Prebiotic relevance

These schemes have been frequently suggested [105-107] as possible mechanisms to achieve the chirally pure starting point for prebiotic molecular evolution toward our present homochiral biopolymers. Demonstrably successftd amplification mechanisms are the spontaneous resolution of enantiomeric mixtures under race-mizing conditions, [509 lattice-controlled solid-state asymmetric reactions, [108] and other autocatalytic processes. [103, 104] Other experimentally successful mechanisms that have been proposed for chirality amplification are those involving kinetic resolutions [109] enantioselective occlusions of enantiomers on opposite crystal faces, [110] and lyotropic liquid crystals. [Ill] These systems are interesting in themselves but are not of direct prebiotic relevance because of their limited scope and the specialized experimental conditions needed for their implementation. 

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. 

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]. 

The prebiotic relevance of these heterogeneous oligonucleotides was shown by using them as templates for the preparation of the complementary oligoguanylates from S -phosphorimidazolide of guanidine (ImpG)... 

Both pathways probably involved quite different conditions, the main difference being the absence of liquid water in the interstellar medium. Nevertheless the basic building blocks and chemical reactions should have been roughly similar, thus leading to important connections between these two routes. While a wide variety of amino acids are prebiotically relevant (as attested by either Urey-Miller experiments or meteorite analysis), we shall focus in this section on a-amino acids (as the most relevant to biochemistry) and closely related compounds. 

Commeyras and co-workers proposed for the first time a prebiotically relevant synthetic pathway [143,144]. They established that the nitrosation of N-carbamoylamino acids 25 (otherwise believed as unreactive in prebiotic environments) by nitrogen oxides (typically obtained by mixing O2 and NO) quantitatively releases NCA. Although first tested in organic solvent [ 143], the reaction proved to work in the solid state [144], and also in aqueous solution [145], where the NCA can be observed for ca. 1 h prior to its conversion into either free amino acids or peptides. 

The prebiotic relevance of this pathway was examined in several papers [144,146,147]. The main requirements of such a pathway are (z) the presence of CAA in the prebiotic pool, (ii) the presence of sufficient amounts of NO in the atmosphere and, (iii) the possibility of a concentration process, e.g. through water evaporation. 

Although amino acids and other biochemical monomers are readily produced from reduced gas mixtures (6), the prebiotic relevance of these model atmospheres... 

From the methodological point of view the hypercycle model is the most impressive example of the possibility of unifying detailed biochemical examinations and mathematical models of chemical reactions. Both the prebiotic relevance of the model and its mathematical structure were studied, sometimes critically (e.g. King, 1981 Miiller-Herold, 1983 Szathmary, 1984). It was suggested that the hypercyclic evolution is not as effective as conventional autocatalysis, and the model should be defined by multiple time singular perturbation theory. 

In all the experiments, the main decomposition products were phosphonates, which are also stable in concentrated solutions of Mg and Ca chlorides. In some experiments, pyrophosphate, and in smaller amounts triphosphate, could also be detected. The authors thus assume that the primeval ocean contained phosphonates as a source of phosphorus for reactions leading to biochemically relevant molecules. Iron meteorites could have delivered sufficient reduced phosphorus (Fe3P) to the primeval Earth, so the question of prebiotic phosphorus chemistry should be looked at in more detail in the future (Pasek and Lauretta, 2005). 

The pH of the oceans forming the primordial soup is important in controlling the charged nature, or otherwise, of the amino and carboxylic acid species and hence their chemistry. Generating reaction schemes for the prebiotic synthesis of molecules requiring basic conditions will not be relevant if the oceans are acidic. Consider dissolving CO2 into water, simply written as ... 

Following Miller s experiment, many chemists successfully synthesized other compounds of biochemical relevance under prebiotic conditions - thereby demonstrating convincingly that several molecular bricks of life might have been present on prebiotic Earth. 

In particular, the aminoacyl phosphate derivatives of 5 -AMP could condense to a polypeptide containing up to 56 residues. This is certainly a very elegant and impressive reaction, hut its relevance from the point of view of prebiotic chemistry is doubtful since, as already mentioned, it is not obvious whether and to what extent this reaction can be extended to co-oligopeptides. Also, of course, AMP cannot be considered (thus far) a prebiotic molecule. Here again we encounter the old problem of the RNA world, the fact that the prebiotic synthesis of mononucleotides is not understood. There is no doubt that if a robust and credible prebiotic route... 

Ribo also suggests that this might be relevant in a prebiotic scenario, at the early stages of the origin of life, since there may well have been vortices determining permanent sign directions in primordial times. 

The availability of building materials (including amino acids) under hypothesized primitive Earth conditions is a major concern in prebiotic chemistry. Major - closely linked to each other - concerns in the field are which compounds can be formed , how much , and through which mechanisms . When estimating the availability of prebiotic organics, two relevant, parallel pathways are usually pointed out ... 

Y. Yamagata, H. Watanabe, M. Saitoh and T. Namba (1991). Volcanic production of polyphosphates and its relevance to prebiotic evolution. Nature (London), 352, 516-519. 

Banfield, J. F. Hamers, R. J. (1997). Processes at minerals and surfaces with relevance to microorganisms and prebiotic synthesis. In Geomicrobiology Interactions between Microbes and Minerals, Vol. 35, ed. J. F. Banfield K. H. Nealson. Washington DC Mineralogical Society of America, pp. 81-122. 

Cyanamide is a relevant molecule in prebiotic chemistry, and it was recently shown that water-ice catalyzes the rearrangement of cyanamide to carbodiimide. Carbodiimide could act as a condensation agent in the assembly of amino acids into peptides. In the peptide synthesis, using substitued carbodiimides as condensation agents, formation of L—L bonds is favored over D—D bonds by a ratio of 6 1. ... 

---