Amino Acids and the Asymmetry of Life: Caught in the Act of Formation

265 Pages·2008·35.44 MB·English

by  Uwe Meierhenrich

Amino Acids and the Asymmetry of Life
Advances in Astrobiology and Biogeophysics
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Uwe Meierhenrich
Amino Acids
and the Asymmetry of Life
Caught in the Act of Formation
Prof.Dr.UweMeierhenrich
Universite´ Nice
CNRSFRE2721
ParcValrose
06108Nice
France
[email protected]
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Foreword
Abookonaminoacidsinthecontextoftheoriginoflifeandlife’shomochirality
is unusual. Professor Meierhenrich has written a very detailed scenario on the at-
tempts of scientists to propose hypotheses taking into account the stereochemical
information given by the analysis of organic molecules. One key tool refersto the
famous statement of Louis Pasteur in his lectures: “Optical activity is a signature
oflife”.
Theauthorpresentsmanyfacetsofthechemicalandphysicalprocessesinvolved
onearthorinspaceandheentersintoafascinatingareaofongoingscientificdebate.
Attimesthediversityofthetopicsinvolvedmightbedemandingforundergraduates
or newcomers to the field, but the book, while challenging, illuminates the way
science is progressing among a wide diversity of techniques, scientific areas, and
hypotheses.
WhenIstartedpreparingmyPh.D.thesisattheColle`gedeFranceinParisunder
the supervision of Dr. Jean Jacques, I was attracted by the possibility of deducing
the shape of molecules, including their absolute configuration, from experimental
data, either spectroscopic or chemical. Organic chemistry remained strongly con-
nectedtopharmaceuticalchemistry,wheremanycompoundsofbiologicalinterest
areasymmetricandopticallyactive.ThemanipulationofmolecularmodelsasDrei-
dingmodels(therewerenocomputersatthattime)allowedforanalysingthethree-
dimensionalbehaviourofmanyrigidormobilestructures.Conformationalanalysis
ofcyclohexaneandpolycyclicsystemssuchassteroidshavebeenrecentlyrational-
izedbyD.H.RBartonandO.Hassel(Nobelprizewinnersin1969).Theprediction
ofthestericcourseofreactionsthatcreateanewasymmetriccenterbecamepossible
andwasbasedonsimpleconcepts.AftermyPh.D.,Iwasinvolvedinafruitfulcol-
laboration with Prof. Alain Horeau, Head of the Organic Chemistry Laboratory at
theColle`gedeFrance.Heproposedachemicalmethodtoestablishtheabsolutecon-
figurationofanasymmetriccenterofsecondaryalcoholsRCHR(R’)OHaswellas
deuteratedprimaryalcoholsRCHD(R)OH.Iwasassociatedwiththelatestdevelop-
mentsoftheHoreau’smethod.Later,Istarted,alwaysinassociationwithA.Horeau,
toapproachthedifficultproblemofefficientlycreatinganasymmetriccenterinan
achiral molecule (asymmetric synthesis). For that purpose, we used chiral auxil-
iaries temporarily connected to a substrate or a reagent. Especially successful was
anasymmetricsynthesisthatprovidedalmostenantiomericallypureasparticacid.
vii
viii Foreword
When I became scientifically independent, I launched two completely different
projectssimultaneouslyinOrsay.Onewastheexplorationofasymmetriccatalysis
usingchiraltransitionmetals.Theotherwastorevisittheoldquestionoftheuseofa
chiralphysicalagentsuchascircularlypolarizedlight(CPL),alreadysuggestedby
PasteurandLeBelin1860–1874.Inasymmetriccatalysis,wemadeimportantdis-
coveriesinthe1970sbydevisingchiralligands(C -symmetricchiraldiphosphines)
2
forrhodiumcomplexes.Thesenewcatalystsallowedpreparationofα-aminoacids
by asymmetric hydrogenation with up to 80% enantiomeric excess (e.e.), values
whichwerethehighestatthattime.Nowadays,someenantioselectivecatalystsare
usedinindustrytopreparechiralcompoundswith99%e.e.
Oursecondprojectwasmoreinlinewiththetopicofthepresentbook.Thepres-
ence of circularly polarized light, a chiral agent, has been suggested to be present
when the prebiotic soup started to produce biomolecules. Pasteur, Le Bel, van’t
Hoff, Cotton, Kuhn, and many others considered this possibility seriously, but it
