Alkaline saline lakes: A chemical evolution experiment evaluating the stability of formaldehyde in an aqueous saline environment.

Formaldehyde condensation in the presence of a mineral catalyst and under alkaline conditions is considered to be a "messy" chemical system due to its dependence on the complex chemical equilibrium between the reaction intermediates, which has a significant impact on the final products. This chemical system is extremely important in prebiotic chemistry and has been proposed as a potential pathway for carbohydrate formation in the early Earth. Saline and soda lakes are alkaline systems that could concentrate and accumulate a wide variety of ions (such as phosphate) and clay minerals, which can catalyze prebiotic chemical reactions. These geological environments have recently been suggested as ideal environments in which prebiotic chemical reactions could have occurred. This study uses Lake Alchichica in Mexico as a physicochemical analog of an early Archean saline lake to examine the stability of formaldehyde in these aqueous saline environments. Formaldehyde decomposes into sugar-like and CHO molecules in alkaline, high-salinity environments depending on the minerals phases present. As phosphate ion (HPO4 2-) is available in the aqueous medium, the results of our experiments also imply that phosphorylation processes may have occurred in these natural settings.

Stability of DL-Glyceraldehyde under Simulated Hydrothermal Conditions: Synthesis of Sugar-like Compounds in an Iron(III)-Oxide-Hydroxide-Rich Environment under Acidic Conditions.

Researchers have suggested that the condensation of low-molecular-weight aldehydes under basic conditions (e.g., pH > 11) is the prebiotic reaction responsible for the abiotic formation of carbohydrates. It has also been suggested that surface hydrothermal systems were ubiquitous during the early Archean period. Therefore, the catalysis of prebiotic carbohydrate synthesis by metallic oxide minerals under acidic conditions in these environments seems considerably more probable than the more widely hypothesized reaction routes. This study investigates the stability of DL-glyceraldehyde and its reaction products under the simulated conditions of an Archean surface hydrothermal system. The Hveradalur geothermal area in Iceland was selected as an analog of such a system. HPLC-ESIMS, UV−Vis spectroscopy, Raman spectroscopy and XPS spectroscopy were used to analyze the reaction products. In hot (323 K) and acidic (pH 2) solutions under the presence of suspended iron(III) oxide hydroxide powder, DL-glyceraldehyde readily decomposes into low-molecular-weight compounds and transforms into sugar-like molecules via condensation reactions.

Computer and Experimental Simulation of Alloxazine Synthesis from Gamma Irradiation of Amino Acids on Iceland Spar: A Prebiotic Chemistry Perspective.

On ancient Earth, environmental conditions favored prebiotic chemical reactions. In the Archean, some molecules with conjugated rings might have been synthesized, displaying structural stability in the Archean in the presence of ionizing radiation and hydration-dehydration events. Additionally, it is suggested that on ancient Earth, calcite was a common mineral promoting organic compound synthesis. In the present work a study of the interaction of amino acid mixtures with the (104) surface of calcite is presented. Our preliminary results show the abiotic synthesis of alloxazine (a flavin with relevant photochemical properties). Computer simulations were performed in HyperChem 8.0.1. by means of MM+ molecular mechanics and PM3 semi-empirical methods, in 27 possible amino acid trimers of alanine, glycine and lysine. Alloxazine formation is possible by the gamma irradiation of amino acids. The computer simulations show that trimers GGG and GGA promote the further transformation from diketopiperazines (DKP's) and KGK to alloxazine. The computer simulations with free radicals are not stable when alloxazine is interacting with the calcite surface. Experiments in anoxygenic environments with hydration-dehydration events in gamma irradiated samples allow the abiotic formation of flavins, DKP's and a heterocycle compound with possible relevance in prebiotic chemistry.

Adenine Adsorbed onto Montmorillonite Exposed to Ionizing Radiation: Essays on Prebiotic Chemistry.

Most adsorption and radiolysis experiments related to prebiotic chemistry studies are performed in distilled water or sodium chloride solutions. However, distilled water and sodium chloride solutions do not represent the composition of the primitive seas of Earth. In this work, an artificial seawater with ion abundances Mg2+ > Ca2+ >> Na+ ≈ K+ and SO42- >> Cl- was used, one that is different from the average composition of seawater today. This artificial seawater is named seawater 4.0 Ga, since it better represents the composition of the major constituents of seawater of primitive Earth. The radiolysis of adenine adsorbed onto montmorillonite was studied. The most important result is that adenine is adsorbed onto montmorillonite, when it is dissolved in artificial seawater 4.0 Ga, and the clay protects adenine against gamma radiation decomposition. However, desorption of adenine from montmorillonite was possible only with 0.10 mol L-1 of KOH. This result indicates that adenine was strongly bonded to montmorillonite. Fourier transform infrared spectroscopy showed that NH2 group and electrostatic interactions, between negatively charged montmorillonite and positively charged adenine, are responsible for adsorption of adenine onto montmorillonite. In addition, X-ray diffractograms showed that adenine enters in the interlayer space of montmorillonite.

