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Chemistry |
Studies on the Origins of Life: The Formation of the RNA World
A likely route to RNA from compounds formed spontaneously on the primitive Earth, is by their selective adsorption on a mineral which catalyzes their condensation to polymers. At Rensselaer we discovered that montmorillonite clay catalyzes the conversion of activated RNA monomers to oligomers. The role of monomer structure, phosphate activating group and mineral catalysis on the formation of RNA under prebiotic and conditions is being explored. .
Initial studies established that some montmorillonite clays catalyze condensation of activated mononucleotides to oligomers which contain about ten monomer units. For example, the self-condensation of the 5'-phosphorimidazolide of adenosine (ImpA) in pH 8 aqueous solution in the presence of montmorillonite results in the formation of 2-10 mers in which 66% of the phosphodiester bonds are 3',5'-linked.
A detailed kinetic analysis of the reaction of ImpA on montmorillonite showed that the montmorillonite enhances the rate of reaction of ImpA by about 1000-fold. In addition, the rates of oligomer formation increase in the order dimer-trimer-tetramer-pentamer and higher. These findings demonstrate that short oligomers will grow more rapidly than new dimers are formed; a result consistent with the formation of longer RNA oligomers on mineral surfaces.
The phosphate activating group has an important role in the oligomer chain length and the regioselectivity of the phosphodiester bond formation. When 4-dimethylaminopyridine was the activating group oligomers as long as twelve mers were formed which contained about 90% 3',5'-linked phosphodiester bonds. Adenine and its derivatives are also activating groups; the most effective being the 1- and 3-methyl derivatives of adenine. Monomers activated with these derivatives yielded oligomers up to 10 mers in length which contained about 88% 3',5'-links. The 2-methyl derivative of adenine and adenine itself were also found to promote the condensation of 5'-AMP to oligomers. These results suggest that adenine and its derivatives, which were also required for prebiotic nucleotide formation, could have been used for the condensation of monomers to oligomers.
The corresponding self-condensation of the 5'-phosphorimidazolide of cytidine (ImpC) results in the formation of 2-15 mers in which about 25% of the oligomers contain 3,5'-linked phosphodiester bonds. The oligomeric mixture formed in this reaction served as the template for the formation of the corresponding oligo(G)s. In addition, it was demonstrated that 2',5'-linked oligo(C)s served as templates for the formation of oligo(G)s. These experiments demonstrate that RNA which contains exclusively 3',5'-phosphodiester bonds is not a prerequisite for template-directed synthesis of the complementary RNA. Thus, the heterogeneous oligomers formed by mineral catalysis could have been replicated.
More recently it was shown that longer RNA oligomers form by the sequential addition of activated monomers to primers bound to montmorillonite. Daily additions of ImpA to a decameric primer resulted in the formation of oligomers which contained more than 50 mers after fourteen days of addition . Since a thirty to fifty mer of RNA is believed to be sufficiently long to have catalytic activity, the next stage of this research is the synthesis of RNA polymers containing up to fifty monomer units by the stepwise addition procedure which contains complementary bases and then to search for catalytic activity in the oligomers formed. It is expected that some of the RNAs produced by mineral catalysis would be able to catalyze the reaction of other oligomers.
This search for catalytic activity will be carried out jointly between the groups of Ferris and Nierzwicki-Bauer (Biology) with strong input from Dr. Marlene Belfort (NY State Health Laboratory, Albany NY) and Dr. Jack Szostak (Harvard Medical School), Associated Scientists on the project.
Photochemistry of Planetary Atmospheres and the Origins of Life
Studies of the atmospheric chemistry of Jupiter at Rensselaer show that the photolysis of methane and ammonia initiate the formation of complex organics there. Light of wavelengths shorter than 150 nm converts methane to acetylene, ethylene and other unsaturated hydrocarbons. The longer wavelength UV light reaching the ammonia clouds dissociates ammonia into radicals which initiate the further reaction of the acetylene and ethylene formed by methane photolysis. The coupled ammonia-acetylene photochemistry yields a series of C, H, and N- containing compounds which are derived from adducts of hydrazine and acetylene .
Nitrogen, the principal constituent in the atmosphere of Titan, is believed to have been formed by the photolysis of an early Titan atmosphere which contained ammonia. The second most abundant atmospheric compound in Titan's atmosphere, methane, is currently undergoing photolysis to form more complex hydrocarbons. It is proposed from studies carried out at Rensselaer that the haze present on Titan is formed by the copolymerization of acetylene with cyanoacetylene. There may be a large reservoir of polymeric material on the surface of Titan that has been formed by photochemical reactions over the past 4.5 Gyr. The physical properties of a simulated Titan haze have been determined.
The photochemical reaction pathways by which these complex organics and polymers are formed on Jupiter and Titan are under investigation. Since solar UV light is believed to have had a profound effect on altering the composition of the atmosphere of the primitive Earth, knowledge of the photochemical pathways on Jupiter and Titan will provide important insight into how atmospheric photochemistry may have proceeded on the early earth.
If the atmosphere of the primitive earth contained methane and ammonia, then some of the photochemical reactions there were similar to those on Jupiter. But the methane and ammonia were present only for a short period of time because they are not regenerated as they are on Jupiter and because the solar UV flux was greater on the primitive Earth than on Jupiter. The continued presence of methane on Titan, after 4.5 billion years irradiation by solar UV, suggests that it is being slowly released from Titan's crust into its atmosphere. Methane may have also been slowly released into the primitive Earth's atmosphere so it may have been present there for a somewhat longer time than predicted by atmospheric modeling experiments.
It is proposed to test the postulate that a reducing atmosphere, methane, ammonia and water, may have been the principal UV absorbing gases in the the initial atmosphere of the primitive Earth. Photolysis of this mixture could have resulted in the rapid formation of complex organics which initiated the origin of life before the methane and ammonia were depleted by the solar irradiation. Since it is known that methane photolysis at wavelengths less that 150 nm generates acetylene and ethylene, mixtures containing these two carbon compounds along with ammonia and water will be photolyzed using UV light of wavelengths longer than 150 nm (at 185 nm) and the higher molecular weight C,H,N,O compounds produced will be determined. These photolyses will be performed in a photochemical flow reactor where it is possible to study small partial pressures of reactant gases yet obtain sufficient material for NMR and FTIR analysis. The use of small partial pressures of reactants is important because it is likely that only a few torr of atmospheric methane and ammonia present on the primitive Earth.
The likely abundance of water in the Earth's primitive atmosphere will limit the production of complex organic and polymers in the atmosphere of the primitive earth because photolysis of water generates hydroxyl radicals which convert methane to alcohols, aldehydes and ketones. The hydroxyl radicals may also accelerate the destruction of ammonia so it is not clear whether the compounds useful for the origins of life will be produced even during the short time that a reducing atmosphere existed on the primitive earth.
William Hagan (St. Rose, Albany), John Delano (SUNY Albany, Geology), Wayne Roberge (Physics), A. Tielens (NASA Ames Research Center) and James Kasting (Penn State), will be important contributors to the project.