Abstract. The RNA World hypothesis proposes that primitive forms of life used polymers resembling RNA both as catalysts and as carriers of genetic information. It has also been suggested that the origin of life occurred in hydrothermal conditions, but this implies that the ester bonds of nucleic acids are sufficiently stable to survive in aqueous conditions at elevated temperatures. Here, we summarize the results of experimental tests of RNA in simulated hydrothermal conditions in which stability is monitored at elevated temperatures and pressures. This perspective provides insight into the evolutionary pathway from small nucleotides to functional RNA molecules and the feasibility of RNA-based life. Simulation experiments of prebiotic RNA formationA major unsolved question in origins of life research concerns the process by which nucleic acids were first assembled and incorporated into the earliest forms of life. Substantial evidence supports the conjecture that the first living organisms passed through a phase in which RNA served both as a catalyst and as a carrier of genetic information [1][2][3][4]. However, it is still unclear how non-biological processes could have synthesized random polymers of RNA-like molecules as a first step toward living systems. The formation of RNA molecules is required not only for the accumulation of RNA but also for RNA to evolve biochemical functions.Several studies simulating the prebiotic conditions on the early Earth demonstrated that RNAlike molecules can be synthesized from chemically activated monomers. Because polymerization of nucleoside 5'-triphosphate is not efficient under these conditions, activated nucleoside 5'-monophosphorimidazolides were used instead, both in the presence and absence of a polynucleotide template [5][6][7][8] (Fig. 1). The formation of oligonucleotides proceeds from the activated nucleotide monomers in the presence of a metal ion catalyst [6] or a clay mineral catalyst [7]. The formation of oligonucleotides in the presence of such prebiotic catalysts leads to the production of oligomers as a Corresponding
The RNA world hypothesis suggests that chemical networks consisting of functional RNA molecules could have constructed a primitive life-like system leading a first living system. The chemical evolution scenario of RNA molecules should be consistent with the Hadean Earth environment. We have demonstrated the importance of the environment at both high temperature and high pressure, using different types of hydrothermal flow reactor systems and high-pressure equipment. In the present study, we have attempted to develop an alternative easy-to-implement method for high-pressure measurements and demonstrate that the system is applicable as an efficient research tool for high-pressure experiments at pressures up to 30 MPa. We demonstrate the usefulness of the system by detecting the high-pressure influence for the self-cleavage of avocado hammerhead ribozyme (ASBVd(−):HHR) at 45–65 °C. A kinetic analysis of the high-pressure behavior of ASBVd(−):HHR shows that the ribozyme is active at 30 MPa and its activity is sensitive to pressures between 0.1–30 MPa. The surprising finding that such a short ribozyme is effective for self-cleavage at a high pressure suggests the importance of pressure as a factor for selection of adaptable RNA molecules towards an RNA-based life-like system in the Hadean Earth environment deep in the ocean.
In a previous study, we have showed that the elongation of an alanine oligopeptide [L-alanyl-L-alanyl-L-alanyl-L-alanine ((Ala))] to higher oligopeptides is enhanced by calcite and dolomite at 275°C, using a mineral-mediated hydrothermal flow reactor system. However, a problem during the use of hydrothermal flow reactor system was that some of the minerals, such as clay, could not be tested due to their clogging in the reactor. In this article, we attempted to analyze the scope of enhancement for the formation of L-alanyl-L-alanyl-L-alanyl-L-alanyl-L-alanine ((Ala)) and higher oligopeptides with different minerals including clay minerals for the elongation of alanine oligopeptide at 175°C. First, carbonate minerals and some clay minerals showed an enhancement of the formation of (Ala) from (Ala). On the contrary, volcanic products showed strong inhibitory activities. According to the pH dependence on the (Ala) elongations, we confirmed that most enhancement and inhibitory activities are due to the pH influence on the elongation of (Ala). However, the enhancement of montmorillonite (Tsukinuno), sphalerite, apatite, tourmaline, calcite (Nitto Funka), and the inhibitory activities by volcanic ash (Shinmoedake), volcanic ash (Sakurajima), dickite, and pyrophillite are not simply due to the pH change in the presence of these minerals. The difference found between the previous and present studies suggests that the interaction kinetics of the aqueous phase with the mineral phase is also an important factor for the elongation of (Ala). These data imply that the environments with pH near neutral to weak alkaline and with minerals might have been useful for the accumulation of oligopeptides in hydrothermal conditions.
The reactivity of gangliosides with superoxide anion (O2(•-)) and hydroxyl radical (HO(•)) was evaluated by ESR spin-trapping using 5,5-dimethyl-1-pyrroline-N-oxide under physiological conditions (1/15 M phosphate buffer, pH 7.4). Gangliosides proved to react with HO(•) but not with O2(•-). The second order rate constants for various gangliosides with HO(•) ranged from 5 × 10(9) M(-1)s(-1) to 16 × 10(9) M(-1)s(-1). The rate constant for tetrasialoganglioside, GQ1b, was about three times higher than that of monosialoganglioside, GM1. The reactivity of gangliosides and asialo-GM1 with HO(•) was in the order: GQ1b >GT1b >GD1a >GD1b = GM1 ≫ asialo-GM1. The observed high reactivity of gangliosides probably involves the sialyl residues, since sialic acid was shown to be more reactive with HO(•) than d-glucose under the same conditions.
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