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Synthesis of Activated Pyrimidine Ribonucleotides in Prebiotically Plausible Conditions

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Synthesis of Activated Pyrimidine Ribonucleotides in Prebiotically Plausible Conditions Vol 459 | 14 May 2009 | doi:10.1038/nature08013 LETTERS Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions Matthew W. Powner1,Be´atrice Gerland1 & John D. Sutherland1 At some stage in the origin of life, an informational polymer must we have discovered a short, highly efficient route to activated pyrimi- have arisen by purely chemical means. According to one version of dine ribonucleotides from these same precursors that proceeds by way the ‘RNA world’ hypothesis1–3 this polymer was RNA, but attempts of alternative intermediates (Fig. 1, green arrows). By contrast with to provide experimental support for this have failed4,5. In parti- previously investigated routes to ribonucleotides, ours bypasses ribose cular, although there has been some success demonstrating that and the free pyrimidine nucleobases. Mixed nitrogenous–oxygenous ‘activated’ ribonucleotides can polymerize to form RNA6,7,itis chemistry first results in the reaction of cyanamide 8 and glycolalde- far from obvious how such ribonucleotides could have formed from hyde 10, giving 2-amino-oxazole 11, and this heterocycle then adds to their constituent parts (ribose and nucleobases). Ribose is difficult glyceraldehyde 9 to give the pentose amino-oxazolines including the to form selectively8,9, and the addition of nucleobases to ribose is arabinose derivative 12. Reaction of 12 with cyanoacetylene 7 then inefficient in the case of purines10 and does not occur at all in the gives the anhydroarabinonucleoside 13, which subsequently under- case of the canonical pyrimidines11. Here we show that activated goes phosphorylation with rearrangement to furnish b-ribocytidine- pyrimidine ribonucleotides can be formed in a short sequence that 29,39-cyclic phosphate 1. In a subsequent photochemical step, 1 is bypasses free ribose and the nucleobases, and instead proceeds partly converted to the corresponding uracil derivative, and synthetic through arabinose amino-oxazoline and anhydronucleoside inter- co-products are largely destroyed. mediates. The starting materials for the synthesis—cyanamide, We had previously shown that in unbuffered aqueous solution, cyanoacetylene, glycolaldehyde, glyceraldehyde and inorganic 2-amino-oxazole 11 adds to glyceraldehyde 9 to give the pentose phosphate—are plausible prebiotic feedstock molecules12–15, and amino-oxazolines including 12 in excellent overall yield20. Our start- the conditions of the synthesis are consistent with potential ing point in the present work was therefore to find a prebiotically early-Earth geochemical models. Although inorganic phosphate is only incorporated into the nucleotides at a late stage of the O O OH sequence, its presence from the start is essential as it controls three 10 OH HO OH reactions in the earlier stages by acting as a general acid/base cata- HO O lyst, a nucleophilic catalyst, a pH buffer and a chemical buffer. For OH 4 (p) prebiotic reaction sequences, our results highlight the importance 9 OH N HO of working with mixed chemical systems in which reactants for a NH H2N CN 2 O –H O OH particular reaction step can also control other steps. 8 2 O 11 HO Because they comprise phosphate, ribose and nucleobases, it is +H O 2 O HO OH 4 (f) tempting to assume that ribonucleotides must have prebiotically N H N NH2 2 NH assembled from such building blocks. Thus, for example, it has previ- HO O 2 O 12 NH2 ously been supposed that the activated ribonucleotide b-ribocytidine- 6 N 29,39-cyclic phosphate 1 must have been produced by phosphorylation X –H O 2 N O of the ribonucleoside 2, with the latter deriving from the conjoining of H –H2O O 3 the free pyrimidine nucleobase cytosine 3 and the furanose form of CN 5 ribose 4 (Fig. 1, blue arrows). This mode of assembly is seemingly NH HO 2 supported by the facts that cytosine 3 can be synthesized by condensa- +H O O N N tion of cyanoacetaldehyde 5 and urea 616 (the hydration products of 2 cyanoacetylene 717, and cyanamide 818, respectively) and pentoses CN O 7 HO2 OH including ribose can be produced by aldol reaction of glyceraldehyde HO OH 8,9 P 9 and glycolaldehyde 10 . The insuperable problem with this –2H2O O O– NH2 approach, however, is that one of the presumed steps, the condensa- Pi HO 11 HO O tion of ribose 4 and cytosine 3, does not work . The reasons for this are O N N N + both kinetic (the N1 lone pair of 3 is unavailable owing to delocaliza- O NH2 –H2O O N O O tion) and, in water, thermodynamic (the equilibrium constant is such HO 13 P thathydrolysis of2 to 3 and4 isfavouredovercondensation).The same OO– 1 is true for ribosylation of uracil, which has also not been demonstrated. Figure 1 | Pyrimidine ribonucleotide assembly options. Previously assumed We have considered a large number of alternative ribonucleotide synthesis of b-ribocytidine-29,39-cyclic phosphate 1 (blue; note the failure of assembly modes, including those that extend back to the same small- the step in which cytosine 3 and ribose 4 are proposed to condense together) molecule precursors as the traditionally assumed route described and the successful new synthesis described here (green). p, pyranose; f, above19. By systematic experimental investigation of these options, furanose. 