1 Department of Physics, Chemistry and Pharmacy, Faculty of Science, SDU2 Department of Physics, Chemistry and Pharmacy, Faculty of Science, SDU
The emergence of RNA chains from prebiotic soup is considered a stumbling block in the RNA world theory (Orgel 2004). Both the activation of RNA monomers and their subsequent oligomerization is hard to achieve in accepted early Earth conditions, thus putting doubt on the prebiotic plausibility of the RNA world concept. Contrary to RNA building blocks, amino acids form quite easily in simulated prebiotic reactions. Also, many prebiotic scenarios for condensation of amino acids into peptides have been proposed and successfully demonstrated experimentally (Rode 1999). We also have growing body of experimental evidence showing various catalytic activities associated with short chain peptides, some of them as small as dipeptides. We have decided to use one of such catalytic pepides (Li et al. 2000, Gorlero et al. 2009) in an attempt to demonstrate possibility of peptide catlysed oligomerization of RNA. As an environment medium we have used water/ice eutectic phase – conditions believed to be present on the early Earth and many icy moons. In this work we describe a prebiotically plausible system in which L-dipeptides containing a histidine residue, primarily the SerHis dipeptide act as catalyst for the formation of RNA oligomers from imidazole derivatives of mononucleotides. The thermodynamic shift towards condensation was achieved using water/ice eutectic phase environment (Monnard and Ziock 2008). To obtain such an environment, a reaction solution is cooled below its freezing point, but above the eutectic point. Under these conditions, most of the water is in the form of ice crystals and the other reactants are upconcentrated in the remaining liquid micro-inclusions, hence creating an environment with low water activity in which condensation reactions can occur. In the above mentioned conditions we were able to succesfully demonstratate formation of short oligomers of RNA. During the oligomerization, an active intermediate (dipeptide-mononucleotide) is produced, which is the reactive species. Details of the mechanism and kinetics, which could be elucidated with a set of control experiments, further establish that the imidazole side chain of a histidine at the carboxyl end of the dipeptide plays a crucial role in the catalysis. Results suggest that the oligomerization catalysis occurs according to a transamination mechanism. Because peptides are much more likely products of spontaneous condensation than nucleotide chains, their potential as catalysts for the formation of RNA is very interesting from the origin-of-life perspective. The ability of simple peptides to catalyze RNA synthesis could represent a link between prebiotic chemistry and the RNA world. Prebiotic soup likely contained complex mixtures of various molecules. Interaction of peptides and nucleotides shows that we should give more consideration to systems chemistry approach in the origin-of-life research.
prebiotic chemistry; RNA world; peptides; origin of life