A. Negrón-Mendoza

Radiolysis of aqueous solutions of ammonium bicarbonate over a large dose range

Oxygen-free aqueous solutions of 0.05 mol dm-3 ammonium and sodium bicarbonate were studied after receiving various doses of 60Co gammas (0.01–400 kGy) or 0.5–20 Gy pulses of 10 Mev electrons. Formate and oxalate were found to be the main radiolytic products, in addition to trace amounts of formaldehyde and an unidentified polymer. A large initial yield of formate in the γ-radiolysis, G(HCOO-)=2.2, is due to the reaction COO- + HCO-3⤦HCOO-+CO-3. The efficiency of organic synthesis within the large dose range studied is low and is explained by efficient pathways leading to the reformation of bicarbonate, where the reaction COO-+CO-3 is particularly significant. Computer fitting of the data obtained gives k(COO- + HCO-3)=(2±0.4) x 103 dm3 mol-1 s-1, k(COO-+CO-3) = (5±1) x 107 dm3 mol-1 s-1, k(NH2+HCO-3)< 104 dm3 mol-1 s-1, and k(NH2+CO-3) = (1.5±0.5) x 109 dm3 mol-1 s-1.

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, pKa of the adsorbate, and Van der Waals interactions.

Radiation chemistry of a multicomponent aqueous system relevant to chemistry of cometary nuclei

SummaryWe have examined a water-dominated multicomponent system after irradiation in the multimegarad dose range with gamma rays from a60Co source at both 77 and 310 K. The constituents were simple organic compounds in the proportions in which they appear in a dense interstellar cloud: HCN/CH3OH/CH3CN/C2H5CN/HCOOH=1∶0.6∶0.2∶0.1∶0.05. The total amounts were adjusted to correspond to a carbon to nitrogen ratio of 1.8 and a water content of about 50% in a cometary nucleus where the dust to volatiles ratio is 1; the total amount of CN-bearing compounds was taken to correspond to 0.4% of the cometary mass. In experiments at 310 K about 40 radiolytic products are identified, among them aldehydes and amino and carboxylic acids. Abundant polymeric material (Mw up to 80,000 daltons) is formed. The basic aspects of radiolysis of the liquid system are present also at 77 K, although at radiation-chemical yields that are lower by one to two orders of magnitude. We have considered the relevance of the present findings to the chemistry of a liquid-water core and the icy layers of a cometary nucleus.

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.

A review of conditions affecting the radiolysis due to40K 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 particular40K, at the epoch of the emergence of life on Earth. The free dispersion of soluble radionuclides constitutes an effectivein 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 from40K 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 × 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 further polymerization, as it has been usually taken for granted.

Radiolysis of cyanocobalamin (vitamin B12)

Abstract Research on the radiolysis of vitamins is of considerable interest since these compounds are important nutritional constituents in foods and in dietetic supplements. In spite of these considerations there are few data and very often difficult to compare for the radiolytic behavior of vitamins. In this work we focused our attention on to the study of the radiolysis of cyanocobalamin (vitamin B 12 ) in solid state and in aqueous solutions. The procedure was followed by HPLC and UV-spectroscopy. The results obtained in aqueous solutions showed a dependence of the decomposition as a linear function of the dose. The G of decomposition for a 1×10 −5 M solution was 3.3. In the solid state the vitamin was very stable towards the irradiation in the conditions used in this study with a G =2.1×10 −3 . A study made with Serratia marcescens as a microbiological contaminant showed that at the sterilization dose there is a destruction of the vitamin in aqueous solution. In the solid state the degree of decomposition was 7%.

Radiation chemistry of overirradiated aqueous solutions of hydrogen cyanide and ammonium cyanide

SummaryThe radiolysis of aqueous solutions (O2-free) of HCN and NH4CN was examined at very large doses of60Co gamma radiation (up to 230 Mrad). In this dose range the cyanide initially present (0.12 M) is decomposed and only its radiolytic products participate in the radiation-induced chemical process. It has been found that the weight of the dry residue containing the mixture of nonvolatile radiolytic products increases as doses increase up to 40 Mrad (up to about 4 g/l), but with further dose increases remains practically unchanged (NH4CN) or decreases slightly (HCN). Carboxylic and amino acids are present in overirradiated samples. At increasing doses their concentrations decrease, with the exception of oxalic and malonic acids, which are continually produced and accumulate. This is also the case with the abundant NH3 and CO2, as well as with several other products that were generated at lower radiation-chemical yields. The molecular weights of the radiolytic products are up to 20,000 daltons throughout the dose range studied. Their amounts gradually change with increasing doses above 30 Mrad: The compounds with Mw between 2,000 and 6,000 daltons become more abundant, while the amounts of polymers with Mw between 6,000 and 20,000 decrease. The relevance of these findings for studies of chemical evolution is considered.

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.

Enhance decarboxylation reaction of carboxylic acids in clay minerals

Abstract Clay minerals are important constituents of the Earths crust. These minerals catalyze reactions in several ways: by energy transfer processes, redox reactions, stabilization of intermediates and by Bronsted or Lewis acidity behavior. Important set of organic reactions can be improved in the precedence of clay minerals. Besides the properties of clays to catalyze chemical reactions, it is possible to enhance some of its reactions by using ionizing radiation. The phenomenon of radiation-induced catalysis may be connected with ionizing process in the solid and with the trapped non-equilibrium charge cariers. In this paper we are reporting the decarboxylation reaction of carboxylic acids catalyzed by clay and by irradiation of the system acid-clay. We studied the behaviour of several carboxylic acids and analyzed them by gas chromatography, X ray and infrared spectroscopy. The results showed that decarboxylation of the target compound is the dominating pathway. The reaction is enhance by gamma radiation in several orders of magnitude.

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.