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Crude Oil 11

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    Theironprotoporphyringroupofcytochrome c iscovalentlylinkedtothe proteinviathioetherbridgesbetweentheporhyrinringandtwocysteine residuesinpeptidechain presumably form additionofthe SH groupacrossthedoublebondof th and4 vinylgroupsoftheproto- porphyrin.  sphaltenesDeposits Asphaltenesareamongtheleastunderstooddepositsoccurringinthe oilfield.Theyarethoughttobethebyproductofcomplexhetero-atomicaro maticmacro-cyclicstructurespolymerizedthroughsulfidelinkages see Figure11-1 .Porphyrinsarecommoninnatureandarepartofalargerclassof chemicalscalledcytochromes coloredbodies ,whichareiron-containing electron-transferringproteins.Theseproteinssequentiallytransferelectrons fromflavoproteinstomolecularoxygen,andtheyallcontainiron-porphyrin prostheticgroups.Theporphyrinringispresentinseveralbiologicalsystems andincludeschlorophyllsofgreenplantcells.Althoughmanycytochromes havebeenhighlypurified,withoneexceptiontheyareusuallyverytightly boundtothemitochondrialmembraneanddifficulttoobtaininsolubleand homogenousform.Theexceptioniscytochromec,whichisveryeasily extractedfrommitochondriabystrongsaltsolutions. AccordingtoAlbertLehninger,   sphaltenesand rudeOil  Fig 11 1Possibleasphalteneorigins HeminCH 2 =CHHCH 3 H~C,, C)~~-CH=CH2 \ i N~_/ H::Fe+  t \ tH~ReducedproductscondensedC==NN___./withsulfurtoformhighmolecular /-1 I~HCHweightandviscouspolymers. t~ ~ t~·   HOOCCH~H2   H2-CH;zCOOHPorphyrinnucleus    213 PolymericFormsDerivedfromProtoporphyrin Giventheinformationprovidedintheprevioustwoparagraphs,andin particularthequotefromLehningeraboutthethioesterformationofapoly mercalledcytochromec,protoporphyrinprovidesasuitablestructuralana logtoasphaltene.Thefactthatsulfurisinvolvedinthepolymerizationisnot altogethersurprising,sincesulfurradicals singleunpairedelectrons)are verycommoninthiselement schemistry.Sulfhydrylradicalsareoften formedfromtheheterolyticcleavageofhydrogensulfide agascommonto crudeoils). Thesesulfhydrylradicalsareenergeticenoughtoreact wit activateddoublebondslikethosefoundontheprotoporphyrinmolecule.Thedouble  oplanerOrbitalOverlaporPi onding   theearliersectionsofthisbookwetalkedabouttherolesofhydro genbonding,Londondispersionforces,andionicinteractionsinaggrega tionphenomenarelatedtocomplexhierarchicalstructures.  n additionaltypeofinteractionisfoundtobeoperativeforsystemsexhibitinghigh degreesofunsaturation,andthisinteractioniscalledpibonding.Themech anismofthisinteractioninvolvestheoverlapofpartiallyfilledelectronic orbitalsbetweentwoconjugated alternatingdoubleandsinglebonded atoms)molecularspecies.Theinterchangeofelectronsbetweenorbitalsofseparatemolecules leadstoaforceofattractionbetweenthetwomolecules.Theelectroninter actionsarequantummechanicalinnature,andadetaileddescriptionofthe magnitudeoftheseinteractionsrequiresquantumcalculationsbeapplied. However,ashasbeenthepracticesofar,theintuitivelyunderstandablecon ceptofblendingorbitalswillbestsuitourtreatmentofthisphenomena. Configurationsofthemoleculesundergoingpibondingmustallowa sufficientlycloseapproachfortheelectroncloudsofthemoleculestoshare electrons.Theplanerconfigurationoftheporphyrinandcertainothercon jugatedmoleculesallowsthesemoleculestobestacked seeFig.11-2).Ifa comparisonofrelativestrengthismadebetweenLondondispersiveattrac tionforces,hydrogenbonding,andpibondingforces,thepibonding strengthwouldfallsomewherebetweenthemasinthefollowingorder: hydrogenbonding>pibonding>inductiveforces. Thuscondensationwithotherelectrophiles e.g.,hydrogensulfide)is possibleandtheproductwillconsistofseveralrepeatingporphyrinunits exhibitingveryhighmolecularweights.Thehighdegreeofunsaturatedring carbons doublebondsbetweencarbons)andtheconjugatednatureis responsibleforextremelyintensecolorationofthesespecies.  n additionalpropertyoftheporphyrinringisitshighdegreeofplanarityderivedfromthe fusedunsaturatedstructure. AsphaltenesandCrudeOil    1J C  6 c 0   0 ;:::; CO   CO > 5   Qj C CO   II : /   l I  Oil u 2 4   >- SectionIII Chapter

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Jul 23, 2017

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