Gamma irradiation of adenine and guanine adsorbed into hectorite and attapulgite.

This study focuses on the radiolysis (up to 36 kGy) of guanine and adenine (nitrogenous bases) adsorbed in hectorite and attapulgite to highlight the potential role of clays as protective agents against ionizing radiation in prebiotic processes. In this framework, the study investigated the nitrogenous bases' behavior in two types of systems: a) aqueous suspension of adenine-clay systems and b) guanine-clay systems in the solid state. This research utilized spectroscopic and chromatographic techniques for its analytical purposes. Regardless of the reaction medium conditions, the results reveal that nitrogenous bases are stable under ionizing irradiation when adsorbed on both clays.

Histidine Self-assembly and Stability on Mineral Surfaces as a Model of Prebiotic Chemical Evolution: An Experimental and Computational Approach.

The abiotic synthesis of histidine under experimental prebiotic conditions has proven to be chemically promising and plausible. Within this context, the present results suggest that histidine amino acid may function as a simple prebiotic catalyst able to enhance amino acid polymerization. This work describes an experimental and computational approach to the self-assembly and stabilization of DL-histidine on mineral surfaces using antigorite ((Mg, Fe)3Si2O5(OH)4), pyrite (FeS2), and aragonite (CaCO3) as representative minerals of prebiotic scenarios, such as meteorites, and subaerial and submarine hydrothermal systems. Experimental results were obtained through polarized-light microscopy, IR spectroscopy (ATR-FTIR), and differential scanning calorimetry (DSC). Molecular dynamics was performed through computational simulations with the MM + method in HyperChem software. IR spectra suggest the presence of peptide bonds in the antigorite-histidine and aragonite-histidine assemblages with the presence of amide I and amide II vibration bands. The FTIR second derivative inspection supports this observation. Moreover, DSC data shows histidine stabilization in the presence of antigorite and aragonite by changes in histidine thermodynamic properties, particularly an increase in histidine decomposition temperature (272ºC in antigorite and 275ºC in aragonite). Results from molecular dynamics are consistent with DSC data, suggesting an antigorite-histidine closer interaction with decreased molecular distances (cca. 5.5 Å) between the amino acid and the crystal surface. On the whole, the experimental and computational outcomes support the role of mineral surfaces in prebiotic chemical evolution as enhancers of organic stability.

Characterization of selective antibacterial peptides by polarity index.

In the recent decades, antibacterial peptides have occupied a strategic position for pharmaceutical drug applications and became subject of intense research activities since they are used to strengthen the immune system of all living organisms by protecting them from pathogenic bacteria. This work proposes a simple and easy statistical/computational method through a peptide polarity index measure by which an antibacterial peptide subgroup can be efficiently identified, that is, characterized by a high toxicity to bacterial membranes but presents a low toxicity to mammal cells. These peptides also have the feature not to adopt to an alpha-helicoidal structure in aqueous solution. The double-blind test carried out to the whole Antimicrobial Peptide Database (November 2011) showed an accuracy of 90% applying the polarity index method for the identification of such antibacterial peptide groups.

Organics produced by irradiation of frozen and liquid HCN solutions: implications for chemical evolution studies.

Hydrogen cyanide (HCN), an important precursor of organic compounds, is widely present in extraterrestrial environments. HCN is also readily synthesized in prebiotic simulation experiments. To gain insight into the radiation chemistry of one of the most important and highly versatile constituents of cometary ices, we examined the behavior of over-irradiated frozen and liquid HCN solutions under ionizing radiation. The samples were exposed to gamma radiation at a dose range from 0 up to 419 kGy. Ultraviolet spectroscopy and gas chromatography were used to follow the process. The analyses confirmed that gamma-ray irradiation of liquid HCN solutions generates several organic products. Many of them are essential to life; we verified the presence of carboxylic acids (some of them members of the Krebs cycle) as well as free amino acids and urea. These are the first studies to reveal the presence of these compounds in experiments performed at low temperatures and bulk irradiation. Organic material was produced even at low temperatures and low radiation doses. This work strongly supports the presumption that, as a parent molecule, HCN played a central essential role in the process of chemical evolution on early Earth, comets, and other extraterrestrial environments.

