Wachtershauser

Until the 1980s, yields of nucleobases obtained in prebiotic syntheses were very small. Thus, some scientists assumed that in earlier phases of molecular evolution, the nucleic acids used other bases in their information-transmitting substances. Piccirilli et al. (1990) suggested isocytosine and diaminopyridine, while Wachtershauser (1988) suggested that the first genetic material possibly consisted only of purines. However, pyrimidine (about a fifth of the total amount of purines present) had been detected in the Murchison meteorite, so that an effective pyrimidine synthesis should have been possible. 

However, there are also biogenesis models which do not require phosphate, such as the inorganic hypothesis of the origin of life proposed by Cairns-Smith (see Sect. 7.1), the thioester world proposed by de Duve (see Sect. 7.4) or the sulphur-iron world suggested by Wachtershauser (see Sect. 7.3). The RNA world (see Chap. 6), however, cannot exist without phosphate. 

The use of reactive surfaces for the specific synthesis of biomolecules, or as a model for replication processes, was first reported by Cairns-Smith and Weiss (see Sect. 7.1) and continued by G. Wachtershauser (see Sect. 7.3), as well as J. Ferris and L. Orgel. It was thus appropriate to study the stabilisation of the reaction partners in enzyme-free self-replication at surfaces with reactive properties. As early as 1995, the group of G. von Kiedrowski (then at Freiburg, Germany) bonded reacting molecules at surfaces and then added the other required reaction components to the system in a stepwise manner (the latter process is referred to as feeding ). 

The still open question, Information or metabolism first has again been discussed by Robert Shapiro. In an article with the title Did This Molecule Start Life A Simple Origin for Life , he again stresses that it is improbable that life could have begun in an RNA world (referred to here as RNA-first ). Shapiro offers his own suggestion in the metabolism debate he assumes that cyclical processes, occurring in small compartments, lead from small molecules to systems of higher complexity. The Shapiro model takes into account aspects of the approaches and hypotheses proposed by Wachtershauser (see Sect. 7.3), de Duve (see Sect. 7.4) and Kauffmann (see Sect. 9.3). In order to avoid one-sidedness, Shapiro s article is accompanied by a short reply An RNA-First Researcher Replies . In this way, the reader is shown in a clear and understandable manner what the differences between the two approaches are (Shapiro, 2007). 

Huber and Wachtershauser (1997) were able to demonstrate the incorporation of CO at an Fe/Ni sulphide catalyst. Orgel, however, doubts that it is justified to draw further conclusions from this result, since this enzyme mechanism may have developed after protein synthesis began or evolved. Orgel considers the use of the mechanism referred to above of CO incorporation in biomolecules to be a result of chemical determinism. 

Cairns-Smith, as the leading proponent of the mineral theory, has also shown interest in both the hydrothermal biogenesis theory (Cairns-Smith, 1992) and the iron-sulphur hypothesis proposed by G. Wachtershauser (see Sect. 7.3). 

The first indication of a possible connection between geological processes occurring at the boundaries between tectonic plates of the mid-oceanic ridges and the biogenesis problem was provided by J. B. Corliss (1981). He considered the hydrothermal conditions to be ideal reactors for abiotic synthesis these ideal conditions were the water temperature gradients, the pH, and the concentrations of solutes in the hot springs. The presence of certain minerals which could act as catalysts, such as montmorillonite, clay minerals, iron oxide, sulphides etc., was also very important. The initial model presented for the hydrothermal synthesis of biomolecules (Corliss, 1981) was modified, particularly by Russell (1989) and Wachtershauser (see Sect. 7.3). 

None of the biogenesis hypotheses previously discussed differs so clearly from all the other models as the chemoautotrophic theory proposed by the Munich patent attorney Dr. Gunter Wachtershauser. 

Wachtershauser gained attention with a fundamental article published at the end of the 1980s (Wachtershauser, 1988a). Active support for his entry into the (almost) closed society of biogeneticists was provided by Karl Popper, whose authority in the philosophy of science was recognized worldwide. Wachtershauser formulated his ideas on the basis of Popper s scientific theory. The new theory (or hypothesis) negates ... 

The RNA world requires a system capable of self-replication as a precondition for the beginnings of life. In contrast, the surface metabolism theory proposed by Wachtershauser postulates that the initial step is metabolism, from which complex replication systems can evolve later. This metabolism would have occurred at the... 

It must be selective and mild (thus, Wachtershauser excludes the influence of solar UV irradiation). 

The reduction potential must suffice for all the reduction reactions involved in the metabolic system (Wachtershauser, 1992). 

Wachtershauser provides a detailed discussion of whether particular crystal modifications of pyrite could have led to homochirality in the biomolecules produced, but his theory appears to be extremely speculative. 

This archaic cycle contains either many, or only a few, realistic steps, depending on one s attitude to Wachtershauser s hypotheses. 

G. Wachtershauser formulated his suggestions on the initial metabolic processes on the primeval Earth (as described above) in the form of six postulates and eleven theses (Wachtershauser, 1990b). Laboratory experiments planned to check, and perhaps confirm, these theories have already been attempted. The reductive Krebs cycle (rTCA cycle) has recently been the subject of much discussion. Smith and Morowitz consider that the rTCA cycle is a universal, possibly primordial, core process which could have provided substances for the synthesis of biomolecules on the young Earth (Smith and Morowitz, 2004). 