Solid adenine and seawater salts exposed to gamma radiation: An FT-IR and EPR spectroscopy analysis for prebiotic chemistry.

Solids of adenine obtained from distilled water and seawater lyophilized solutions were γ irradiated at a 94.52 kGy dose. Results indicate that pure solid adenine had a low degradation rate, likewise the solid containing seawater salts. However, EPR spectroscopy analysis suggests a high interaction of the radiation with seawater salts, by radical formation in sulfate ions. These outcomes are of interest for prebiotic chemistry, since ions could have played important roles in chemical evolution. In addition, Martian soil is rich in sulphate salts, thus these salts could protected organic molecules being degraded by γ-radiation.

Salinity Effects on the Adsorption of Nucleic Acid Compounds on Na-Montmorillonite: a Prebiotic Chemistry Experiment.

Any proposed model of Earth's primitive environments requires a combination of geochemical variables. Many experiments are prepared in aqueous solutions and in the presence of minerals. However, most sorption experiments are performed in distilled water, and just a few in seawater analogues, mostly inconsistent with a representative primitive ocean model. Therefore, it is necessary to perform experiments that consider the composition and concentration of dissolved salts in the early ocean to understand how these variables could have affected the absorption of organic molecules into minerals. In this work, the adsorption of adenine, adenosine, and 5'AMP onto Na+montmorillonite was studied using a primitive ocean analog (4.0 Ga) from experimental and computational approaches. The order of sorption of the molecules was: 5'AMP > adenine > adenosine. Infrared spectra showed that the interaction between these molecules and montmorillonite occurs through the NH2 group. In addition, electrostatic interaction between negatively charged montmorillonite and positively charge N1 of these molecules could occur. Results indicate that dissolved salts affect the sorption in all cases; the size and structure of each organic molecule influence the amount sorbed. Specifically, the X-ray diffraction patterns show that dissolved salts occupy the interlayer space in Na-montmorillonite and compete with organic molecules for available sites. The adsorption capacity is clearly affected by dissolved salts in thermodynamic terms as deduced by isotherm models. Indeed, molecular dynamic models suggest that salts are absorbed in the interlamellar space and can interact with oxygen atoms exposed in the edges of clay or in its surface, reducing the sorption of the organic molecules. This research shows that the sorption process could be affected by high concentration of salts, since ions and organic molecules may compete for available sites on inorganic surfaces. Salt concentration in primitive oceans may have strongly affected the sorption, and hence the concentration processes of organic molecules on minerals.

Effects of temperature during the irradiation of calcium carbonate.

Calcium carbonate received gamma irradiation at different doses (0-309kGy) and temperature regimes (77-298K) to study the effects of irradiation temperature. The changes were followed by EPR spectroscopy. We observed the formation of a composite EPR spectrum, even at low radiation doses and temperature. There was a strong effect on the evaluation of the radicals formed as a function of irradiation temperature, probably due to the diffusion in the frozen powder and the recombination of some radicals at room temperature.

Calcium carbonate as a possible dosimeter for high irradiation doses.

The aim of this work is to analyze the interactions of 5MeV electron beam radiation and a 290MeV/u Carbon beam with calcium carbonate (powder) at 298K and at different irradiation doses, for the potential use of calcium carbonate as a high-dose dosimeter. The irradiation doses with the electron beam were from 0.015 to 9MGy, and with Carbon beam from 1.5kGy to 8kGy. High-energy radiation induces the formation of free radicals in solid calcium carbonate that can be detected and measured by electron paramagnetic resonance (EPR). An increase of the EPR response for some of the free radicals produced in the sample was observed as a function of the irradiation dose. These measurements are reproducible; the preparation of the sample is simple and inexpensive; and the signal is stable for several months. The response curves show that the dosimeter tends to saturate at 10MGy. Based on these properties, we propose this chemical compound as a high-dose dosimeter, mainly for electron irradiation.

The gamma ray response of alanine film dosimeters at low temperatures.

The response of alanine film EPR dosimeters was studied for low temperature gamma irradiation conditions (77-293 K) in the dose interval from 6.3 to 80 kGy. It was found that the response of the dosimeter decreases with decreased irradiation temperature and saturates at lower doses for lower irradiation temperatures. The analysis of the EPR signal suggests that the radical species formed at low temperature are the same as those used for dosimetry at room temperature, but with different concentrations. Their concentrations evolve as the temperature of the sample increases until the usual EPR signal used at room temperature is obtained.