Prebiotic thermal polymerization of crystals of amino acids via the diketopiperazine reaction.

In this work, we continue our studies on the thermal prebiotic oligomerization of amino acids. The next step is to consider all four types of electromagnetic interactions that our model may admit. In addition, only the polymerization of amino acids via the formation of diketopiperazine, which arises from the cyclodehydration of two amino acids, will be considered. By assuming that only one residue group of two will predominate in the diketopiperazine molecule, it is possible to reduce the three-body problem to a simpler situation with the two objects that we have already solved.

Optical absorption and thermoluminescence in single NaCl:Cu crystals exposed to 60Co and UV light.

Optical absorption (OA) and thermally stimulated luminescence measurements were performed on NaCl:Cu+(0.04 and 0.08%) crystals blocks grown by the Czochralski technique. The NaCl:Cu+ crystals were exposed to gamma rays from a 60Co source (0.954-30 kGy) as well as UV radiation. The radiation-induced defects were mainly F, Cu- and Cu+ centres, with absorption bands located at 464, 256.7 and 236 nm, respectively. The absorption bands were found to be independent of the Cu impurity concentration. As the gamma-dose irradiation increased, the absorption band at 256.7 nm decreased while the band at 236.3 nm increased highly along with the 256.7 nm band. The F-centres produced at high gamma-radiation dose while thermally bleached showed an increase of the Cu+ OA bands with a simultaneous decrease of Cu- absorption band. The bleaching with F-light showed the participation of the F centre generated by gamma radiation on the ion valence changes of the doping impurity as well as on the TL phenomenon. The TL measurements in NaCl:Cu crystals with both impurity concentrations demonstrated that the Cu+ concentration has a strong influence on the intensity and shape of the glow peaks.

Sites of adsorption of adenine, uracil, and their corresponding derivatives on sodium montmorillonite.

Clay minerals are considered important to chemical evolution processes due to their properties, ancient origin, and wide distribution. To extend the knowledge of their role in the prebiotic epoch, the adsorption sites of adenine, adenosine, AMP, ADP, ATP, Poly A, uracil, uridine, UMP, UDP, UTP and Poly U on sodium montmorillonite are investigated. X-ray diffraction, ultraviolet and infrared spectroscopy studies indicate that these molecules distribute into the interlamellar channel and the edge of the clay crystals. Monomers are adsorbed predominantly in the interlamellar channel, whereas polymers adsorb along the crystal edges. Such behavior is discussed mainly in terms of bulk pH, pK(a) of the adsorbate, and Van der Waals interactions.

The behavior of single crystals of NaCl: Ca2+, Mn2+ exposed to gamma rays.

The behavior of single crystals of NaCl: Ca(2+), Mn(2+) exposed to gamma rays was explored for its potential usage as a dosimeter. The study was focused to the effect of dose and dose rate. The crystals were analyzed using thermoluminescence (TL). The productions of irradiation defects in the solid were correlated with the glow curve. F-centers were measured as function of the dose. The bleaching of the F-centers produced a decrease of the peak of the glow curve. The results showed that the response is linear in an interval up to 60 Gy.

Iron salts in solid state and in frozen solutions as dosimeters for low irradiation temperatures.

The aim of this work is to study the irradiation of iron salts in solid state (heptahydrated ferrous sulfate) and in frozen acid solutions. The study is focused on finding their possible use as dosimeters for low temperature irradiations and high doses. The analysis of the samples was made by UV-visible and Mössbauer spectroscopies. The output signal was linear from 0 to 10 MGy for the solid samples, and 0-600 Gy for the frozen solutions. The obtained data is reproducible and easy to handle. For these reasons, heptahydrate iron sulfate is a suitable dosimeter for low temperature and high irradiation doses, in solid state, and in frozen solution.

Dependence of thermoluminescence response of calcium sulphate activated by dysprosium on the temperature irradiation.

