Recent Efforts Towards a Systems-Level Hypothesis for an Early RNA World

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Chemistry Department Faculty Citations Chemistry

2015

Reaching Back to Jump Forward: Recent Efforts towards a Systems-Level Hypothesis for an Early RNA World

Greg Springsteen Furman University

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Recommended Citation Springsteen, Greg, "Reaching Back to Jump Forward: Recent Efforts towards a Systems-Level Hypothesis for an Early RNA World" (2015). Chemistry Department Faculty Citations. 347. https://scholarexchange.furman.edu/chm-citations/347

This Citation is made available online by Chemistry, part of the Furman University Scholar Exchange (FUSE). It has been accepted for inclusion in Chemistry Department Faculty Citations by an authorized FUSE administrator. For terms of use, please refer to the FUSE Institutional Repository Guidelines. For more information, please contact [email protected]. DOI:10.1002/cbic.201500233 Highlights

Reaching Back to Jump Forward:Recent Efforts towards aSystems-Level Hypothesis for an Early RNA World Greg Springsteen*[a]

Experimental approaches toward discovering the chemical ori- group in 1970 published aremarkable synthesis of acytidine gins of life are often criticized for their lack of holistic guiding nucleoside from ribose, cyanamide, and cyanoacetylene hypotheses;though justifiable, these assessments are mitigat- (Scheme 1A).[8,9] The direct condensation of ribose and acanon- ed somewhat by the general recognition that this is areally, ical pyrimidine heterocycle had never been observed under really hard problem. prebiotically plausible conditions (andstill hasnot). However, For the moment, let us set aside the challenges of demon- Orgel’s work side-stepped this difficult nucleosidation by clos- stratingthe template-catalyzed polymerization of an RNA repli- ing the pyrimidine ring on aprebuilt ribose scaffold;interest- cator that avoidsend-product inhibition, and focus solely on ingly,modernbiology uses the same strategy. apotential prebiotic synthesis of asingle pyrimidine nucleo- However,from the perspective of jumping directly from side. The challenges inherent to the construction of just this small molecules to RNA monomers, this remarkable synthesis monomer are formidable. The heterocyclic, saccharide, and leaves some gaping holes. For example, how is ribose selected phosphate chemical components of the monomer seemingly from the multitude of sugars produced from formaldehyde in requiremultiple incompatible chemistries—and,bythe way, aformose reaction?How is acontinuous synthesis of nucleo- these final building blocks do not react even when collected side maintained when the reactants, cyanamide and cyanoace- together. tylene,must be added sequentially,and without overlap,to However, this paradigm regardingthe prebiotic synthesis of this ribose?How is the primary a-cytidine product epimerized pyrimidine nucleosides haschanged in the last few years, pre- to the canonical b-cytidine (Scheme 1A)? dominantly due to the efforts of the Sutherland group,[1–6] The work of Sutherland’s group has unified disparate issues which culminated in arecent paper in Nature Chemistry.[7] The within this prebiotic pyrimidine synthesis in acontext that po- narrative of these discoveries beginswith the work of alegen- tentially connects pyrimidine formation with biologically rele- dary figure in origins of life chemistry,Leslie Orgel, whose vant amino acid andglycerolsyntheses. In 2009, the Suther-

Scheme1.A) a-Cytidine is synthesizedbytreating ribosewith cyanamide, followed by cyanoacetylene and then hydrolysis. b-Arabinocytidine and a-cytidine- 5P are produced by replacing ribose with arabinose and ribose-5P,respectively(~10–30 %yield of furanose anomers).[8] B) The canonical b-cytidine is generated from arabinose-3P (~4% overall yield).[1, 10] C) The Sutherlandgroup discovered that aselectivesource of pentoses was no longer necessary when the reaction was startedwith the small molecules glyceraldehydeand 2-amino-oxazole (glycolaldehyde +cyanamide; ~ 15%yield).

land lab demonstrated a b-cytidinenucleotide synthesis from [a] Dr.G.Springsteen the simple sugar precursors glycolaldehyde (C )and glyceral- Department of Chemistry,Furman University 2 3300 Poinsett Highway,Greenville, SC 29613(USA) dehyde(C3), thus avoiding the necessity for aselective ribose E-mail:[email protected] synthesis (Scheme 1C).[3] The condensation of glycolaldehyde

