Prebiotic chemistry

Prebiotic chemistry of natural heterocycles, among them purines, pyrimidines, sugars, and riboflavin 97T11493. 

It remains an open question as to whether this newly identified species of amino acid was important in the construction of peptide nucleic acids (see Sect. 6.7) in the prebiotic chemistry of the RNA world phase of biogenesis (Meierheinrich et al 2004). 

The discovery of this amino acid in outer space supports the theory that organic molecules from space must have played a central role in prebiotic chemistry on Earth (Kuan et al., 2003). 

Several institutes throughout the world immediately began to carry out experiments on prebiotic chemistry. At this point, we need to realize that the prebiotic synthesis of protein building blocks is only a first step towards solving the biogenesis problem. Put simply, it is a method for making bricks which will later be used in building a multi-storey office block ... 

Although the Miller-Urey experiments of 1953 are of only historic interest today, they do mark the beginning of prebiotic chemistry and modern biogenesis research. 

However, the amino acids were not necessarily formed in the gas phase. John Oro, one of the pioneers in prebiotic chemistry, carried out syntheses in the liquid phase by reacting HCN, NH3 and H2O at 353 K. The results were confirmed by Lowe et al. (1963) and developed further ten years later, Jim Ferris took them up and did considerable further work (Ferris et al., 1973, 1974). In all these simulation experiments, the simplest amino acids (glycine, alanine and in small amounts aspartic acid and a-aminobutyric acid) are, as expected, the main products the yields of glycine are around 1%, those of the other amino acids much lower. 

The prebiotic chemistry of the nucleic acid bases is still the subject of debate among experts. One of the most mindful critics is Robert Shapiro, professor of chemistry at New York University and a DNA expert. His book Origins-A Sceptic s Guide to Creation of Life on Earth includes a critical analysis of the results previously obtained in biogenesis research (Shapiro 1986). Shapiro s has been the critical voice in the community of biogenesis researchers for many years. He identifies the weak points in some of the audacious hypotheses, which are often raised to the status of theories even though they involve many open questions. 

Carbohydrates are still difficult to deal with in prebiotic chemistry although the first laboratory syntheses of carbohydrates were described more than 150 years ago, their prebiotic synthesis is still unclear. 

Earlier studies showed that reactions of sugars with ammonia lead to small molecules such as amines or organic acids. A. L. Weber has reported important autocatalytic processes occurring when trioses are allowed to react with ammonia under anaerobic conditions, such reactions provide products which are autocatalyt-ically active. Their autocatalytic activity was determined directly by investigating their effect on an identical triose-ammonia reaction. Both an increase in the triose degradation rate and an increased rate of synthesis of pyruvate, the dehydration product of the triose, were observed. Such processes may have been of importance for prebiotic chemistry occurring on the primeval Earth (Weber, 2007). 

Nearly 50 years ago, T. Volker (1960) reviewed the complex reaction possibilities of polymeric HCN, not from the point of view of prebiotic chemistry, but from that of industrial chemistry. 

Since no efficient reactions for the synthesis of high-energy phosphates under prebiotic conditions had been found, Keefe and Miller (1995) came to a negative, pessimistic conclusion with respect to previous results in this sector of prebiotic chemistry. Some studies on the problem have in fact shown positive results. 

A. W. Schwartz gave a detailed account of the phosphate problem with the title Phosphorus in Prebiotic Chemistry in February 2006 (Schwartz, 2006). 

There is no doubt that volcanic activity had various effects on the chemical processes occurring on the young Earth. Two articles deal with simulation experiments intended to study how volcanic exhalations may have affected prebiotic chemistry. 

The importance of this sensational discovery for biogenesis research only became apparent in the next few years (Lahav, 1999). It is clear that prebiotic chemistry is much more complex and versatile than was thought about 50 years ago, when Stanley Miller carried out his first successful amino acid syntheses. Experiments similar to the ones described above, as well as new ones which must first be devised, could help to slowly close the wide gap which still exists between the prebiotic and the living worlds (Wills and Bada, 2000). 

A detailed account of the prebiotic chemistry of the amino acids and peptides as well as a perspective on protometabolism has been provided by R. Pascal et al. (2005). 

However, the question must always be asked as to whether these processes could have taken place on the primordial Earth in its archaic state. The answer requires considerable fundamental consideration. Strictly speaking, most of the experiments carried out on prebiotic chemistry cannot be carried out under prebiotic conditions , since we do not know exactly what these were. In spite of the large amount of work done, physical parameters such as temperature, composition and pressure of the primeval atmosphere, extent and results of asteroid impacts, the nature of the Earth s surface, the state of the primeval ocean etc. have not so far been established or even extrapolated. It is not even sure that this will be possible in the future. In spite of these difficulties, attempts are being made to define and study the synthetic possibilities, on the basis of the assumed scenario on the primeval Earth. Thus, for example, in the case of the SPREAD process, we can assume that the surface at which the reactions occur could not have been an SH-containing thiosepharose, but a mineral structure of similar activity which could have carried out the necessary functions just as well. The separation of the copy of the matrix could have been driven by a periodic temperature change (e.g., diurnal variation). For his models, H. Kuhn has assumed that similar periodic processes are the driving force for some prebiotic reactions (see Sect. 8.3). 

