Counterarguments Against the Biotic Theory

A common criticism on the biotic or selection theories is based on experiments and a qualitative model of Goldanskii and Kuz'min (1988, 1991) and argues that pure homochiral protein macromolecules and pure polynucleotide chains are not self-replicable in a racemic medium. In short: life requires homochirality! Models of biogenesis do all demand a supply of sufficient enantioenrichments in precursor molecules. These enantioenrichments are required to trigger self-organization far from thermodynamic equilibrium (Krueger and Kissel 1989) in order to generate living structures. Manifold laboratory experiments on the generation of polymers that are composed of enantiopure monomers determined the effect of "enantiomeric cross inhibition" (Lundberg and Doty 1957; Idelson and Blout 1958; Joyce et al. 1984; Wachtershauser 1991; Rein 1992). The statement "life requires homochi-rality" is supported by the "enantiomeric cross inhibition", an effect that is often encountered in the scientific literature and deserves an explanation.

3.4.3.1 Enantiomeric Cross Inhibition of Polypeptides

The effect of "enantiomeric cross inhibition" was discovered at the University of Boston by M. Idelson and E. R. Blout, who investigated the polymerization of an L-glutamate derivative7 that forms polypeptides with up to 1 500 monomer units. Under a variety of experimental conditions kinetic data were recorded: solvent, pH values, and temperature were varied and molecular initiators were added in order to fully understand the polymerization reaction and to elucidate its mechanism. In the context of this book, it is of crucial importance that Idelson and Blout (1958) also added defined quantities of D-glutamate7 to the reaction mixture of L-glutamate7 in

7 Y-benzyl-l-glutamate-N-carboxyanhydride (NCA) derivative order to study the effect of the optical configuration of the reactants on the polymerization. Kinetic data were recorded and the molecular weights of the formed polymers were determined. The result of these studies is that the presence of small amounts of D-glutamate decreased the polymerization rate and lowered the molecular weight of the polypeptide formed as depicted in Fig. 3.2. This effect was later called the "enantiomeric cross inhibition": the cross enantiomer (here: D-glutamate) inhibits the chain formation of L-glutamate. In other words: D-glutamate incorporated at the end of an L-glutamate oligopeptide acts as chain terminator. Also the inverse effect was observed namely that a small quantity of L-glutamate7 decreased the polymerization rate of a growing D-glutamate7 polypeptide.

One year before, Lundberg and Doty (1957) had investigated the rate of polymerization of D,L-mixtures of the glutamate derivative showing a pronounced decrease in the propagation rate for the racemic mixture relative to that of the pure L-enantiomer. While the growing chain was non-helical, the lowering was less than a factor of two and was thereby accounted for in terms of a preference of the chain to add the isomer corresponding to its terminal residue. At a later stage of the reaction process, when the growing chain had reached the helical configuration, the lowering of the rate was greater than a factor of two and this indicated a preference exerted by residues lying behind the terminal one. For the L-glutamate derivative the helical

Fig. 3.2 Effect of optical isomer incorporation on the degree of polymerization and molecular weight. The more the initial enantiomer composition deviates from a racemic 50% d- and 50% l-mixture (see x-axis), the higher the degree of polymerization of the created polymer (see y-axis on the right) and the higher the molecular weight of this polymer (see y-axis on the left). As expected, the pure d-glutamate derivative has the same degree of polymerization as the pure l-glutamate derivative. However, when as little as 5% (n/n) of the l-glutamate is polymerized with 95% (n/n) of d-glutamate (or vice versa) the polymerization rate is reduced to 1/3 of the value obtained for each enantiopure isomer. At equimolar proportions of d-glutamate and l-glutamate the polymerization rate is 1/17 that of the pure enantiomers (Idelson and Blout 1958)

Fig. 3.2 Effect of optical isomer incorporation on the degree of polymerization and molecular weight. The more the initial enantiomer composition deviates from a racemic 50% d- and 50% l-mixture (see x-axis), the higher the degree of polymerization of the created polymer (see y-axis on the right) and the higher the molecular weight of this polymer (see y-axis on the left). As expected, the pure d-glutamate derivative has the same degree of polymerization as the pure l-glutamate derivative. However, when as little as 5% (n/n) of the l-glutamate is polymerized with 95% (n/n) of d-glutamate (or vice versa) the polymerization rate is reduced to 1/3 of the value obtained for each enantiopure isomer. At equimolar proportions of d-glutamate and l-glutamate the polymerization rate is 1/17 that of the pure enantiomers (Idelson and Blout 1958)

sense of the polymer occurred after about four residues were added. Moreover, evidence accumulated that the helical configuration of a polymer was substantially less stable when the polypeptide contained both d- and L-residues. For the D,L-glutamate reaction mixture, a polymer in the preformed helical configuration composed of a single enantiomer reacted about five times faster with its own enantiomer than with the opposite optical isomer (Lundberg and Doty 1957).