wasW.Kuhn,who,in1929–1930,firstperformedapartialasymmetricphotolysis
ofaracemiccompound(N,N-dimethylα-azidopropionamide).Therecoveredma-
terialwasopticallyactive,althoughofverysmalle.e.Thiscaseofkineticresolution
wasnotfollowedbyotherexamplesuntil1974,whenweclearlyshowedthatkinetic
resolutionofcamphorcanbecarriedoutwithCPLandaffordedrecoveredcamphor
with 20 % e.e. at high conversion. The key factor for a successful experiment is
toselectacompound withagoodgfactor(g=Δε /ε).Laterseveralgroupsestab-
lishedthatCPLcanalsobeusedtoperformsimilarexperimentsonaminoacids.In
1971,wedescribedthefirstexampleofanasymmetricsynthesisbyusinganachiral
substrate and CPL. A photolytic cyclization gave rise to hexahelicene, which was
slightly enriched in one enantiomer. This was easy to measure with a polarimeter
because hexahelicene has a huge specific rotation. With regard to the question of
whether CPL is able to produce optically active compounds, we learned that the
answer is “yes”. But what is its prebiotic significance? This question still remains
unanswered.
We were also involved in a third project that may be of interest for prebiotic
chemistry:theamplificationofasmallenantiomericexcessbycatalyticorstoichio-
metricmethods.Infact,weshowedin1986thatitispossibletouseanenantioim-
purecatalysttocreateaproductofhigherenantiomericexcessthantheoneofthe
catalyst.Thiseffectwasnamedthe“nonlineareffect”anditcanbeusefulinauto-
catalyticreactions.In2007,weestablishedexperimentally,asproposedbyI.Ugiin
1977,thatsomeracemicreagentscanincreasethee.e.ofpartiallyresolvedsamples
thankstoaselectivedestructionofsomeracemiccontentinthesamples.
Thepresentbookdiscussesmanywaystoinitiateordevelopopticalactivityfrom
achiral or racemic materials. The outlined space exploration and the study of chi-
ral molecules are certainly of great interest to better understand the origin of ho-
mochirality of biological systems. The underlying problem, however, may remain
unsolved for a long period of time, if not forever. The scientific subject is intel-
lectually appealing, and it has great potential for attracting young people towards
chemistryaswellasthephysicalandnaturalsciences.
Orsay,France HenriB.Kagan
May2008
Foreword ix
“Now,ifyou’llonlyattend,Kitty,andnottalksomuch,I’ll
tellyouallmyideasaboutLooking-glassHouse.First,there’s
theroomyoucanseethroughtheglass–that’sjustthesame
asourdrawingroom,onlythethingsgotheotherway.Ican
seeallofitwhenIgetuponachair–allbutthebitbehindthe
fireplace.Oh!IdosowishIcouldseethatbit!Iwantsomuch
toknowwhetherthey’veafireinthewinter:younevercan
tell,youknow,unlessourfiresmokes,andthensmokecomes
upinthatroomtoo–butthatmaybeonlypretence,justto
makeitlookasiftheyhadafire.Wellthen,thebooksare
somethinglikeourbooks,onlythewordsgothewrongway;I
knowthat,becauseI’vehelduponeofourbookstotheglass,
andthentheyholduponeintheotherroom.”
“HowwouldyouliketoliveinLooking-glassHouse,
Kitty?Iwonderifthey’dgiveyoumilkinthere?Perhaps
Looking-glassmilkisn’tgoodtodrink–Butoh,Kitty!nowwe
cometothepassage.Youcanjustseealittlepeepofthe
passageinLooking-glassHouse,ifyouleavethedoorofour
drawing-roomwideopen:andit’sverylikeourpassageasfar
asyoucansee,onlyyouknowitmaybequitedifferenton
beyond.Oh,Kitty!howniceitwouldbeifwecouldonlyget
throughintoLooking-glassHouse!I’msureit’sgot,oh!such
beautifulthingsinit!
Let’spretendthere’sawayofgettingthroughintoit,
somehow,Kitty.Let’spretendtheglasshasgotallsoftlike
gauze,sothatwecangetthrough.Why,it’sturningintoasort
ofmistnow,Ideclare!It’llbeeasyenoughtogetthrough”–
Shewasuponthechimney-piecewhileshesaidthis,though
shehardlyknewhowshehadgotthere.Andcertainlytheglass
wasbeginningtomeltaway,justlikeabrightsilverymist.
InanothermomentAlicewasthroughtheglass,andhad
jumpedlightlydownintotheLooking-glassroom.Thevery
firstthingshedidwas...
LewisCarrollin
ThroughtheLooking-Glass,andWhatAliceFoundThere