that the formation of such complex gigantic inorganic clusters was only possible due to the Author contributions: L.C. designed research; H.N.M., C.M., W.X., and R.P. performed utilization of a number of common building blocks (“Mo ,”“Mo ,” research; H.N.M. and C.M. contributed new reagents/analytic tools; H.N.M., C.M., D.- 1 2 L.L., R.P., and L.C. analyzed data; and H.N.M., C.M., and L.C. wrote the paper. and “Mo6”) able to form embedded autocatalytic sets (Fig. 1). This is because in general the formation mechanism of large The authors declare no competing interest. clusters via the polymerization of molybdenum oxide cannot This article is a PNAS Direct Submission. explain why only very specific products are formed since the This open access article is distributed under Creative Commons Attribution License 4.0 (CC BY). utilized building-block library could in principle form thousands Data deposition: All relevant data present in this publication can be accessed at https:// of structures of comparable stability via a combinatorial explosion github.com/croningp/clusterautocatalyticsets. Crystallographic data for compound 1 have process. Overcoming thermodynamic and geometry-imposed been deposited with the Cambridge Crystallographic Data Centre, https://www.ccdc.cam. limitations requires the transfer of key molecular templating in- ac.uk/structures/, under the CCDC code 1916468. formation during the formation cycle, leading to full conversion 1To whom correspondence may be addressed. Email: [email protected]. and finally production of very specific giant molecules. For ex- This article contains supporting information online at https://www.pnas.org/lookup/suppl/ ample, a solution of sodium molybdate at pH 1.7 always produces doi:10.1073/pnas.1921536117/-/DCSupplemental. a cluster containing 36 molybdenum atoms {Mo36} in the absence www.pnas.org/cgi/doi/10.1073/pnas.1921536117 PNAS Latest Articles | 1of7 Downloaded by guest on September 24, 2021 A key feature of self-replicating and autocatalytic sets is that during the initial stages of the reaction, the process occurs pri- marily via an uncatalyzed pathway. However, once the product in solution reaches a critical concentration, then the autocatalytic cycle begins to operate. Therefore, the presence of presynthe- sized {Mo36}, at the beginning of the reaction (t = 0), should result in no kinetic lag/induction period in the rate profile for the reaction, and an increase of the initial rate. In order to dem- onstrate this effect, Na2MoO4‧2H2O and HCl were reacted un- der identical conditions described previously, but with the −3 addition of 1–3 mL of preformed {Mo36} 1.6 × 10 M (see SI Appendix, section 3–3 for details and SI Appendix, Fig. S6). As Fig. 1. Size-nuclearity correlation and hypothesized embedded autocata- shown in SI Appendix, Figs. S7 and S8, autocatalyst saturation − lytic network. (A) Comparative representation of molybdenum-based family: occurs after addition of 3 mL of preformed {Mo } (1.6 × 10 3 3– VI 8– 36 [PMo12O40] , {PMo12}; [Mo 36O112(H2O)16] ,{Mo36}; [H3Mo57M6(- 22– VI V 42– M), inducing maximization of the self-propagated rate of NO)6O183(H2O)18] ,{Mo57M}; [Mo 72Mo 60O372 (CH3COO)30 (H2O)72] , 14– 16– {Mo36}, as is expected (16). {Mo132}; [Mo154O462H14(H2O)70] ,{Mo154}; [H16Mo248O720(H2O)128] , 48– In order to explore the mechanism of self-assembly of the {Mo248}; [H16Mo368O1032(H2O)240(SO4)48] ,{Mo368}. The molybdenum-blue VI V VI V structure reported here, [H16Mo 100Mo 24Ce4O376(H2O)56(PMo 10Mo 2 clusters, and to understand why only a finite number of very 5– O40)(C6H12N2O4S2)4] ,{Mo124} or compound 1 is highlighted in the purple complex products are observed, we developed a stochastic model box. (B) Hypothesized embedded autocatalytic set which funnels mass from to simulate the formation of the clusters using a kinetic Monte small {Mo1} (yellow polyhedra) monomers into corner-shared (light red), Carlo approach, based on mass action (17). In principle this edge-shared (dark red) dimers {Mo2}, {Mo6} (blue/cyan polyhedra) building modeling approach requires estimates of the reaction rate con- blocks, {Mo36} templates and {Mo132} Keplerate ball and {Mo154} stants for every possible reaction between molecular intermedi-