Mapping the Landscape of Potentially Primordial Informational Oligomers: Oligodipeptides and Oligodipeptoids Tagged with Triazines as Recognition Elements
Department of Chemistry and The Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA.Angewandte Chemie International Edition (Impact Factor: 11.26). 03/2007; 46(14):2470-7. DOI: 10.1002/anie.200603207
(Chemical Equation Presented) Pairing up: Oligodipeptide, oligodeoxy-dipeptide, or oligodipeptoid backbones tagged with the 2,4-diaminotriazine nucleus pair strongly with complementary DNA and RNA. This is in sharp contrast with the behavior of the 2,4-dioxotriazine nucleus, which does not act as a nucleo-base in these systems.
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- "The same reaction performed at room temperature afforded glycine, parabanic acid, hydantoin and hydantoin derivatives as the only recovered products (spark discharge performed under an inert argon atmosphere only yielded triazines). Triazines are prebiotically relevant, since these compounds embedded in peptidic oligomers can establish strong interactions with nucleobases  . The authors suggested a reaction pathway involving the formation of cyanoacetylene and cyanoacetaldehyde for the synthesis of cytosine and uracil, while triazines were most probably obtained by a sequence of urea transformations with biuret (aminocarbonyl urea) and isocyanic acid HCNO as key intermediates . "
ABSTRACT: The presence of organic molecules in meteorites clearly indicates the occurrence of a large panel of chemical reactions in space conditions. The scenarios in which these transformations take place are diverse and fascinating: proto-stellar nebulae, dense or rarefied clouds of interstellar and cosmic dust particles, comets, meteorites, proto-planets and asteroids. High energy particles (cosmic rays and solar winds), heat, electromagnetic radiations, and radioactive decays continuously interact with simple chemical precursors to yield new complex derivatives. Some of these reactions are more relevant than others in the process of origin of life. The prebiotic chemistry in space conditions finally determines the synthesis of molecules that may play a key role in the organization of the first genetic and metabolic systems. Once synthesized some molecules can be transported through the universe until habitable planets. The description of the full set of these reactions is extremely complex and necessarily incomplete. In this review, some relevant prebiotic processes in space conditions are described with particular attention to the catalytic role played by stellar objects in the transformation of ubiquitous chemical precursors, such as formamide, formaldehyde and hydrogen cyanide. Thus, amino acids, nucleobases, sugars, lipids and carboxylic acids emerge as very easily synthesizable molecules in the universe ready to join in the first living cell.
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ABSTRACT: The RNA World hypothesis suggests that RNA, or a proto-RNA, existed in an early form of life that had not yet developed the ability to synthesize protein enzymes. This hypothesis, by some interpretations, implies that nucleic acid polymers were the first polymers of life, and must have therefore spontaneously formed from simple molecular building blocks in the "prebiotic soup." Although prebiotic chemists have searched for decades for a process by which RNA can be made from plausible prebiotic reactions, numerous problems persist that stand in the way of a chemically-sound model for the spontaneous generation of an RNA World (e.g., strand-cyclization, heterogeneous backbones, non-selective ligation of activated nucleotides). The Molecular Midwife hypothesis, proposed by Hud and Anet in 2000, provides a possible solution to several problems associated with the assembly of the first nucleic acids. In this hypothesis, nucleic acid base pairs are assembled by small, planar molecules that resemble molecules which are known today to intercalate the base pairs of nucleic acid duplexes. Thus, the validity and merits of the Molecular Midwife hypothesis can be, to some extent, explored by studying the effects of intercalation on the non-covalent assembly of nucleic acids. In this thesis, I explore the role of the sugar-phosphate backbone in dictating the structure and thermodynamics of nucleic acid intercalation by using 2′,5′-linked RNA intercalation as a model system of non-natural nucleic acid intercalation. The solution structure of an intercalator-bound 2′,5′ RNA duplex reveals structural and thermodynamic aspects of intercalation that provide insight into the origin of the nearest-neighbor exclusion principle, a principle that is uniformly obeyed upon the intercalation of natural (i.e. 3′,5′-linked) RNA and DNA. I also demonstrate the ability of intercalator-mediated assembly to circumvent the strand-cyclization problem, a problem that otherwise greatly limits the polymerization of short oligonucleotides into long polymers. Together, the data presented in this thesis illustrate the important role that the nucleic acid backbone plays in governing the thermodynamics of intercalation, and provide support for the proposed role of intercalator-mediated assembly in the prebiotic formation of nucleic acids.
Article: Prebiotic synthesis of nucleic acids[Show abstract] [Hide abstract]
ABSTRACT: The origin of the first RNA polymers is central to most current theories regarding the origin of life. However, difficulties associated with the prebiotic formation of RNA have lead many researchers to conclude that simpler polymers, or proto-RNAs, preceded RNA. These earlier polymers would have been replaced by RNA over the course of evolution. A remaining difficulty for this theory is that the de novo synthesis of a feasible proto-RNA has not yet been demonstrated by plausible prebiotic reactions. This thesis focuses on two problems associated with prebiotic proto-RNA synthesis: The formation of nucleosides and the necessity of reversible backbone linkages for error correction in nucleic acid polymers. "The Nucleoside Problem", or the lack of success in forming pyrimidine nucleosides by plausible prebiotic reactions, represents a significant stumbling block to the RNA world hypothesis. Nearly four decades ago Orgel and coworkers demonstrated that the purine nucleosides adenosine and inosine are synthesized by heating and drying their respective bases and ribose in the presence of magnesium, but these reaction conditions do not yield the pyrimidine nucleosides uridine or cytidine from their respective bases. In this thesis a potential solution to The Nucleoside Problem is hypothesized based upon a proposed chemical mechanism for nucleoside formation. This hypothesis is supported by the successful synthesis of 2-pyrimidinone nucleosides by a plausible prebiotic reaction in good yield, demonstrating that pyrimidine nucleosides could have been available in the prebiotic chemical inventory, but that uridine and cytidine were likely not abundant. Reversible backbone linkages are necessary to provide a mechanism for error correction in non-enzymatic template-directed syntheses of proto-RNAs. In this thesis, acetals are explored as low-energy, reversible linkage groups for nucleosides in polymers. The synthesis of glyoxylate-acetal nucleic acids (gaNAs) through simple heating-drying reactions from neutral aqueous solutions is demonstrated, and these linkages are shown to be hydrolytically stable under a considerable range of solution conditions. Computational models demonstrate that the glyoxylate linkage is an excellent electronic and isosteric replacement for phosphate. Molecular dynamics simulations also indicate that a gaNA duplex would have structural properties that closely match a phosphate-linked RNA helix, suggesting the possibility for cross-pairing between gaNAs and RNAs, allowing for sequence transfer and genetic continuity through the evolution from proto-RNAs to RNA. The principles illustrated in this thesis by 2-pyrimidinone nucleoside and gaNA synthesis can be extended to other prebiotic condensation reactions. Factors affecting condensation yield, such as thermodynamics, kinetics, reactant solubility, and salt effects, are summarized herein. Ph.D. Committee Chair: Hud, Nicholas V.; Committee Member: Fox, Ronald F.; Committee Member: Lynn, David G.; Committee Member: Powers, James C.; Committee Member: Wartell, Roger M.; Committee Member: Williams, Loren D.
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