Agrawal, M. C.

Upadhyay, Santosh K.

English

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IndianJ+urnalof ChemistryVol. 15AJMay 1977,pp. 416·419Kinetics of Os(VIII) -catalysedOxidationof Somea-AminoAcids by FerricyanideSANTOSH K. UPADHYAY & M. C. AGRAWALDepartmentof Chemistry,Harcourt Butler TechnologicalInstitute, Kanpur 208002Received 23 June 1976; accepted30 September 1976The ratesof Os(VIII)-catalysedferrocyanideoxidationof a-aminoacidssuchas glycine,L-Ieucine,L-phenylalanine,DL-a-alanineandDL-valinehavebeenmeasuredspectrophotometri-cally. Thereactionsfollowcomplexkineticsbeingfirst orderwith respecto bothaminoacidandthecatalyst.Theeffectof alkaliis slightlypositiveat high concentration(>O·2M). Al-thoughtherateis independentof ferricyanideion concentration,it showsspecificferrocyanideioncatalysis.Theenergiesof activationhavebeenfoundwithintherange12·9-14·3kcalmole-l.A suitablemechanisminvolvingthe formationof an intermediateaminoacid-ferrocyanide"complexis proposed.IDATION of ~-aminoacidsby a varietyofxidantshasbeenstudiedin detailby severalnvestigators1-s.Lambertand Jones6 studiedthe u catalysedoxidationof someaminoacidsbyalkali 'e ferricyanideand reportedthat the re-action was extremelyslow even at high ionicstrenghs (4·SM KNOa). The strongcatalyticeffectof Os III) on ferricyanideoxidationsin alkalinemedia"is well documented7-9•In this paper thekinetis of Os(VIII)-catalysedoxidationof glycine,L-leucie, L-phenylalanine,DL-~-alanineand DL-valine,arereportedanda mechanismof thereactionis sugested. It has beenobservedthat the pre-sence,of a trace amountof Os(VIII) (10-6M) issufficint to catalysethe oxidationof ~-aminoacidsby al line ferricyanideto a reasonablerate.Mateials and MethodsThereagentsemployedwereglycine,DL-~-alanine,L-leucie, DL-valine,L-phenylalanine(all BDH sam-ples) otassiumferricyanide(recrystallized)andsodiun hydroxide(both BDH, AR samples)andosmiu tetroxide(Johnson Matthey grade). Allthe s lutions were preparedin doubly distilledwater. Otherreagentsusedwereof AR grade.The kineticswerefollowedby determiningferri-cyani spectrophotometricallyat variousintervalsoftim ona BauschandLombspectronic-20spectro-photo,eterat 420nm andkeepingthe ferricyanideion co centrationbelow5·0x10-4M.Stoihiometry- The reactionmixturecontainingaknow excessof ferricyanideover aminoacid waskept 40°in the presenceof 3,9X 10-6M Os(VIII)and 02M alkali for 12 hr. The amountof ferri-cyani left revealedI: 2 stoichiometrybetweenamino"acidandferricyanide.The reactionmayberepresnted by the followingequation:RCH H2COOH+2Fe(CN)~-+20H-=RCOCOOH+NHa+2Fe(CN):-+HPWherei.,RrepresentsH, CHa, (CH3)2CHCH2'(CH3)2CHand C HsCH2for glycine,~-alanine,leucine,valineand p enylalaninerespectively.Pro uct analysis- The presenceof ketoacidsinthe re ctionproductswasdetectedby variousspot416tests10,and the results are in agreementwithearlierwork on the oxidationof aminoacidsbyOs(VIII)11·12.ResultsEffect of varying [jerricyanide]- There was agradualincreasein the rate of disappearanceofferricyanideat high [aminoacid],and absorbanceversustimeplotsrepresentedcurveswith increasingslopevalue. However,at differentconcentrationsof ferricyanidethesecurveswereparallelto eachother,and the half-lifeperiodsin ferricyanideweredependenton the initial [ferricyanide](Table 1).The oxidationrate appearsto be independentof[ferricyanide],thecatalyticinfluenceof