Prebiotics additives

Reports of the synthesis of cytosine from cyanoacetylene (or its hydrolysis product cyanoacetaldehyde) with cyanate, cyanogens or urea show that these substances react faster with nucleophilic compounds to give side products than to give the required main product. In addition, the formation of cytosine requires concentrations which are unrealistic in prebiotic environments. 

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 idea that inulin-type fructans are fermented by bacteria colonising the large bowel is supported by many in vitro (both analytic and microbiological) and in vivo studies, which, in addition, confirm the production of lactic and short-chain carboxylic acids as end products of the fermentation (Tanner, 2005). Furthermore, it was shown inhuman in vivo studies that this fermentation leads to the selective stimulation of growth of the bifidobacteria population, making inulin-type fructans the prototypes of prebiotics (Roberfroid, 1997 Roberfroid, 2001). 

Several factors indicate that the amino acids detected in all of these carbonaceous chondrites are indigenous and that they must have originated abiotically. First, the presence of protein and non-protein amino acids, with approximately equal quantities of D and L enantiomers points to a nonbiological origin and precludes terrestrial contamination. In addition, the non-extractable fraction of the Murchison is significantly heavier in 13C than terrestrial samples. Finally, the relative abundances of some compounds detected resemble those of products formed in prebiotic synthesis experiments. The aliphatic hydrocarbons are randomly distributed in chain length, and the C2, C3, and C4 amino acids have the highest concentrations (i.e., the most easily synthesized amino acids with the least number of possible structures are most abundant) [4]. 

The ability to catalyse the evolution or oxidation of H2 may have been exploited by the earliest life forms as H2 would have been present in the early prebiotic environments. The origins of the proton-dependent chemiosmotic mechanism for ATP synthesis may also reflect the formation of proton gradients created by hydrogenases on either side of the cytoplasmic membrane. In addition, it has been speculated that the coupling of H2 and S metabolisms was also of fundamental importance in the origin of life. These two processes seem intimately coupled in the bifunctional sulfhydrogenase found in Pyrococcus furiosus (a combination of subunits for hydrogenase and sulfite reductase) which can dispose of excess reductant either by the reduction of protons to H2 or S° to H2S (Ma et al. 1993 Pedroni et al. 1995). 

The bases of nucleic acids can also be considered prebiotic compounds. A possible prebiotic route to adenine has been described (Oro, 1960 Ord and Kimball, 1961 1962 see also Shapiro, 1995), as shown in Figure 3.4. For details see also Miller s review (Miller, 1998). Guanine, and the additional purines such as hypoxan-thine, xanthine, and diaminopurine could also have been synthesized by variations of the above synthesis (Sanchez et al, 1968). 

Of all mentioned prebiotic membranogenic molecules, the ones that have gained more attention in the literature are long-chain fatty acids. In addition to their prebiotic relevance, these compounds are relatively simple from the structural point of view, and most of them are easily available. We will see in the next chapter that these vesicles have acquired a particular importance in the held of the origin of life. In fact, the hrst inveshgations on self-reproducing aqueous micelles and vesicles were carried out with caprylate (Bachmann et al, 1992) and most of the recent studies on vesicles involve vesicles from oleic acid/oleate (for simplicity we will refer to them as oleate vesicles). In this section, I would like to illustrate some of the basic properties of these surfactant aggregates. 

In the case of the matrix effect, contrary to the autopoietic experiments described earlier, there is no need of water-insoluble precursors - it is the very addition of the same surfactant to an already existing family of vesicles that brings about the multiplication of the same size distribution. All that is needed is an initial narrow distribution of vesicles, and a continuous addition of fresh surfactant. Methods to obtain narrow size distributions in the case of spontaneous vesiculation have been described (Domazou and Luisi, 2002 Stano et al., in press). In fact a prebiotic scenario may be conceived where the fresh surfactant is continuously synthesized in situ, and thanks to the matrix effect the same sizes are propagated over and over again. Of course there is no way to demonstrate that this is what really happened in prebiotic times - it is fair, however, to claim that, given the simpUcity of the process, there is a reasonable probability that a process of this sort may have occurred (Luisi et al, 2004). 

Sea animals are rich in soluble dietary fibers, proteins, minerals, vitamins, antioxidants, phytochemicals, and polyunsaturated fatty acids, with low caloric value. Polysaccharides from marine animals have been reported to possess biological activities with potential medicinal values in addition to their current status as a source of dietary fibers and prebiotics. Moreover, they have a lot of dietary fiber, which lowers blood cholesterol, and iodine, which improves metabolism, vascular and cardiac action, body temperature, and perspiration regulation, and are effective in... 

The time period in which prebiotic synthesis of organic compounds took place is frequently misunderstood. The earth is 4.5 x 109 years old, and the earliest fossil organisms known, the Warrawoona microfossils and stromatolites, are 3.5 x 109 years old.10 The difference is 1.0 x 109 years, but the time available for life to arise was probably shorter. It probably took a few hundred million years for organisms to evolve to the level of those found in the Warrawoona formation. In addition, if the earth completely melted during its formation, then the time available would be further shortened by the time needed for the earth to cool down sufficiently for organic compounds to be stable. 

According to our definition a multicomponent reaction (MCR) comprises reactions with more than two starting materials participating in the reaction and, at the same time, the atoms of these educts contribute the majority of the novel skeleton of the product (Scheme 3.1) [1], For example, adenine may be formed by the addition of five molecules of isocyanic acid, a reaction of possible high prebiotic relevance [2]. 

Hydroxymethyluracil 30, a component of the present-day DNA of Bacillus subtilis bacteriophages [103], was obtained by electrophilic addition of formaldehyde to the C5-C6 double bond of a preformed uracil ring (which is probably the reason for the absence of uracil in the reaction mixture). Thymine was then obtained from 5-hydroxymethyluracil by the hydride shift mechanism shown in Scheme 18 involving formic acid as a product of formaldehyde oxidation. This is the only prebiotic synthesis of thymine so far described starting from one-carbon atom precursors as simple as formamide and formaldehyde. 

This unexpected consequence of the efficient reaction of carbon dioxide can be expressed in an other way NCAs can be considered as the most activated amino acid species achievable in water in the environment of the primitive Earth. The only exception would be species bearing a chemical protection of the a-amino groups that are unlikely because peptide elongation would have been complicated by the necessity of an additional deprotection step. From a prebiotic perspective, there is consequently no need to search for activated amino acid derivatives with a degree of activation higher than NCAs (thermodynamic limit in Fig. 3). 

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