1School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, UK. 239 ©2009 Macmillan Publishers Limited. All rights reserved LETTERS NATURE | Vol 459 | 14 May 2009 plausible synthesis of 11. Constitutionally, 11 is the condensation We then investigated whether the subsequent reaction of 11 with product of 8 and 10, and although there exists, in the conventional glyceraldehyde 9 would be tolerant to the residual presence of phos- chemical literature, a procedure to bring about this condensation, it phate. In the absence of phosphate, the ribose and arabinose amino- requires strongly alkaline conditions21. Because we wanted to oxazolines 21 and 12 are the major products, and the xylose derivative generate 11, and then allow it to react with 9, which is unstable to 22 is a minor product (Fig. 3a)20. The lyxose amino-oxazoline 23 is alkali, under the same conditions, neutral-pH reaction conditions formed in intermediate amounts as an equilibrating mixture of had to be found. pyranose and furanose isomers. All of the pentose amino-oxazolines We initially investigated the reaction with 8 and 10 in a 1:1 ratio have the potential to be converted reversibly into one or other of the starting at neutral pH in unbuffered aqueous solution. Only a small 5-substituted 2-amino-oxazoles 24 and 25 by phosphate catalysis (by amount of 11 was produced under these conditions, and 1H NMR chemistry similar to that underlying the conversion of 16 to 11), but to spectra were indicative of the formation of a variety of carbonyl addi- differing extents depending on their stability. After one day in the tion adducts and other intermediates, for example 14–18 (Fig. 2a, b). presence of phosphate, all of the amino-oxazolines showed some con- The carbonyl addition adducts 14 were presumably formed reversibly, version to the corresponding 5-substituted 2-amino-oxazole (24 or and so did not represent material irretrievably committed to other 25), but the lyxose amino-oxazoline 23 proved the least stable and products, but rather intermediates stalled en route to 11. At low con- underwent the greatest conversion (Fig. 3b). We then took a crude centrations of hydroxide, it appeared that two additional types of sample of 11 that had just been prepared from cyanamide 8 and reaction needed to make 11 were very sluggish: intra-adduct attack glycolaldehyde 10 in the presence of phosphate, and added glyceralde- of the glycolaldehyde-derived hydroxyl group on the cyanamide- hyde 9 to it. After overnight incubation, 1H NMR analysis revealed that derived nitrile carbon (for example 14 (n 5 0) R 15), and C–H depro- although all four amino-oxazolines were still formed, the lyxose deri- tonation leading to aromatization (17 R 11). vative 23 was selectively depleted and was now a minor product along Denied the opportunity of using hydroxide as a specific base cata- with the xylose derivative 22 (Fig. 3c). With two of its stereoisomeric lyst to accelerate these slow steps, we sought a general base catalyst that relatives now minor products, the path from the arabinose amino- could provide the same acceleration, but at neutral pH. Inorganic oxazoline 12 to ribonucleotides looked clearer. Selective crystallization phosphate seemed to be ideal in this regard because its second pKa of ribose amino-oxazoline 21 offers a further means of enriching 12 value is close to neutrality. Furthermore, as phosphate is ultimately such that it becomes the major product in solution20,22. needed in some form to make activated nucleotides, we decided to We then proceeded to the second stage of pyrimidine nucleobase include it from the start of the assembly sequence. We repeated the assembly. Although our focus was on the chemistry of the key arabinose earlier reaction of cyanamide 8 and glycolaldehyde 10, but in the amino-oxazoline 12, the corresponding chemistry of the ribose amino- presence of 1 M phosphate buffer at pH 7.0. 1H NMR analysis revealed oxazoline 21 was also studied (Supplementary Information). It had that 2-amino-oxazole 11 was produced in .80% yield (75% isolated earlier been shown that in unbuffered aqueous solution, 12 reacts with yield) (Fig. 2c). With an excess of 8 over 10, the synthesis of 11 still an excess of cyanoacetylene 7 giving b-arabinocytidine 26,(Fig.4a)23. takes place in the presence of phosphate, but is followed by slower The yield of 26 was relatively low, however, and we used 1HNMR phosphate addition to residual 8 giving the intermediate adduct 19, analysis to determine why.
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