The peptide-catalyzed stereospecific synthesis of tetroses: A possible model for prebiotic molecular evolution

Abstract

Using a water-based prebiotic model of sugar synthesis involving glycolaldehyde self-condensation, we demonstrate that homochiral L-dipeptide catalysts lead to the stereospecific syntheses of tetroses. The asymmetric effect is largest for erythrose, which may reach a D-enantiomeric excess of >80% with L-Val-L-Val catalyst. Based on results obtained with various peptides, we propose a possible catalytic-reaction intermediate, consisting of an imidazolidinone ring formed between the two nitrogen atoms of the peptide catalyst and the C1 of one glycolaldehyde molecule. The study was motivated by the premise that exogenous material, such as the nonracemic amino acids found in meteorites, could have participated in the terrestrial evolution of molecular asymmetry by stereospecific catalysis. Because peptides might have formed readily on the early Earth, it is possible that their catalytic contribution was relevant in the prebiotic processes that preceded the onset of life.prebiotic chemistry ͉ chirality ͉ catalysis A lthough the chemical environments that led to life's origin on the Earth are unknown, our knowledge of current and extinct life processes allows us to postulate some possible working assumptions. For example, we may describe life as an autocatalytic chemical process where simple molecules, such as amino acids and nucleotides, assemble into larger proteins and nucleic acids that, in turn, control their metabolism and synthesis (1). In these terms, we may also identify key requirements that make such processes possible. One such prerequisite is the exclusive one-handedness of the chiral constituents of proteins and nucleotides (L-amino acids and D-sugars) that is essential to extant biopolymers' structure and function and whose origin is unknown.Several general theories have been brought forward in regard to the origin of this terrestrial homochirality (2). Some propose that life first arose from a racemic environment and later evolved to chiral homogeneity; this ''biotic'' premise circumvents the ease with which prebiotic amino acids and sugars would have lost any incremental gain in asymmetry to racemization. Abiotic theories, on the other hand, propose that some asymmetry preceded the onset of life, overcame racemization, and contributed to homochirality. The finding in meteorites of amino acids resistant to racemization and displaying enantiomeric excesses (ee) that, if less extensive, have the same sign (L-) as in terrestrial amino acids (3, 4) supports, albeit does not prove, the latter hypothesis.Apart from either hypothesis, because abiotic syntheses of chiral molecules would produce only racemic mixtures if unaided by catalysts and meteorites' composition shows that any preexisting ee available for the formation of biomolecules would not have been large (4), it is reasonable to assume that the development of chiral homogeneity was an evolutionary process that involved catalysis. This inference, in turn, leads to the question of whether the unique nonracemic amino acids of meteorites might have play...

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