NANOPARTICLE-BASED WORLD – ORIGIN OF LIFE THEORY

Many theories are currently being accepted as possible explanations for the origins of life on early Earth (e.g. Metabolism-first world, Zinc world, Thioester world, RNA world and others). However, discovered enzyme-mimetic properties of nanoparticles may lead to reconsideration of these theories. Recently, we postulated that one of the stages of chemical evolution could have been associated with nanozymatic activity of different nanoparticles that could form primitive prebiotic photodynamic redox chemical networks with proto-enzymatic activity. Our theory (Nanoparticle-based World) assumes that chemical evolution passed through the nanoparticle-assisted stage and could have led to sustainable metabolism. Formamide-based prebiotic chemistry assumes that formamide could have accumulated in sufficient amounts to serve as a feedstock and reaction medium for the synthesis of the first biogenic molecules. In this work, UV-induced formation (prebiotic scenario) of nanoparticles with xanthine oxidase like activity was demonstrated. All reactants (thiol-containing compounds, metals ions - zinc and cadmium, ammonia and fomamide) were highly probably present on early Earth.


INTRODUCTION
Many theories like Metabolism-first world [1], Zinc world [2], Thioester world [3], RNA world [4], Lipid world [5] and others, are currently being considered the explanation of the origin of life on Earth. Scientists all around over the world still discuss the physical and chemical conditions on early Earth, sources of energy, the feedstock molecules and transformation of non-living matter (biogenic building blocks) to self-sustaining autocatalytic networks (sustainable life). We believe that all the right places for the emergence of life contained mainly sulfur (especially thiols), metals ions, ammonia, and formamide environment. Formamide-based prebiotic chemistry assumes that formamide could accumulate in high enough amounts to serve as a feedstock and reaction medium for the synthesis of the first biogenic molecules like nucleobases, nucleosides, amino acids, sugars, amino sugars, carboxylic acids and others [6]. In our previous work, we showed that thermal treatment (160 °C, 24 hours) of formamide in presence of various clays (Ni/Fe meteoritic material, Fe2O3) caused formation of adenine, guanine, cytosine, thymine, uracil, purine, urea, hypoxanthine, guanidine and glycine [7]. In this work, we proved that formamide (50%) is a great solvent (environment) for UV-induced formation of ZnCd quantum dots (QDs) from very simple mixture of precursors containing thiols (e.g. mercaptosuccinic acid), metals ions (Zn 2+ and Cd 2+) and ammonia (NH3, in range 0 -2%). Also it was proved that these ZnCd QDs mimic enzyme xanthine oxidase (XO) under visible light conditions. And that is why we assume that first proto-metabolism pathway could have been associated with purine proto-metabolism catalyzed by nanoparticles (nanozymes). Caetano-Anolles G. et al. strongly suggested that first metabolic pathway in prebiotic chemistry was related to the synthesis of nucleotides for the RNA world, which was linked to the purine metabolic subnetwork [8]. Sun M. et al. showed that CdTe QDs can specifically recognize and cleave the restriction site GATATC in double-stranded DNA, mimicking an enzyme restriction endonuclease [9]. These findings support the assumption that abiotic artificial materials (nanozymes) were apparently connected to the RNA world. The functions of ribozymes in modern organisms support the hypothesis that life passed through an RNA world before the emergence of proteins or DNA. Moreover, numerous microorganisms are capable of nanoparticle synthesis in order to remove undesired metal species [10][11][12][13][14]. This ability of microorganisms can also support the hypothesis that chemical evolution of life passed through the abiotic artificial material (nanozymes). Nanozymes as alternatives to natural enzymes excel mainly due to their stability, chemical diversity, and functionality in non-aqueous solvents [15]. Currently, nanozymes are prepared mainly from metal or carbon materials [16]. The enzymatic activity of inorganic materials starts to be considered highly important in the context of the new theories about the origin of life [17][18][19]. In this work, we present and discuss one of the possible scenarios of prebiotic preparation of ZnCd QDs nanozymes (mimic XO under visible light) with connection to first proto-metabolism based on purines catabolism in formamide environment.

Chemicals
Standards and other chemicals were purchased from Sigma-Aldrich (St. Louis, MO, USA) in ACS purity.

Enzymatic reactions
Xanthine oxidase-like activity of ZnCd QDs after visible light irradiation by table 6800K (6W) LED lamp (EMOS, Prerov, Czech Republic) was investigated by highly water-soluble tetrazolium salt that produces a formazan dye upon reduction with a superoxide anion. The reaction was monitored at 440 nm by UV/Vis spectrophotometer Tecan Infinite 200 M PRO (TECAN, Switzerland) using UV-transparent 96 wellplate with a flat bottom by CoStar (Corning, USA). The signal intensity was proportionate to the amount of superoxide anion. Standard enzyme of xanthine oxidase (positive control) was purchased from Sigma-Aldrich.

RESULTS AND DISCUSSION
The hydrogen cyanide (HCN) is most abundant three-atoms compound in interstellar environment and together with three-atoms water (H2O) formed formamide (CHONH2) [6]. Codella C. et al proved according interferometric measurement that formamide (potentially crucial building block of life [21][22][23][24]) is efficiently formed in the gas phase around Sun-like protostars [25]. The formamide has been also detected in comets, in the solid phase on grains around the young stellar object, in the galactic center sources Sagittarius and in the interstellar medium [26][27][28][29][30]. This work was focused on one-pot synthesis of the ZnCd QDs in formamide environment and subsequently investigation of their proto-metabolic activity (mimicking enzymes -xanthine oxidases). Photo-chemical processes leading to formation of the ZnCd QDs are described in our previous work [31]. Several plausible scenarios involve prebiotic reactions in warm mineral and metal rich environment (involving mainly Fe, Cu but also Zn, Cd, Ag, Co, Ni, Pb, Mo etc. [32,33] and their combination with chemistries of sulfiderich hydrothermal vents [34] or lagoons exposed to UV radiation [35]. Here, the UV-induced reaction was studied in 50% formamide environment. Formamide-based prebiotic chemistry suggests a plausible reaction route for the synthesis of nucleobases and others biogenic molecules [36]. Therefore, we have tested the Formamide-based prebiotic chemistry together with formation of ZnCd QDs (mimicking enzyme XO) could be associated with first self-sustaining autocatalytic networks.

CONCLUSION
The present work demonstrates that formamide is an excellent solvent (environment) for light (UV)-driven synthesis of ZnCd QDs in the whole spectral range (emission from blue to red). Also, it was proved that ZnCd QDs mimic enzyme xanthine oxidase under visible light irradiation. We believe that all the cellular subsystems could have arisen simultaneously through common formamide-based chemistry and the key reaction steps could have been driven by light. Therefore, we assume that first proto-metabolism pathway could be associated with purine catabolism provided by nanoparticles (nanozymes) like QDs or other types of nanoparticles/nanomaterials. All precursors (metals ions, sulfur-containing compounds and ammonia), together with formamide, are abundantly represented in space environment. We suggest named all these nanoparticle-mediated reactions in the context origin of life as Nanoparticle-based world [36].