Radiation dosimetry is a very important issue in space research and in experiments that try to simulate chemical processes that may occur in cometary nucleus, interstellar grains, and other extraterrestrial environments, due to their irradiation by cosmic rays. The temperature effect is an important factor that has not been considered in many of these experiments. In this work, this effect was studied in TLD dosimeters exposed to gamma rays. The irradiations were done from 77 to 298 K in a gamma cell unit with a dose rate of 1.0 Gy/s. Results obtained for CaSO4:Dy show that there is a considerable effect in the evaluation of the dose as function of the irradiation temperature.

Dependence of irradiation temperature in the response of iron salts.

A potential dosimeter based on aqueous frozen solutions and solid-state salt are presented for the evaluation of the energy transferred during the interaction of high-energy radiation with matter at low temperature. The foundation of these dosimeters, both the solid state and the frozen solutions, is based on the measurement of the change of the iron oxidation state. The systems were irradiated with gamma radiation at different doses (up to 10 MGy), and at different temperatures (from 77 to 298 K). The irradiated samples were analysed by UV-spectroscopy and Mössbauer spectroscopy. A theoretical model was developed for the chemical reactions system. This model reproduces the experimental effects produced by the irradiation in aqueous solutions of ferrous salt. The results showed that the response of the dosimeters depends on the irradiation temperature. At low-radiation doses, the response was linear. In particular, this work can be applied to low-temperature dosimetry can be specially applied to simulation experiments of extraterrestrial bodies, as well as in general to space research.

Evaluation of the dosimetric response for CaSO4:Dy at low temperature.

Homemade solid state CaSO4:Dy detectors were tested to evaluate their response to gamma radiation at liquid nitrogen temperature (77 K). The dosemeters were irradiated with doses between 12 and 1071 Gy. For this study these dosemeters were exposed to gamma rays with a dose rate of 1.19 Gy.min(-1). The analysis for these crystals was made by thermoluminiscence. The dose response at liquid nitrogen temperature was linear in the dose range studied and it is about 20% lower with respect to the response at room temperature. The response is reproducible with the same geometric set-up.

Behavior of adenine in Na-montmorillonite exposed to gamma radiation: implications to chemical evolution studies.

Adenine is an important compound in biological systems, such as genetic and energy utilization processes. Adenine is readily formed in prebiotic conditions. Its synthesis and stability in environmental conditions are of paramount importance in chemical evolution processes. Clay minerals might have played an important role in the early Earth. Clays are known to have a high affinity for organic compounds, and they may provide protection to adsorbed molecules against high-energy radiation. The purpose of this work is to testthese assumptions. We study the stability of adenine under irradiation, in aqueous solution and also adsorbed in a clay mineral. The recovery of adenine after a gamma irradiation was higher in the system containing clay in relation to a system without clay. Results show that adenine is readily adsorbed in the clay, and that the clay act as surface protector toward the degradation of adenine by the radiation.

Biased polymers in the origin of life.

We delve into the study of a Markov chain formalism applied to the thermal prebiotic oligomerization of amino acids. We find for the case of only two types of electromagnetic interactions, that the steady state attainment by the Markov chain is a built in mechanism limiting the expected variability in sequences in a population of polymers. Such result may be of importance as it makes more accessible the replication of a minimal chemical machinery compatible with life.

A simple model of the thermal prebiotic oligomerization of amino acids.

We construct a probabilistic model with the aid of the Markov chain formalism to describe and give a physico-chemical justification to an oligomerization process of a set of amino acids under certain prebiotic conditions. Such chemical process shows a remarkable bias in the polymer products that our model can explain. Some predictions and limitations are also discussed.

A review of conditions affecting the radiolysis due to 40K on nucleic acid bases and their derivatives adsorbed on clay minerals: implications in prebiotic chemistry.