Using experimental conditions similar to those described above, Huber and Wachtershauser were able to detect the formation of peptide bonds in reactions involving three amino acids the main product, however, was only a dipeptide. 

The results obtained appeared quite promising, but the real sensation was the detection of pyruvate, the salt of 2-oxopropanoic acid (pyruvic acid), which is one of the most important substances in contemporary metabolism. Pyruvic acid was first obtained in 1835 by Berzelius from dry distillation of tartaric acid. The labile pyruvate was detected in a reaction mixture containing pure FeS, 1-nonanethiol and formic acid, using simulated hydrothermal conditions (523 K, 200 MPa). The pyruvate yield, 0.7%, was certainly not overwhelming, but still remarkable under the extreme conditions used, and its formation supports Wachtershauser s theory. Cody concludes from these results that life first evolved in a metabolic system prior to the development of replication processes. 

About three years after Wachtershauser s first publication appeared, an article by Christian de Duve and Stanley Miller was published in the Proceedings of the National Academy of Sciences under the title Two-Dimensional Life the title alluded to the theory of reactions at positively charged pyrite surfaces (de Duve and Miller, 1991). Their criticisms of the chemoautotrophic theory were directed particularly towards certain kinetic and thermodynamic aspects, but also to theoretical statements for which no experimental support was available. 

A detailed theoretical study of the properties of the redox system FeS/FeS2 was carried out in the Department of Geosciences of SUNY Stony Brook (Schoonen et al., 1999). The authors conclude that the hypothetical reduction of CO2 (by the FeS/FeS2 redox pair) formulated in Wachtershauser s early work, and the carbon fixation cycle on the primeval Earth associated with it, probably could not have occurred. This judgement is made on the basis of a theoretical analysis of thermodynamic data other conditions would naturally have been involved if CO had reacted rather than C02. It is not known whether free CO existed in the hydrosphere, or if so, at what concentrations. 

Photoelectrochemical experiments on the pyrite/H2S system, as well as theoretical considerations, led Tributsch et al. (2003) to the conclusion that CO2 fixation at pyrite probably could not have led to the syntheses proposed by Wachtershauser. The reaction mechanism involved in such reactions is likely to be much more complex than had previously been assumed. The Berlin group supports the objection of Schoonen et al. (1999) that, apart from other points, the electron transfer from pyrrhotine to CO2 is hindered by an activation energy which is too high. There is, thus, no lack of different opinions on the model of chemoautotrophic biogenesis hopefully future studies will shed more light on the situation ... 

If CO2 is replaced by CO in the pyrite/H2S system, the conditions become much more favourable, as the positive results obtained by Huber and Wachtershauser (1998) and Cody (2000a) show. 

Further experiments by Huber and Wachtershauser on chemoautotrophic biogenesis under hydrothermal conditions have shown that a number of a-amino acids and a-hydroxyacids could have been formed, subsequent to the binding of carbon (in the form of CO and CN ) to catalytically active transition metal precipitates. The general structure of such compounds is R-CHA-COOH, with R = H, CH3, C2H5 or HOCH2 and A = OH or NH2. 

The reactions were carried out at temperatures between 353 and 393 K and CO pressures up to 75 atm reaction times were between 20 h and 10 days. Of vital importance are the catalytically active precipitates of Ni or Ni/Fe with carbonyl, cyano and methylthio ligands as carbon sources. Calcium or magnesium hydroxide were used as buffers to prevent the system from becoming too acidic (Huber and Wachtershauser, 2006). 

Finally, an observation on Wachtershauser made by R. Shapiro in his book Planetary Dreams, he recounts Leslie Orgel s judgement on Wachtershauser s ideas, ... that he1 considered the work to be the most important finding in the last century (Shapiro, 1999). The Science journalist M. Hagmann (Zurich, Switzerland) has provided a comprehensive profile of Wachtershauser (Hagmann, 2002). 

Pyrite is not only one of the key compounds in Wachtershauser s theory, but could also have fulfilled an important function for phosphate chemistry in prebiotic syntheses. A group in Rio de Janeiro studied the conditions for phosphate sorption and desorption under conditions which may have been present in the primeval ocean. In particular, the question arises as to the enrichment of free, soluble inorganic phosphate (Pi), which was probably present in low concentrations similar to those of today (10 7-10 8M) (Miller and Keffe, 1995). Experiments show that acid conditions favour sorption at FeS2, while a weakly alkaline milieu works in an opposite manner. Sorption of Pi can be favoured by various factors, such as hydrophobic coating of pyrite with molecules such as acetate, which could have been formed in the vicinity of hydrothermal systems, or the neutralisation of mineral surface charges by Na+ and K+. 

When the Fe2+/Fe3+ system is included, the thioester hypothesis, with its roots in sulphur chemistry, shows clear links with the iron-sulphur world of Wachtershauser s chemoautotrophic biogenesis model (see Sect. 7.3). 

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While a passionate enthusiast for evolutionary theories, and their experimental testing, Gunther Wachtershauser is in his professional life a patents lawyer. 

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