ferrocyanideformedis alsoevident. It wasverifiedby carryingout investigationin the presenceof excessferro-cyanidewherethezeroorderplotswerereasonablylinearcurves(Fig. 1).0.4'"uCj 031O.2~,0.1Time(min)Fig. 1- Zero order rate plots in ferricyanideat 35° andin the presence of 5x 10-32\1 Fe(CN)~- for (A) leucine,(C) phenylalanine,(D) alanineand tE) valine {[Fe(CN)~-]=5·0 x 10-32\1; [amino acid]=4·0 x 10-32\1; [OH-J =0'2M;[OS04J =3·9 x 1O-6M}!:fi'll-----~- {1!UPADHYAY & AGRAWAL: Os(VIIIj-CATALYSED FERRICYANIDE OXIDATION OF IX-AMINO ACIDSTABLE 1 - EFFECT OF VARYING [REACTANTSJ ON RATE AT 35° IN THE PRESENCE OF 3-9 X to-8MOsO,104 X 103 X10 X [owJ t! (min)[Fe(CN):J[amino acidJMMM LeucineGlycinePhenylalanineAl iValine3·04·2 0783463,5,125744·0356568 91105·0374,-591035288·65·,*-5·05 0*Effect of alkali on the rate of oxidation of glycine and leucine was studied at (J. =l'2M (NaCl04) while on therest of amino acids it was studied at (J. =0·4M (NaCl04)·TABLE 4 - EFFECT OF ADDED SODIUM PERCHLORATE AT 35°TABLE 2 - EFFECT OF VARYING [OS04J ON THE REACTIONRATE AT 35°TABLE 3 - EFFECT OF VARYING [Fe(CN)t-J ON THEREACTION RATE AT 35°{Fe(CN)l-J =5·0 X 10-4M; [OWJ =0'2M; [amino aci<l]=4·0 X 10-3M}Valinett (min)Phenyl- Alaninealaninet1 (min)70 - 105 125 20853 63 81 93 150~ % ~ U lW31 32 44 43 8422 23 28 32 57Leucine GlycineLeucine Glycine Phenyl- Alanine Valinealanine434662751200·2373451. 043SO940·627818668417None 434662741 03·03-303364,012828545·1716391041022·343-123,905·467,80108 X[OS04JM[NaClOJM{[Fe(CN):-J =5·0 X 10'4M; [OW] =0'2M; [OS04J =3·9 X 10-8M; [amino acid] =4·0 X 10-3M}{[Fe(CN)n =5·0 x 10-4M; [OWJ =0·2M; [OS04J =3·9 X 10-4M; [amino acid] =4·0 X 10-8M}104X t1 (min)[Fe(CNW] _.~---------------M Leucine Glycine Phenyl- Alanine ValinealanineEffect of varying [amino acid]- Studiescarriedout at variousinitial concentrationsof the aminoacids revealedthat an increasein [aminoacid]resultedin a decreasein half-lifeperiod (tl/2) inferricyanide(Table 1). The plots of log (1Itl/2)(proportionalto pseudozeroorderrateconstantinferricyanidejversuslog [aminoacid] were linearwith slopevaluesof .....,1 therebyestablishingfirstorderdependencewith respectto aminoacid.Effect of varying [OH-]- The effect of in-creasing[alkali]waslesspronouncedin oxidationofphenylalanine,iX-alanineand valine. However,inthe caseof glycineand leucinetl/2 decreasedsigni-ficantlywith increasing[alkali](Table1).Effect of varying [Os(VIII)] - There was amarkedcatalyticinfluenceof osmiumtetroxideonthe reactionrate. The resultsin Table 2 showanear proportionaldecreasein tl/2 in ferricyanidewith increasingin [Os(VIII)] indicatinga directdependenceon the [catalyst].Effect of ferrocyanide ions - The oxidationwasfoundto be highlysusceptibleto ferrocyanideionswhichstronglyacceleratedthe rate (Table3). Asferrocyanideis also a productof the reactionitsacceleratinginfluenceis encounteredin everykineticrun. Theplot of [Fe(CN)~-]against11tl/2wasalmos~linearwith a positiveintercepton Y-axis.Effect of varying ionic strength- The effect ofvaryingionicstrengthwasstudiedby carryingoutthe investigationsin the presenceof differentamountsof NaCI04• The results(Table4) showaslightpositivesalt effect.Effect of varying temperature- The oxidationofaminoacidswas studiedat 30°,35°,40°,45°and50° and 1Itl/'}.valueswereobtained. The activa-tion energies(~Et) calculatedfrom the Arrheniusplots (log lit versusliT) were 12·9±0-1, 13·8± 0-1, 13·5f 0,2, 14-6±0-2 and 14·3±0·2 kcalmole-1for theoxidationof leucine,glycine,phenyl-alanine,alanineandvaline,respectively_DiscussionMechanism- The resultsshowthattheoxidationof glycine, alanine, valine, leucine and phenyl-INDIAN J. CHEM., VOL. 1SA,MAY 1977alanin proceedsby a commonmechanism.It iswell k own that aminoacidsexist as dipolarionsin aq .ous solution. Thesedipolarions dissociateto a ino acid anions and the pK values forRCH ;COO-~ RCHNH2COO-+H+system arereportd13 to be .....,9·60for all aminoacidsstudiedhere cept phenylalaninefor which the value is9·13a 25°. Sincestudieswerecarriedout in 0·2MNaOH it may be assumedthat the aminoacidswere ompletelydissociatedinto their anions. Atthesa e timeOs(VIII) in suchmediais reported14to exit as ROsOi. Now, as experimentalresultsshow rst orderdependencein catalystand aminoacid, their interactionappearsto constituteaprimay ratedeterminingreaction. It is mentionedin lite·ature12,15that Os(VIII) formscomplexeswithamino.acid in alkaline solution which are sub-sequetly degradedto keto acids and ammonia.It is somentionedthat liberationof ammoniais afuncti n of therateof oxidationand,therefore,thereactin maybe representedin generalby Eqs. (1)and ( ) (Scheme1).As Os(VIII)] presentin the reactionmixtureis10-3t mesthat of aminoacid,reactions(1)and (2)represnt nearly 0·1% oxidationof amino acidsonly.. However,in the presenceof ferricyanideafasti teractionbetweenOs(VI) speciesandFe(CN)~-takes laceto regenerateOs(VIII) asshownin step(3) in Scheme1.0.100.08.~~A,....., O,06r /~g'" l. 0.04The rate law (6)predictsthat Os(VIII)-catalysedoxidationof amino acids by alkalineferricyanidewill followa first orderdependencewith respecttoaminoacid and Os(VIII). The effectof ferrocya-nide on reactionrate would be proportionalto{constant+[Fe(CNH-]}.As has beenmentionedinthe earlierpart of discussion,the rate determiningreactionwould involve interactionbetweennega-tivelychargedaminoacidanionandHOs05(step-1).For this step a positivesalt effect is expected.'Our experimentalresultsarein completeagreementwith the abovefacts.SPectroPhotometricevidencefor the formation ofjerrocyanide-aminoacid comPlex- At 290nmwhereferrocyanideabsorbsfairly strongly,the additionof aminoacidsin thepresenceof alkalicausedabout30% reductionin the absorbancedue to ferrocya-nide. This fall in absorbanceis, however,not somarked when [amino acid]:[Fe(CN)t-] ratio isgreaterthanunity. As aminoacidshavenegligibleabsorbanceat this wavelength,the formationof atransient[aminoacid-Fe(CN)t-]complexwhich isweakly absorbingin comparisonto Fe(CN):- isvisualized.It is reportedin literaturethatOs(VIII)-catalysedoxidationsby ferricyanideeithershowa retardinginfluenceon the additionof ferrocyanide7•8or showa decreasein rateat theendof everykineticmn18•The slight deviationfrom linearity of the plotsshownin Fig. 2 could thereforebe attributedtothe fall in [Os(VIII)] caused'bythe reverseof step(3)at high [ferrocyanide].A slightincreasein therateof oxidationof glycineand leucinewith an increasein [alkali] can beexplainedon the basis of the fact that at high[alkali] the uncatalysedoxidationof glycineandleucine may becomeprominentresulting in anincreasei1'1the reactionrate. In the caseof ala-nine, valine and phenylalaninethe uncatalysedoxidationevenat high concentrationsof alkali