One of the founding fathers of prebiotic chemistry, Stanley Miller, took up this question. The PNA polymer consists of ethylenediamine monoacetic acid (EDMA) units, which can also be called A-(2-ami noclhyl )-gl yci nc (AEG). 

Although the pre-RNA world is now much more the centre of scientific attention in prebiotic chemistry, there have been several attempts in recent years to understand the synthesis of oligonucleotides from the normal nucleotides by using simulation experiments (Ferris, 1998). In condensation reactions in aqueous media, there is always competition between synthesis and hydrolysis synthesis is generally only successful when supported by catalysts. 

The RNA world was the first biological world. If this is the case, we can learn or predict but little about the prebiotic chemistry of the RNA world from the biochemistry of today perhaps only the fact that the formation and polymerisation of nucleotides were once prebiotic processes. Thus Orgel is not of the... 

One of the most important questions in prebiotic chemistry is as follows Which reactions of simple building block molecules are robust enough that they can survive great changes in pressure, temperature, pH and irradiation and still lead to more complex systems ... 

Chemical reactions similar to the Fischer-Tropsch synthesis have been discussed for some years in connection with prebiotic chemistry they are described as Fischer-Tropsch type reactions (FTT). In its technically optimized form, the FIT... 

Prebiotic chemistry must cope with many problems a particularly difficult one is contamination. Prebiotic experiments often lead to the formation of important molecular species in extremely low concentrations. The successes of the synthesis may sometimes appear sensational, but there is always the danger that artefacts may be involved. Control experiments carried out with ultrapure deionised water showed that, at higher temperatures (>373 K), synthetic polymers in components of the apparatus could provide a source of organic contaminants such as formate, acetate or propionate ions. Stainless steel had a catalytic effect on the decomposition of formate, so that the use of titanium alloys in the apparatus is recommended. 

These experiments were, however, not completely accepted by some chemists working in the area of prebiotic chemistry these critics were unhappy with some synthetic conditions used, such as the KCN concentrations (0.1-0.2 M at 373 K). They felt that cyanide in such concentrations would have been relatively rapidly hydrolysed at 373 K in addition, CO pressures of 75 atm were considered incommensurate with real conditions in hydrothermal systems. 

The importance of the thioesters was realized at the beginning of the 1950s by Theodor Wieland from the University of Frankfurt am Main (Wieland and Pflei-derer, 1957), who used aminoacyl mercaptans as activated amino acids in peptide syntheses (see Sect. 5.3). Thus, 30 years later, this area of basic research came to be useful for prebiotic chemistry. 

The thioester world postulated by de Duve should in fact be called the sulphur-iron world , since iron ions are essential for the redox processes occurring in such a thioester world, de Duve (1991, 1996) asks a question which is vital for the whole of prebiotic chemistry where did the redox equivalents necessary for the construction of biomolecules on the primeval Earth come from This question becomes largely irrelevant if the strongly reducing atmosphere postulated by Miller/Urey and... 

We do not yet know what the optimal temperatures for the evolution of life were. It is generally accepted that the prebiotic chemistry on the primeval Earth must have taken place at moderate temperatures. It is, however, also possible that various... 

The clay mineral montmorillonite, which is often used in different prebiotic syntheses, is probably now the most important mineral for experiments on prebiotic chemistry. It has shown its abilities in the area of simulation experiments on the formation of primitive cellular compartments montmorillonite accelerates the spontaneous conversion of fatty acid micelles to vesicles. Clay particles are often incorporated into the vesicle, just as is RNA, which is adsorbed at such clay particles. If the vesicles have been formed, they can continue to grow if fatty acids are fed to them via micelles. If the vesicles are pressed through 100 nm pore filters, they divide without dilution of their contents. 

Of the three extraterrestrial targets in our solar system, the Saturnian moon Titan is the least likely to provide signs of life. To quote Christopher McKay from the NASA Ames Research Center, Titan is an interesting world. For example, its organic haze layer could be an example of the prebiotic chemistry which led to life on Earth . Direct links to extraterrestrial life have not, however, yet been found, as water (one of the main preconditions for life) has not been detected on Titan, apart from traces of water vapour in the higher layers of the Titanian atmosphere (Brack, 2002). 

Of the major solids formed from melts, many, but not all, at equilibrium, the overwhelming influence is of cooperative interaction between ionic units of similar shape and size as we see in crystals. Trace elements apart from forming isolated minerals are fractioned in bulk oxides, for example, in particular orders as the melt solidifies, and this reduces the relative availability of some elements such as Cr and Ni (see Williams, and Williams and Frausto da Silva (1999) in Further Reading). Again the interaction of selective molten minerals and water creates extremely reactive environments and such environments still exist, especially in the deep sea black smokers (hydrothermal vents), around which particular mixed minerals form, which could also have been involved in prebiotic chemistry and are still involved in the peculiarities of life in these smokers . In Figure 1.6 we summarise... 

This pathway sets the direction for the remainder of this chapter but has the same basic strategy for the understanding of meteorite chemistry, comet chemistry, planetary atmospheric chemistry, prebiotic chemistry and ultimately the chemistry of an organism. This pathway is the molecule-up view of the Universe. 

The temperature of the planet within a habitable zone controls the rate of chemical reactions, the availability of chemicals and the phase in which the chemicals will be found. These have profound consequences for prebiotic chemistry and the origins of life. 

Protection of prebiotic chemistry from the highly energetic UV radiation... 

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