Temperature, primitive earth

This synthesis of amino acids, called the Strecker synthesis, requires the presence of NH4+ (and NH3) in the primitive ocean. On the basis of the experimental equilibrium and rate constants it can be shown16 that equal amounts of amino and hydroxy acids are obtained when the NH4+ concentration is about 0.01 M at pH 8 and 25°C with this NH4+ concentration being insensitive to temperature and pH. This translates into a pNH3 in the atmosphere of 2 x 1(U7 atm at 0° and 4 x 10-6 atm at 25°C. This is a low partial pressure, but it would seem to be necessary for amino acid synthesis. Ammonia is decomposed by ultraviolet light, but mechanisms for resynthesis are available. The details of the ammonia balance on the primitive earth remain to be worked out. 

Since high concentrations of HCN were not plausibly present on the primitive Earth, several experiments were performed using dilute solutions of HCN in alkaline media at high temperature (< 0.1 M HCN solutions) [52]. Under these experimental conditions, the purine and pyrimidine derivatives were not directly recovered from the reaction mixtures, the chemical information... 

In the 1960s, Schwartz described the phosphorylation of adenosine with trimetaphosphate to yield 2 - and 3 -AMP. The strong alkaline conditions used for this transformation were not likely to occur on the primitive Earth [137]. Similarly, all natural ribonucleosides were phosphory-lated to corresponding 2 - and 3 -nucleotide monophosphates with sodium trimetaphosphate at high pH and temperature [138,139]. When the reaction was performed under similar experimental conditions at lower pH, 2/,3/-cyclic phosphate nucleotides were recovered as the major products [140]. Magnesium ion catalyzes this transformation in neutral water solution [141]. 

The primitive earth long remained covered in darkness, wrapped in dense burning clouds into which water vapor poured continuously from volcanic emissions. When temperatures finally cooled sufficiently, the clouds began to melt into rain. At first, falling on incandescent rock, the rain evaporated, but the evaporation... 

The steady state concentrations of HCN would have depended on the pH and temperature of the early oceans and the input rate of HCN from atmospheric synthesis. Assuming favorable production rates, Miyakawa et al (30) estimated steady state concentrations of HCN of 2 x 10 M at pH 8 and 0°C in the primitive oceans. At 100° C and pH 8 the steady state concentration was estimated as 7 x 10 M. HCN hydrolyzes to formamide which then hydrolyzes to formic acid and ammonia. It has been estimated that oligomerization and hydrolysis compete at approximately 10 M concentrations of HCN at pH 9 (31), although it has been shown that adenine is still produced from solutions as dilute as 10 M (32). If the concentration of HCN were as low as estimated, it is possible that HCN tetramer formation may have occurred on the primitive Earth in eutectic solutions of HCN-H2O, which may have existed in the polar regions of an Earth of the present average temperature. High yields of the HCN tetramer have been reported by cooling dilute cyanide solutions to temperatures between -10° C and -30° C for a few months (31). Production of adenine by HCN polymerization is accelerated by the presence of formaldehyde and other aldehydes, which could have also been available in the prebiotic environment (29). 

Extraterrestrial materials consist of samples from the Moon, Mars, and a variety of smaller bodies such as asteroids and comets. These planetary samples have been used to deduce the evolution of our solar system. A major difference between extraterrestrial and terrestrial materials is the existence of primordial isotopic heterogeneities in the early solar system. These heterogeneities are not observed on the Earth or on the Moon, because they have become obliterated during high-temperature processes over geologic time. In primitive meteorites, however, components that acquired their isotopic compositions through interaction with constituents of the solar nebula have remained unchanged since that time. 

Noble gas clathrates will not now form on the Earth, as can be seen from the air pressure decomposition temperatures in Table 2.7. They might, however, form in cooler regions of the primitive solar nebula (see Limine Stevenson, 1985). Sill and Wilkening (1978) note that for pressures in a plausible model nebula, pure ice clathrates of Ar, Kr, and Xe could form at 40, 45, and 62 K, respectively. 

As the tree shows, the true relatives of the archaea are not the bacteria but the eukaryotes. However, because the archaea branch-off closest to the root of the tree, the suggestion is that they are the most primitive of the three kingdoms of organisms whereas the eukaryotes are the least primitive (or the most derived). Placement of the archaea closest to the universal ancestor is supported by the fact that many archaea inhabit extreme environments involving high temperature, low pH, high salinity, and so on. Thus, archaea may well be evolutionary relics of the Earth s earliest forms of life. 

---