1Origin of Life: IMonomers to PolymersSynthesis of MonomersLife arose early on Earth (within 0.7 ¥ 109 y) 1. Conditions1. Liquid Water2. Reducing or Neutral Atmosphere3. Energy Sources22. Originally thought atmosphere wasNH3, CH4, H2O, H2Miller -Urey ExperimentNow Believe CO2, H2O, N23. Energy SourcesUltraviolet Light (No Ozone)LightningGeothermal (Lava, Hot Springs, Vents, …)Miller -Urey Experiment3Relative Yield27021145464210.42266613310006456211786COMPOUNDGlycineSarcosineAlanineN-methylalanineBeta-alanineAlpha-amino-n-butyric acidAlpha-aminoisobutyric acidAspartic acidGlutamic acidIminodiacetic acidIminoacetic0propionic acidLactic acidFormic acidAcetic acidPropionic acidAlpha-hydroxybutyric acidSuccinic acidUreaN-methyl ureaHow did Amino Acids form in Miller -Urey Experiment?Strecker SynthesisCH4, H2, NH3 + Energy H2CO, HCN, HC3N,e.g. Glycine Synthesis Urea (H2 NCONH2)ReactiveH2CO + NH3 + HCN N C C ≡ N + H2OHHHHAminoacetonitrile4N C C ≡ N + H2O + H2O HHHHN C C + NH3HHHHOO HglycineH2CO C = O form AldehydeHHMore complex group - other aldehydes more complex amino acidsLower yield if atmosphere was N2, CO2, H2O(If H2/CO2 > 2, get good yield)Problems with Miller - UreyAtmosphere was N2, CO2, H2ONH3, CH4 would react N2, CO2Try N2, CO2, H2O in Miller - Urey simulationOnly get trace amounts of glycineNeed CH4 to get more complex amino acidsNeed H2/CO2 > 2 to get much of any amino acid5Miller - Urey with Cosmic RaysA group in Japan has obtained good yields ofamino acids from slightly reducing gases(CO2, CO, N2, H2O)When they used high energy protons(simulate cosmic rays)Apparently not Strecker Synthesis(Low abundance of aminoacetonitrile)Building Blocks of Nucleic AcidsNot formed in Miller - Urey But some intermediates were1. Ribose Sugar: 5 • H2CO + Heat H10C5O5 [Clay Catalyst]2. Basesa) Purines 5 HCN H5C5N5(Adenine)b) Pyrimidines HC3N + Urea H5C4N3O (Cytosine)(1995) Cyanoacetaldehyde + Urea Uracil63. PhosphateRock ErosionLess understood than amino acidsOther Possibilities:Seafloor VentsInterstellar MoleculesCometsAlternative Delivery Molecular clouds - strongly reducing, contain manymolecules used in Miller-Urey (H2, NH3, H2O, CH4)and intermediates (HCN, H2CO, HC3N) and possiblyglycine Problem: These would not have survived in part ofdisk where Earth formed But interstellar ices comets Evidence from similar molecules (e.g. C2H2, CH4, HNC, …) Clearly indicates interstellar chemistry7Cratering record on moon, …fi heavy bombardment early in historyComets and their debris could have broughtlarge amounts of “organic” matter to Earth(and maybe oceans)Some evidence for non-biological amino acidsin layer depostied after asteroid impact 65million years agoSources of Organic MoleculesQuantitative comparison by Chyba & Sagan, Nature1992, Vol. 355, p. 125Currently, Earth accretes ~ 3.2 ¥ 106 kg y-1 frominterplanetary dust particles (IDP)~ 10 % organic carbon fi 3.2 ¥ 105 kg y-1~ 103 kg y-1 comets~ 10 kg y-1 meteorites~ 103 ¥ more at 4.5 ¥ 109 yr ago (?)(cratering record)UV + reducing atmosphere 2 ¥ 1011 kg y-1But if H2/CO < 0.1 IDP’s dominant source~8So if atmosphere very neutral, IDP’sMost of mass in IDP’s in range of size ~ 100 mmmass ~ 10-5 gComplex structure - composites of smaller grainssome carbon richEnhanced deuterium low TAlso found in interstellar molecules? fi connection back to interstellar chemistry2 kinds(mass ranges) can supplyorganic matter1.Interplanetary dust particles (m < 10-5 g)2.Smaller meteorites (m < 108 g)~~Mass Accretion Rate on Earth9Alternative SitesLocally reducing environments1. Ocean ventsSources of CH4 and H2SCurrent Vents have ecosystems based onenergy from chemicals - not photosynthesisH2S Bacteria Clams, Tube WormsPre-biotic amino acid synthesis?2. Inside EarthMany bacteria now known to live deep(~ 2 miles) in Earth. Again, on chemicals,adapted to high temperature genetic makeupvery ancient3. Hot SpringsBacteria may be important in precipitatingminerals. again adapted to high T andancient10Synthesis of PolymersM1 + M2 P + H2O more likely in liquid H2OSolutions Remove H2O (Drying, Heat)Sydney Fox ProteinoidsEnergy Releasing Reactions (H2NCN or HC3N) Catalysts: ClaysMonomers polymersA problem:Peptide bond requires removal of H2OThis would be hard in primordial seaNeed special molecules to do what Ribosome does inliving cellsInput of energyorDry environment (dry land)Imagine drying tidepool + geothermal heatHeat + amino acids peptide bondsSidney Fox “proteinoids”or catalyst - clay, energy-rich bonds…11Problem greater for nucleic acidsSugar + base + heat some nucleosidesActivated nucleosides + phosphoric acid + Zn+2polymers up to 50 nucleotideslinkages (mostly) correctnucleosidenucleotideNucleic acids more complexMonomers of nucleic acids12Synthesis of AdenosineBase on 1’ Carbon Why?Adenine + ribose sugar adenosine + H2OAlso phosphates 3’ & 5’ carbonsOtherwiseLeslie Orgel has had some success ingetting high percentage of correct linkages,in presence of Zinc ions.Misalignment13The Odds• We need to get an “interesting” polymer– Enzyme– Self replicator• Properties of polymer depend on– Order in which monomers combine• If we combine monomers at random,– How likely to get something interesting?14Statistics of an unlikely eventRandom reactions in primordial soup?Unlikely event versus many trialsProbability Consider tossing 10 coinsProbability of all heads = product of prob.P = 1 1 1 1 12 2 2 2 2 ( )( )( )( )( )…( ) 1210= 11024Probability of getting 10 amino acids proteinChosen from 20 in a particular order 120( )10= 1 1 ¥ 1013But if you try many times, the chance ofsuccess is higherP(r) =n!r! (n - r)!pr (1 - p)n-rr = # of successes p = prob. Of success on each trialn = # of trialsn! = n (n-1) (n-2) … 1e.g. make n = (flip all 10 coins 1024 times)1pP(1) =n!1! (n - 1)!1n( )( )1- 1nn - 1= 0.37Chance of one or more successes = 0.63For reasonable chance need n ~1p15How many do we have to get right?1. How many atoms? Lipids 102 - 103 Enzymes, RNA 103 - 105Bacterial DNA 108 - 109Bacterium 1011 -
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