A similar stereoselectivity in the polymerization of glutamate derivatives has been reproduced by others and was described for a variety of other amino acids including a-alanine, leucine, valine, isoleucine, and phenylalanine (Borchers et al. 2004) These data clearly indicate that the presence of a D-amino acid in an a-helix or P-sheet of mostly L-enantiomers has a distorting and destabilizing effect.

In order to investigate the stereoisomer distribution of tryptophan monomers in formed oligomers, the team of Pier Luigi Luisi at ETH in Zurich followed an innovative approach. One of the tryptophan enantiomers was isotopically labelled and after oligomer formation by a polycondensation reaction, ion monitoring mass spec-troscopy was used to investigate the stereoisomer incorporation. The results show that homochiral oligo-tryptophan was 8.3 and 40 times more frequent in heptamers with n = 7 and decamers with n = 10, respectively, than would have been expected for a statistical distribution. Conversely, heptamers and decamers with approx. equal proportions of d- and L-enantiomers were underrepresented (Blocher et al. 2001).

It would be of great interest to see in future experiments whether mixtures of different amino acids also lead to a preferential formation of homochiral oligopeptides consisting of more than one type of amino acid, a result which we would expect to be demonstrated.

3.4.3.2 Enantiomeric Cross Inhibition of Nucleotides

As in proteins, the chiral monomer units of DNA and RNA are enantiopure, each of the nucleotides containing either D-deoxyribose or D-ribose. The importance of chi-ral purity during polymerization was also found by the experimental investigation of this genetic material, particularly in studies on the template-directed oligomeriza-tion of nucleotides (Bonner 1995a): the oligomerization of an activated guanosine mononucleotide proceeded readily if the monomers were of the same stereochem-ical configuration as the template, and was far less efficient if the monomers were of the opposite handedness (Joyce et al. 1984). More recent research activities shifted to a study of nucleotide analogues such as pyranosyl-RNA (Beier et al. 1999), threofuranosyl nucleic acids TNA (Schoning et al. 2000), and peptide nucleic acids PNA that had been proposed as initial pre-RNA genetic materials (Sutherland 2007). In Chap. 8 of this book we will have a closer look at these candidates in pre-RNA oligonucleotide chemistry. Anyhow, short tetramers of pyranosyl-RNA did not exhibit "enantiomeric cross inhibition" because the oligomerization of homochiral tetramers was not, or only weakly, inhibited by the presence of the non-oligomerizing diastereoisomers (Bolli et al. 1997). However, the rate of oligomerization was significantly reduced when only heterochiral tetramers were available for co-oligomerization (Borchers et al. 2004).

Due to these experimental results on tetranucleotides, the chiroselective replication of biopolymers became an attractive model for explaining homochirality in nature. Saghatelian et al. (2001) theoretically simulated a 32-residue peptide replicator (see also Siegel 2001) that was calculated to amplify homochiral products from a racemic mixture of peptide fragments. This approach used a chiroselective autocatalytic cycle. Similar amplifying models will be discussed in more detail in Chap. 10.

Summarizing, the enantiomeric cross-inhibitory effect was measured and present for peptides and for oligonucleotides, except short pyranosyl-RNA tetranucleotides. It provides evidence for the assumption that big chain length biopolymers would preferably arise in an enantioenriched environment during processes of chemical evolution and that life could arise in a greatly superior way from an environment possessing a certain enantiomeric excess. Life in its primitive form seems to require an ongoing source for the accumulation of chiral molecules on early Earth (Bonner 1995a). As a consequence of this, modern theories on the origin of life were supposed to follow abiogenic models explaining the origin of the molecular parity violation long before biological evolution. The next chapters will hence be dedicated to detailed and topical explanations of modern abiogenic theories on how life's asymmetry originated.

Was life's asymmetry transferred from chiral minerals to racemic carbon-based molecules? Or was it written into the atomic nuclei of these molecules in form of the weak force? Or did chiral fields, e.g. in interstellar space, violate molecular symmetry in prebiotic times long before the actual origin of life on Earth? Our journey through these theories will come to an end by presenting information on the chiral molecular ingredients of extraterrestrial samples.

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