ChemBioChem 2015, 16,1411–1413 1411 2015 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Highlights with cyanamide produces 2-aminooxazole, which subsequently ductionand hydrolysis (a Kiliani–Fischer synthesis) can be re- condenses with glyceraldehyde, and then cyanoacetylene to peatedtoproduce glyceraldehyde, aC3 sugar,from glycolalde- form the anhydro-arabino-nucleoside (I,Scheme 1C). Although hyde. Bothsugars were detected as their oxazolidinones, dependentupon aspecific order of reactant addition, each which were produced by spontaneous condensation of the step in the synthesis of I proceeds in amild aqueous environ- aldehydes with isocyanic acid (Scheme 2C). These oxazolidi- ment and in remarkably good overall selectivity and yield. A nones are not productive in pyrimidine synthesis, and thus re- regioselective phosphorylation of I is achievedbyusing ammo- searchefforts were begun to find aprebiotically plausible sac- nium phosphate suspended in heated urea/formamide. Spon- rificialreductant that would avoid isocyanic acid formation, taneous phosphorolysis generates the b-cytidine nucleotide and the resulting irreversible glycol- and glyceraldehyde se- containing an active cyclic phosphate diester (II,Scheme 1C). questration.[6,7] Hydrogen sulfide was found to be both asuita-

Beginning the synthesis with C2 and C3 sugar fragments, ble and plausible reagent. rather than ribose, was asignificant advance in demonstrating The reaction of hydrogensulfide with cyanohydrin in the aplausible prebiotic synthesis of pyrimidine nucleosides. Sub- presence of catalytic CuI produces glycolaldehyde in over 40% sequently,the Sutherland group sought to reach even further yield (Scheme 3A).[6] As oxazolidinone formationisprevented, back to asugar C1 feedstockthat is compatible with the HCN the generation of other carbonyl compounds such as glycer- chemistry required for thegeneration of cyanoacetylene and aldehyde (by reductive homologation of glycolaldehyde, cyanamide reactants.[5–7] The oxidation of HCN to cyanoacetyl- Scheme3B) as well as acetaldehyde and formaldehyde (by re- ene and cyanamide can be achieved by electric dischargein ductionofglycolaldehyde)becomes more evident. Interesting- the gas phase or photo-oxidation in aqueous solution.[9] HCN ly,subsequentaddition of 1equiv of cyanide and excess am- is also vital to numerousfoundationalsyntheses in prebiotic monia at pH 9slowly produces the nitrile analogues of serine, chemistry including HCN polymerization to purines and the glycine,and alanine from glycolaldehyde, formaldehyde, and Strecker reaction to aminoacids. acetaldehyde, respectively (Scheme 3C). Additionally,the ami- The reductionofHCN to the aldehydic oxidation state re- nonitriles are more resistant to reduction than the cyanohy- quired for sugar chemistry is challenging;the reductantmust drins, thus providing arationale for the generation of nucleo- be able to form, but not over-reduce,the imine/aldehyde tides from the reduction of cyanohydrins (NC-CH2-OH) and product. The Sutherland group settled on adisproportionation amino acids from the hydration of aminonitriles(NC-CH2-NH2). of HCN catalyzed by alight-activated CuI complex These results potentially link amino acid and nucleoside syn- (Scheme 2A).[5] The products of the disproportionation are thesis to common buildingblocks and acommonenviron- formaldehyde and isocyanic acid. Subsequent reaction of the ment. formaldehyde with cyanidegenerates cyanohydrin, which is In their most recent work,[7] the Sutherland group has reduced and hydrolyzedtoglycolaldehyde, aC2 sugar shownthat glyceraldehyde, produced from the reductive ho- (Scheme 2B). This procedure of cyanideattack followed by re- mologation of glycolaldehyde, tautomerizes to dihydroxyacetone in 60 %yield in phosphate buffer (Scheme 4). Reduction of dihydroxyacetone produces both acetone and glycerol, again in high yields. Glycerol was quantitatively phosphorylat- ed into amixture of phosphate isomersbyheatingwith ammonium phosphate in urea/formamide. Glycerol phosphates are the fatty ester/ether linkersrequired for membrane synthesis across all of biology,and represent another cog in apoten-

Scheme2.A) An irradiated (254 nm) copper(I)-catalyzed disproportionation of hydrogen cyanide produces formaldehydeand isocyanic acid.B)Reduc- Scheme3.A) The reduction of cyanohydrin produces glycolaldehyde in 42% tive homologation of formaldehyde and glycolaldehyde with cyanide gener- yield (after 2h), and acetaldehyde in 15 %yield (after 6h). B) Glyceraldehyde ates glycolaldehyde (15 %) and glyceraldehyde (1–5%), respectively.C)Gly- is produced in 17%yieldbythe reductive homologation of glycolaldehyde. colaldehyde and glyceraldehyde are trapped by isocyanic acid as oxazolidi- C) The aminonitrile precursors of amino acids are generated from the nucle- nones. ophilic addition of cyanide to an imine.

