ChemicalYearInReview3.docx
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ChemicalYearInReview3.docx
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ChemicalYearInReview3
ChemicalYearInReview2008
SteveRitter,NathanBecker,andTchadBlaireditedandproducedC&EN'sChemicalYearInReview
C&ENproduces51issueseachyearcontainingseveralhundredarticlesonimportantresearchadvancesinchemistry.OurannualChemicalYearInReviewincludesourchoicesforsomeofthesuperlativeachievementsthatwefeaturedthisyearandprovidesanopportunitytoreflectontheramificationsofthesedevelopments.Thesechoices,displayedlargelyinchronologicalorder,arenecessarilysubjective,andwedonotpretendthattheyarecomprehensive.Indeed,thesestudiesrepresentonlyafewexamplesofthemanywaysinwhichchemistsarepushingtheboundariesofwhatweknowandwhatwecando.
1.SurpriseFromSN2Snapshots
Byanalyzingsnapshotsofsingle-moleculecollisions,aninternationalresearchteamuncoveredunanticipateddetailsaboutthemechanismoftheclassicbimolecularnucleophilicsubstitution(SN2)reaction(C&EN,Jan.14,page9;Science2008,319,183).InadditiontoprovidingdirectevidenceforthetraditionalSN2mechanisminthegasphase,RolandWesterandcolleaguesattheUniversityofFreiburg,inGermany,togetherwithWilliamL.HaseandcolleaguesatTexasTechUniversitydetectedanadditionalunexpectedpathway.Inthis"roundaboutmechanism"thenucleophilebumpsintothesubstrate,spinningthesubstrate360ºbeforesubstitutionandleaving-groupdisplacementoccur.Tovisualizethesedetails,theFreiburgteamcrossedbeamsofreactantsinavacuumanddetectedthedirectionalandspeeddistributionsoftheproducts.Thedatasuggestedatleasttworeactionmechanisms,oneofwhichisthetraditionalSN2pathway.Atomic-levelchemicaldynamicssimulationsbytheTexasTechscientistswereinaccordwiththatconclusion,buttheyalsodiscoveredthatsomedatawereduetotheroundaboutmechanism."Beforeperformingthesimulationswewouldneverhavethoughttheroundaboutmechanismwasinplay,"Hasesaid.Theseresults"challengesomeofourcherishedmodelsofelementaryreactionmechanisms,"saidBenjaminJ.Whitaker,areactiondynamicsexpertattheUniversityofLeeds,inEngland.
2.PeekingAtCatalyticReactions
Withanewnuclearmagneticresonanceimagingtechnique,researcherswereabletogetaneyeoncatalytichydrogenationreactionstakingplaceinsidemicroreactors(C&EN,Jan.28,page13;Science2008,319,442).Thescientistsexpectthemethodtobeusefulforstudyingandoptimizingindustrialcatalystsandcatalyticreactors.AlexanderPines,Louis-S.Bouchard,andcoworkersatLawrenceBerkeleyNationalLaboratoryandtheUniversityofCalifornia,Berkeley,developedthemethodbytakingadvantageoftheparaformofhydrogen(p-H2)toamplifytheNMRsignalofpropanemadebythehydrogenationofpropene.Onitsown,p-H2hasnoobservableNMRsignal,butinahydrogenationreactionthepairedhydrogenatomsbecomemagneticallyinequivalent,resultinginanenhancedNMRsignal.Theresearchersusedthemethodtomappropaneformationandtherebypinpointactiveregionsinthemicroreactorcatalystbed.
3.MeasuringAtomicFriction
Forthefirsttime,scientistsdeterminedtheforcesneededtomoveasingleatomacrossasurface.Withthisadvance,it'snowpossibletoquantifyfrictionattheatomiclevel.Knowinghowtightlyanatomoramolecule"sticks"toasurfacewillbeinvaluableinnanoelectronicsdesignandinbioengineering.AndreasJ.HeinrichandMarkusTernesofIBM'sAlmadenResearchCenter,inSanJose,Calif.;physicsprofessorFranzJ.GiessibloftheUniversityofRegensburg,inGermany;andcolleaguesusedamodifiedatomicforcemicroscopeatultrahighvacuumandatemperatureofonly5Ktomeasureboththelateralandverticalforcesexertedonacobaltatomandcarbonmonoxidemoleculeasthemicroscopetipdraggedthemacrossaplatinumorcoppersurface(C&EN,Feb.25,page6;Science2008,319,1066).Sincethe1990s,manygroupshavesucceededinmanipulatingsurfaceatoms,butmeasuringtheforcesassociatedwiththatmovementissomething"wealldreamedabout,"saidLudwigBartels,achemistryprofessorattheUniversityofCalifornia,Riverside.
4.
MolecularTargetForDEETDiscovered
SinceWorldWarII,peopleacrosstheglobehaveliberallyappliedN,N-diethyl-m-toluamide,betterknownasDEET,towardoffmosquitoesandotherinsectsthatnotonlyareannoyingbutcantransmitdiseases.ButnooneknewexactlyhowthemostcommonlyusedactiveingredientintopicalbugrepellantsworkeduntilLeslieB.VosshallandcolleaguesatRockefellerUniversityreportedthatDEETinhibitscertainmosquitoandfruitflyolfactoryreceptors(C&EN,March17,page42;Science2008,319,1838).PreviousworkbyotherresearchershadshownthatDEETaffectsinsects'abilitytosniffouthumanodors,suchasthosecausedbylacticacid.ButVosshallandcolleagueswerethefirsttopinpointDEET'smoleculartargets:
olfactoryreceptorsthatformacomplexwithacoreceptorcalledOR83b.Tofindthesereceptors,theresearcherscombinedageneticapproachwithaninvitroreconstitutionofinsectodorantreceptors.KnowinghowDEETaffectsreceptorscouldleadtonewinsectrepellentsthat,forexample,couldmoresafelybeusedonyoungchildren.
