Agrawal, M. C.

Upadhyay, Santosh K.

English

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Is•• .ro.r...l of Cbeml_tryVoL ISA, AU~U8t 1977, pp. 709-712Kinetics of Os(VIII) -catalysed Alkaline Hexacyanoferrate(III)Oxidation of Some a-Amino Acids in Presence ofExcess of FerrocyanideSANTOSH K. UPADHYAY & M. C. AGRAWALChemistry Department, Harcourt Butler Technological Institute, Kanpur 208002Received 15 December 1976; accepted 24 February 1977The Os(VIII)-catalysed oxidation of some a-amino acids such as c-Jeuctae, L-l1lutamic acid,L-l1lutamine and L-asparatic acid by alkaline hexacyanoferrate(III) in the presence of excess ferro-cyanide has been studied spectrophotometrically. The oxidation follows complex kinetics beingfirst order in amino acid and zero order in hexacyanoferrate(III) at lower concentrations of aminoacid. Atextremely large concentration of amino acid ([AA]: [Fe(CN)~-]= 80) the reaction becomesnearly independent of [amino acid] and almost second order in [Fe(CN):-l. The order in Os(VIII)has been found to be unity in every case. Asmall variation in ferrocyanide ion concentration is bestexpressed as rate cc{constant + [Fe(CN)~-]}. A suitable mechanism consistent with the kineticdata has been proposed.EARLIER. while studying the kinetics ofOS(VIII)-catalysed oxidation of IX-amino acidsby Ierricyanide, it was observed that concen-tra tion versus time plots were not linear due to strongcatalytic influence of ferrocyanide. As the ferro-cyanide is a product of reduction of ferricyanide,its accelerating influence was encountered in everykinetic run which showed curves with increasingslopes. Therefore, in the present study the kineticsof OS(VIII)-catalvsed oxidation of some <x-aminoacids such as t-leucine, t-glutamic acid, L-glutamineand Lvasparatic acid by ferricyanide were investi-gated spectrophotometrically in the presence ofexcess of ferrocyanide to avoid auto-catalysis.Materials and MethodsSolutions of L-Ieucine, L-glutamic acid, L-glutaminet-asparatic acid and potassium ferrocyanide (allfrom BDH) were freshly prepared in doubly distilledwater. Preparation of all other solutions and theexperimental procedure adopted were identical tothose reported earlier".Stoichiometry - The reaction mixture containinga known excess of ferricyanide over amino acidswas kept at 40° in the presence of 0'1M alkali and3·90x lO--M Os(VIII) (in case of leucine, [Os(VIII)]= 1·95 X la-1M) for 6 he The amount of ferri-cyanide left revealed 1:2 stoichiometry betweenamino acid and Ierricyanide as shown in Eq. (1),RCHNH1 COOH + 2 Fe(CN):- + 20H - ""' RCOCOOH +NH. + H20 + 2Fe(CN).4- ... (1)where R represents (CHa)iCHCHs, -CHIlCOOH,-CHzCHzCOOH and -CH2CH2CONHz groups forleucine: aspara tic acid, glutamic acid and glutaminerespectively.Product analysis - The presence of correspondingketo acids as the reaction products was detectedby spot testss, in agreement with the earlierwortc3·'.ResultsEffects of varying fJerricyanide] and [amion acid]-The kinetics of oxidation of <x-amino acids (AA)were investigated at several initial concentrationsof the reactants in excess Ierrocyanide, The con-centration versus time plots in Ierricyanide werelinear (Fig. 1) suggesting an independent nature ofthe rate in ferricyanide. The pesudo-zero order rateconstants (ko) in ferricyanide were nearly constant(Table 1) at all [ferricyanide], which furtherestablished zero order dependence in ferri-cyanide.The results of varying [amino acid] are given inTable 1. The plots of ko versus [amino acid] showeda deviation from linearity (Fig. 1) at higher [aminoacid], while the plots of 1/kg versus 1I[ amino acid] werelinear (Fig. 2). It, therefore, appears that the orderin amino acid falls from unity at higher [amino acid].0.6o.