isnegligibleand therefore,the rates are almostin-dependentof [alkali].4.0 6.0 8.0 10.0 12.0~e (eN>:] x104Fig. 2 - Plot of lit! versus [Fe(CN):-] for (A) leucine,(C) phenylalanine,(D) alanine and (E) valine {[Fe(CN)~-]=5·0 x 10-'M; [amino acid]=4·0 x 10-3M; [OH-] =0'2M; COsO,]=3·9 x 10-tlM}... (1)k_, (slow)AA +OS (VIII) =---,.Complex (X)k,k (fast)(X) ~ Oxidation products+Os(VI) ... (2)k, (fast)Os(VI) +2Fe(CN)~- ) 2 Fe(CN):- + Os(VIII) ... (3)k_. (fast)Amino acid+Fe (CN):- ~Complex (Y) ... (4)k.k. (slow)s(VIII) --~ Oxidation products+Os(VI)+Fe(CN)~- ... (5)Scheme1ationof Fe(CNH-in step(3)posesproblemsin th reactionasit is foundto catalysetherateofoxidaion. It appearsthat Fe(CN)t takespart inthe I' actionby formingcomplexeswith aminoacidswhic are againproneto oxidationby oxidizingagent. Iron(II) complexesof amino acids arerepored in literature16whereasiron(III) doesnotappa ntly formcomplexeswith aminoacidsabovepH 7 In somecasescomplexesof Fe(CN):-withamin acidsarealsoreported17.It .s more probablefor the Os(VIII)-catalysedreactin to proceedwith the formationof mixedligan ferrocyanide-aminoacid complex (step-4)whic maygetoxidizedin a ratedeterminingstepbyOs(V II) (step-5).Os(VI) formedis thenreoxidizedto o (VIII) by ferricyanidein a fast step(step-3).Ap .lying the steadystateconditionwith respectto ( ), (Y) and Os(VI), the rate law equationforthe sappearanceof Fe(CN):-maybe obtainedas:dldt[e(CN)~-]= [Aminoacid][Os(VIII)]{[kl+k4kS/k.tX[Fe(CN):-]} ...(6)wherJ k_1<{k2 and k-4,?>ks [Os(VIII)] have beentaker;!as suitableapproximations.418UPADHYAY & AGRAWAL: Os(VIII)-CATALYSED FERRICYANIDE OXIDATION OF ex-AMINO ACIDSAcknowledgementThanks are due to the CSIR, New Delhi,and the ChemicalSociety,London. for financialassistance.References1. BACON, R. G. R, HANNA, W. J. W. & STEWART, D.,j. chern.Soc. (C), (1966),1388.2. CHANDRA, G. & SRIVASTAVA, S. N., J. inorg.nucl. Chern.,34 (1972),197.3. SHEIKH, R A. & WATERS, W. A., J. chern.Soc. (B),(1970),988.4. BOSE, A. R., MEHROTRA, R. M. & MUSHRAN, S. P.,Indian]. Chern.,11 (1973),896.5. CLARKE, T. G., HAMPSON, M. A., LEE, J. B., MORLEY,]. R & SCANLON, B., j. chern.Soc. (C), (1970),815.6. LAMBERT, D. G. & JONES, M. M., J. Am. chem.Soc.,88(1966),4615.7. JINDAL, V. R., AGRAWAL, M. C. & MUSHRAN, S.P.,j. inorg. nucl. Chem.,33 (1971),2469.8. JINDAL, V. K., AGRAWAL, M. C. & MUSHRAN, S. P., J.chern.Soc. (A), (1971),622.9. SOLYMOSI, F., Naturwiss,44 (1957),374;ChemistAnalyst,51 (1962),71.10. FEIGL, F., Spottestsin organicanalysis(Elsevier, London),1960,236: 382.11. HAKE, T., Lab. Invest.,14 Pt 2 (1965),1208.12. NYILASI, J. & ORSOS, P., Acta chim.Acad. Sci., HU1ig.,75 (1973),405.13. GREENSTEIN, J. P. & WINITZ, M., Chemistryof aminoacids,Vol. 1 (John Wiley, New York), 1961,486.14. GRIFFITH, W. P., Q. Rev., 19 (1965),254.15. NYILASI, J. & SOMOGYI, P., Ann. Univ. Sci., BudapestRolando Nominatae,6 (1964),139; Chem.Abstr., 62(1965),16363g.16. ALBERT, A., Biochem.j., 47 (1950),531.17. MALIK, W. U. & ASLAM, M., Indian J. Chern.,8 (1970),736.18. SINGH, N. P., SINGH, V. N., SINGH, H. S. & SINGH, M. P .•Aust. J. Ckern.,23 (1970),921.419

Kinetics of Os (VIII)-catalysed Oxidation of Some ɑ-Amino Acids by Ferricyanide

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Kinetics of Os (VIII)-catalysed Oxidation of Some ɑ-Amino Acids by Ferricyanide

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