This paper describes the possible effects of ionizing radiation arising from long-lived soluble radionuclides within clays, in particular 40K, at the epoch of the emergence of life on Earth. The free dispersion of soluble radionuclides constitutes an effective in situ irradiation mechanism that might have acted upon adsorbed nucleic bases and their derivatives on clays, inducing chemical changes on these organic molecules. Several types of well documented reactions for radiolysis of nucleic acid bases and their derivatives are known, even at low doses (i.e., 0.1 Gy). For example, estimates with a dose rate calculated from 40K from deep sea clays at 3.8 Ga ago, indicates that over a period of 1000 years the amount of organic material transformated is 1.8 X 10(-7) moles/kg-clay. Although ionizing radiation may also induce synthetic reactions with prebiological interest, all in all these considerations indicate that nucleic acid bases and their derivatives adsorbed on clays were exposed for long periods to degradation conditions. Such situation promotes decomposition of organic molecules rather than protection of them and enhancement of farther polymerization, as it has been usually taken for granted.

Radiolysis of aqueous formaldehyde relevant to cometary environments.

The radiation chemistry of aqueous solutions of formaldehyde was studied in order to obtain an insight into the possible role of ionizing radiation on cometary environments. Aqueous solutions of 1.0 mol dm-3 formaldehyde were exposed to gamma-radiation in the dose range from 0.01 to 1200 kGy at 298 K. The radiation chemical yield of decomposition of formaldehyde was determined to be: G(-CH2(OH)2)-26.3 +/- 1.2. The high radiation chemical yield of decomposition was explained by a chain reaction initiated by the radical CH(OH)2 with formaldehyde. Computer fitting of the experimental data gives k(CH(OH)2 + CH2(OH)2)- 8.0xl0(1) dm3 mol-1 s-1. In the computer treatment of experimental findings we used 54 equations to consider the radiolysis of water and 11 reactions for the radiolysis of aqueous formaldehyde. Based on previous estimates of the total dose of ionizing radiation that comets have accumulated over 4.6 billion years, we predict a radiation damage-depth curve of formaldehyde in comet nuclei.

Radiation-induced syntheses in cometary simulated models.

The behavior of an aqueous-dominant multicomponent cometary model is examined at high doses of ionizing radiation. The system is composed of a water mixture of HCN (0.2 mol dm-3), CH3CN (0.04 mol dm-3), C2H5CN (0.02 mol dm-3), CH3OH (0.12 mol dm-3) and HCO2H (0.01 mol dm-3. It was exposed to gamma rays at doses up to 18.5 MGy. The chemical kinetic database used in the computer treatment of experimental data consists of 79 reactions. A complex mixture of products has been synthesized: gases, amino acids, carboxylic acids and polymeric material. The results suggest that the pristine material in cometary nuclei may have been chemically altered by the action of cosmic rays and embedded radionuclides.

Radiolysis of aqueous solutions of acetic acid in the presence of Na-montmorillonite.

The gamma-irradiation of 0.8 mol dm-3 aqueous, oxygen-free acetic acid solutions was investigated in the presence or absence of Na-montmorillonite. H2, CH4, CO, CO2, and several polycarboxylic acids were formed in all systems. The primary characteristics observed in the latter system were: (1) Higher yield of the decomposition of acetic acid; (2) Lower yield of the formation of polycarboxylic acids; (3) No effect on the formation of methane; (4) Higher yield of the formation of carbon dioxide; and (5) The reduction of Fe3+ in the octahedral sites of Na-montmorillonite. A possible reaction scheme was proposed to account for the observed changes. The results are important in understanding heterogeneous processes in radiation catalysis and might be significant to prebiotic chemistry.

The gamma-irradiation of aqueous hydrogen cyanide in the presence of ferrocyanide or ferricyanide: implications to prebiotic chemistry.

The gamma-irradiation of 0.1 M, O2-free, aqueous HCN was studied in the presence of ferrocyanide or ferricyanide in the concentration range 10(-3) - 10(-5) M. This study was carried out in order to get an insight into the possible role that cyanocomplexes of iron may have played in promoting prebiotic syntheses via the free-radical oligomerization of HCN. It was found that ferrocyanide or ferricyanide have no effect on the irradiation of 0.1 M HCN solutions at concentrations < or = 10(-4) M. At high concentrations, 10(-3) M, they lead to a marked decrease in the conversion of HCN. There was no significant difference due to the oxidation state of iron used, particularly at high doses > or = 100 kGy.