5.ImagingTechniqueTracksSugarsInLiveAnimals
Itisnowpossibletoimagecarbohydratesastheyareproducedoncellsurfacesofliveanimals,thankstoaresearchteamattheUniversityofCalifornia,Berkeley(C&EN,May5,page8;Science2008,320,664).Cellglycans(oligosaccharides)arekeymarkersandmediatorsofmanyphysiologicalprocesses,butpreviouslytheycouldn'tbevisualizedinlivingorganisms.CarolynR.Bertozziandcoworkersfedazide-derivatizedsugarstozebrafish,whichincorporatedthemodifiedsugarsintotheirglycans.Theresearchersthenuseddifluorinatedcyclooctynereagentswithattachedfluorescentprobestomarktheglycanssotheycouldbevisualizedbyopticalimagingtechniques.Theapproachmakesitpossibletoobservepatternsofglycanexpressionthatpreviouslywouldhavebeenundetectable.Thescientistsdemonstratedthemethodbyusingittodetectchangesinglycanbiosynthesisinspecificlocationsinlivezebrafish.Theabilityofthemethod"todeterminehowglycansaredistributedspatiallywithinatissueandintimecanhelptorevealtheirfunctionalrolesandtoidentifycell-surfacemarkersindicativeofthephysiologicalstatusofcells,"commentedglycan-imagingspecialistMaryL.KraftoftheUniversityofIllinois,Urbana-Champaign.
6.Two-For-OneCatalyst
Byentrappingasolublerhodiumcomplexinsideaporousmatrixofsilveratoms,ateamofIsraelichemistsinitiatedanewapproachtoheterogeneouscatalysis(C&EN,Sept.1,page9;J.Am.Chem.Soc.2008,130,11880).Chemistsoftenharnesssolublehomogeneouscatalystsforsurface-basedheterogeneousreactionsbyfixingthecatalystonanorganicpolymeroraninorganicoxidesuchassilica—materialsthatarenonconducting.Thecatalyst-in-metalcompositedevelopedbyItzikYosef,RaedAbu-Reziq,andDavidAvnirofHebrewUniversityofJerusalemmarksthefirsttimeahomogeneouscatalysthasbeen"heterogenized"inaconductingmaterial.Trappingtherhodiumcomplexina"seaofelectrons"enhancesitscatalyticactivity,presumablybyalteringitselectrondonor-acceptorpropertiesandmodifyingsubstratebinding.Tomakethematerial,theresearchersaddedzincpowdertoasolutioncontainingarhodiumphosphinecyclooctadienecatalystandsilvernitrate.Thezincreducesthesilvercations,andassilvercrystallitesform,theyaggregateandprecipitateoutofthesolution,takingthecatalystwithit.Thiscatalystentrapmentprocess"isawatersheddevelopmentinscience,"commentedorganometallicchemistHowardAlperoftheUniversityofOttawa.Sincetheoriginalreport,Avnir'sgrouphasusedthetechniquetoentrapanacidicpolymerinsilverasanacidcatalyst,entrapactiveenzymesinsilverandingold,anddopesilverwithdifferentreagentstoimproveitscatalyticperformance.
7.
Peptide-ProducingPowerhouses
Whenbacteriawanttobuildintricatepeptides,theyemploymegacatalystscallednonribosomalpeptidesynthetase(NRPS)enzymes.Justoneoftheseenormousassembly-lineenzymescanpossessasmanyas40catalyticsites,locatedinsequentialmodulesthatlengthenthechainonepeptidebondatatime.BecausemanyofthepeptidesproducedthroughNRPSenzymesarepotentantibiotics—thinkpenicillinorvancomycin—researcherswanttofigureouthowtoharnesstheenzymestoproduceexistingantibioticsandengineernewones.Tothatend,biochemistMohamedA.MarahieloftheUniversityofMarburg,inGermany,andcolleaguespublishedthefirstX-raycrystalstructureofanentireNRPSmodule(C&EN,Oct.6,page48;Science2008,321,659).Shortlythereafter,ateamofchemistsledbyGerhardWagnerandChristopherT.WalshofHarvardMedicalSchoolpublishedanuclearmagneticresonancestructureofcomponentsofasimilarmodule(Nature2008,454,903).ThetwostructuresrepresentthefirstviewsofhowseparatecatalyticdomainsareorganizedwithinanNRPSenzyme,notedJanetL.Smith,abiochemistattheUniversityofMichigan,AnnArbor.Thefindingswill"allowustofigureouthowtoengineerNRPSenzymestodoevenmorevariedchemistry,"shesaid.
8.
Gibberellin'sRoleInPlantPuberty
Makinglemonsandgrapesplumper,persuadingasparagustosendoutmoreshoots,andwakingupawidevarietyofseedsfromdormancyareallpartoftheplanthormonegibberellin'sjobdescription.Buthowthisditerpenoidcarboxylicacidgetsdowntobusinesshaslongbeenamystery.Thisyear,Japanesescientistsprovidedsomekeyinformationbyre
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