ChemBioChem 2015, 16,1411–1413 www.chembiochem.org 1412 2015 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Highlights

Scheme5.The synthesis of pyrimidine nucleotides, amino acids, and glycerol phosphate from cyanohydrin. The following reaction types are employed:1)Re- ductionwith NaSH, cat. CuI,and irradiation at 254 nm, 2) reaction with cyanamide (potentiallysourced from calcium ferrocyanide), 3) reaction with cyanoacetylene (potentially sourced from HCN and calcium carbide), 4) dehydrative phosphorylation, accomplishedexperimentallywith inorganic phosphate in hot urea/formamide, and 5) Strecker syntheses with HCN and ammonia (potentially sourcedfrom the hydration of magnesium nitride).

allowed this isolatedchemistry to occur before recombination. Though this argumentisnot, on its surface, as “aesthetic” as the field might hope, it stillrepresents the most holistic, exper- imentally supported hypothesisonthe origins of an RNA world directly from small buildingblocks.

Acknowledgements

G.S acknowledges support from the NSF and the NASA Astrobiol- ogy Program under the NSF Center for ChemicalEvolution, CHE1004570. Scheme4.Glyceraldehyde at RT in pH 8phosphatebuffer tautomerizes to dihydroxyacetone in ~60%yieldafter 20 days. Reduction of dihydroxyacetone generates glycerol in ~35%yield and acetonein~30%yield. Phos- Keywords: glyceraldehyde · glycolaldehyde · origins of life · phorylation with ammonium phosphate at 120 8Cfor 7days produces glyceraldehyde phosphateisomersinnear quantitative yield. a) NaSH, 10 mol% pyrimidines · RNA world

CuCN, hn (254 nm);b)HOPO3À in urea/formamide. [1] A. A. Ingar,R.W.Luke, B. R. Hayter, J. D. Sutherland, ChemBioChem 2003, 4,504 –507. [2] C. Anastasi, M. A. Crowe, M. W. Powner,J.D.Sutherland, Angew.Chem. tial systems-level hypothesis for the origin of multiple biologi- 2006, 118,6322 –6325. cal polymers from asingle source. The acetone also provides [3] M. W. Powner,B.Gerland, J. D. Sutherland, Nature 2009, 459,239 –242. an entry into valine and leucine amino acid syntheses through [4] M. W. Powner,J.D.Sutherland, Angew.Chem. 2010, 122,4745 –4747. two and three, respectively,reductive homologationswith cya- [5] D. Ritson, J. D. Sutherland, Nat. Chem. 2012, 4,895–899. [6] D. J. Ritson,J.D.Sutherland, Angew. Chem. Int. Ed. 2013, 52,5845 – nide and sodium sulfide. 5847; Angew.Chem. 2013, 125,5957 –5959. The Sutherland group has proposed ascenario to link three [7] B. H. Patel,C.Percivalle, D. J. Ritson,C.D.Duffy,J.D.Sutherland, Nat. major prebiotic syntheses back to acommon environment Chem. 2015, 7,301 –307. containing cyanide and hydrogensulfide. Experimental verifi- [8] R. A. Sanchez,L.E.Orgel, J. Mol. Biol. 1970, 47,531–543. [9] J. P. Ferris,R.A.Sanchez, L. E. Orgel, J. Mol. Biol. 1968, 33,693–704. cation of the scenariodemonstratesasuccession of selective, [10] C. M. Tapiero,J.Nagyvary, Nature 1971, 231,42–43. high-yielding reactions.The overall scheme still requiresmulti- ple incompatiblechemistries (arguably 1–5, Scheme 5) that re- Manuscript received:May 7, 2015 quire separation prior to pooling. Ascenario is described[7] in Accepted article published:May 25, 2015 which streams, fed by rainand mineral deposits, might have Final article published:June 1, 2015

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