Catalyzed Synthesis of Zinc Clays by Prebiotic Central Metabolites

Abstract

How primordial metabolic networks such as the reverse tricarboxylic acid (rTCA) cycle and clay mineral catalysts coevolved remains a mystery in the puzzle to understand the origin of life. While prebiotic reactions from the rTCA cycle were accomplished via photochemistry on semiconductor minerals, the synthesis of clays was demonstrated at low temperature and ambient pressure catalyzed by oxalate. Herein, the crystallization of clay minerals is catalyzed by succinate, an example of a photoproduced intermediate from central metabolism. The experiments connect the synthesis of sauconite, a model for clay minerals, to prebiotic photochemistry. We report the temperature, pH, and concentration dependence on succinate for the synthesis of sauconite identifying new mechanisms of clay formation in surface environments of rocky planets. The work demonstrates that seeding induces nucleation at low temperatures accelerating the crystallization process. Cryogenic and conventional transmission electron microscopies, X-ray diffraction, diffuse reflectance Fourier transformed infrared spectroscopy, and measurements of total surface area are used to build a three-dimensional representation of the clay. These results suggest the coevolution of clay minerals and early metabolites in our planet could have been facilitated by sunlight photochemistry, which played a significant role in the complex interplay between rocks and life over geological time.

Figure 1

Powder XRD and TSA characterization of sauconite synthesized at pH₀ 6.3 and 90 °C during 20 h under variable succinate concentration. (A) (A) XRD diffractograms for [succinate] listed above each trace. Numbers, e.g., (0 0 1), indicate basal reflections of the identified phases. (B) The layer to layer distance for 2:1 sauconite (d _{0 0 1}, red solid triangle) and TSA (black solid circle) for traces in (A) vs [succinate].

Figure 2

Powder XRD and d _{0 0 1} values for 2:1 sauconite synthesized at pH₀ 6.3 with 0.10 M succinate during 20 h under variable temperature. (A) XRD diffractograms under the temperatures listed above each trace. The gray line at the bottom labeled “seeded” corresponds to an experiment at 70 °C started after adding a single particle from sauconite synthesized at 90 °C. An oval shaped particle weighing 0.7 (±0.1) mg and symmetry axes of length 313 (±9) and 144 (±7) µm was used for seeding. (B) Values of d _{0 0 1} (red solid triangle) for traces in (A) with no seed vs temperature.

Figure 3

TEM images and energy dispersive X-ray spectra (EDS) of air-dried whole mount synthesized sauconite with 1.0 M succinate at pH₀ 9.0 after 6 h reflux at 90 °C. (A) Aggregates of sauconite nanocrystals. Gel-like material shows nucleation of sauconite (right side of image). (B) Higher magnification of the gel-like sauconite showing the presence of unknown zinc-containing nanocrystals in addition to sauconite. (C) Lattice fringe image of zinc nanoparticles. (D) Representative EDS spectra of the composition of sauconite nanocrystals (upper) and zinc nanoparticles (lower). The presence of Cu is due to the carbon coated copper grid used for EDS analysis.

Figure 4

TEM images of ultrathin sections of synthesized sauconite with 1.0 M succinate at pH₀ 9.0 after 20 h at 90 °C. (A) Overview of the area in the section containing aggregates of sauconite crystals. (B) Close-up of an aggregate of sauconite in the center of (A) showing the disorganized arrangement of individual 2:1 layer silicates with the occasional short-range stack of coherent layers.

Figure 5

(A) Snapshot from a tomogram of an isolated aggregate of sauconite nanocrystals from a less dense area in the section for 20 h synthesis with 1.0 M succinate at pH₀ 9.0 and 90 °C (B) 3D reconstruction of (A). See Video M4 in the Supporting Information.

Figure 6

(A) Cryo-TEM image of sauconite synthesized with 1.0 M succinate at pH₀ 9.0 after 6 h at 90 °C. The presence of irregular-shaped, possibly gel-like particles seen in Fig. 3A,B in their dispersed state. The initiation of the formation of sauconite nanocrystals within these particles can be seen in the center of the image. (B) Cryo-TEM image of 20 h synthesis as for Figure showing isolated, disorganized aggregates of sauconite nanocrystals also seen in Fig. 4. This sample in a diluted dispersed suspension consisting of pure sauconite nanocrystals.

Figure 7

Snapshots from tomograms and 3D reconstructions for 20 h synthesis with 1.0 M succinate at pH₀ 9.0 and 90 °C from cryo-TEM. (A) Raw image showing the individual aggregates of sauconite nanocrystals. (B) Reconstructed image of (A). (C,D) Different views of reconstructed aggregates of sauconite nanocrystals seen in (A) and (B) showing the 3D arrangement and orientation of individual nanocrystals. See Videos M2 and M3 in the Supporting Information.

Figure 8

Time series of powder XRD diffractograms and DRIFT spectra registered at 0, 1, 2, 6, 15, and 20 h for sauconite synthesis with 1.0 M succinate, pH₀ 9.0 at 90 °C. (A) XRD diffractogram. (B) DRIFT spectra including an inset with features at 20 h. (C) Time correlation of d _{0 0 1} values for 2:1 sauconite (red solid triangle) in (A), TSA (black solid circle) in (A), and the area for C-H stretching (ν_C-H, teal solid diamond) integrated between 2937 and 2991 cm⁻¹ in (B), after baseline correction with a two point algorithm.

Figure 9

Model displaying the stacking order of layers in a 2:1 trioctahedral sauconite structure al synthesized during 20 h using 1.0 M succinate at pH₀ 6.3 and 90 °C. The layer to layer distance d _{0 0 1} = 15.68 Å is represented to be about 3-times larger than the length of succinate that appears longitudinally oriented perpendicular to tetrahedral aluminum centers.