••ue~ o.~••.04( e.0.1A 8Fig. 1- Zero order rate plots of (A) glutamic acid(B) asparatic acid, (C) leucine and (D) glutamine in [fern:cyanide] at 35° {Initial [Fe(CN):-] = 5·0 x 104M; [Fe(CN)4-j= S'Ox1O-'M; [OH-] = 0'1M; [AA) "" .'OxtD--M; ~C}[OS04) = 3·90 x tD--M for A, Band D and 1'95 X1o-IM for709INDIAN J. CHEM.• VOL. 15A. AUGUST 1977TABLE 1 - EFFECT OF VARYING (Fe(CN):-J AND [AMINO ACID) AT' 3S0{[OH-]= 0'1M; [Fe(CN):-] = 5·0 X10-aM; and [Os(VIII)] = 3,90 x lo-eM for glutamic acid. glutamine and asparatic acidand 1·95 x 10"'M for leucine}10'[Fe(CN)t"]M101[AA]MGlutamine3·03·5.•.·0.•.·55·05·05·05·05·0807.06.05.0onS?'" 4.00-'&3.0.•.·0.•.·0.•.·04·0.•.·02·.•.3·25·68·0LeucineH81,301·281,321·230,90H51,581·8616.014,012.010.0••b"80 '0.J<-A 6.0BC· 4.002.010'k. (mole Iitre"! min-I)GlutamicacidAsparaticacid1·701·771·731-851,84H31·402,482·951·601-671,661,6.•.1,67HO1,362·0...2,341·191·20H61·241·15O'M0·801·331,557.0',----'-----------------,~~~~~~~~~~o2.0 4.0 6.0 B:O 10.0[AA] II 10' or I/[u] 11102 MFig. 2 - Plots of So or 11k. versus [AA] or I/[AA] at 35·{Initial [Fe(CN):-] = 5·0xlO-'M; [Fe(CN):-] =5·0xlQ-1M;[OH-] = 0'1M;[ AA] = .•.·0 x lo-aM; and [OsOJ = 3·90 xlo-'M for glutamic acid (A). asparatic acid (B) and gluta-mine (D) and 1·95 x 10-- M for leucine (C)] -It was observed that at very high [amino acid].the concentration versus time plots were not linear.while l/absorbance versus time plots were almostlinear (Fig. 3) suggesting a second order de-pendence in ferricyanide. Thus; it seems that t~eorder in amino acid falls from unity to zero whilethat in ferricyanide increases from zero to two withlarge increase in amino acid concentrations.Effect o/varying [Os(VIII)J - Studies were carriedout at various concentrations of Os[(VIII)]. Theplots of ko versus [Os(VIII)] were linear emergingfrom the origin and established first order depen-den ce in Os(VIII). . . .Effect 0/ varying [ferrocyan.ae] - Th.e oxidationof .u-amino acids was studied over a WIde range of[ferrocyanide]. At low initial [ferrocyanide] ~herethe 'concentration versus time plots were non-linearwHh i.n<:reasi~g s.lopi: value •. th~half-li.£~ periods7106.0••vCC£l~II>.0sFig. 3 - Plots of l/absorbance versus time at 35' for(A) glutamic acid. (B) asparatic acid. (C) leucine and(D) glutamine at initial [Fe(CN):-J = 5·0 x 10--M. [Fe(CN):-]= 5·Ox1Q-1M. [OH-] = O'lM; and [OsOJ = 3·9Ox10 ..•.Mfor A. B and D and 1·95 x 10"'M for C {[Glutamic acid] =3·0 x lO-IM; [asparatic acid] = 2·0 X 1O-'M; [leucine]= 4·0xlO-·M; and [glutamine] = 4'0 x lO-IM} .O,12~r-------~------,Fig. 4 - Plots of 1/t1/2 of (A) glutamic acid; (B) asparaticacid. (CjJeucine and (D) glutamine versus [Fe(CN):-l at 35'{[OH-] = .(I'2M; . [ps(VIII)J= 3-90x lO-W; .' [A,A]= .4·()x lO-'M and [Fe(CN):-l- 5·0 x 10-'M} '.. '"UPADHYAY & AGRAWAL: HEXACYANOFERRATE(III) OXIDATION OF at-AMINO ACIDiGlutamic Asparatic Glutaminek,AA + Os(VIlI) ~ complex (Y) ......... slow ... (5)acid acid k.,2·00 1·38 0·96k,2'80 2·00 1·15(Y) -+ keto acid + NH. + Os (VI) ......... fast ... (6)3·00 2'30 1·53 k.3·23 HO 1·70 AA + Os (VI) -+ keto acid + NH. + Os (IV) ... slow ... (7)3·10 2·39 1-61 Scheme 22·97 2-13 1·55TABLE 2 - EFFECT OF VARYING [Fe(CN):-] ON THERATE CONSTANTAT 35°{COHo]= 0'1M; [AAJ = 8·0 x 10-3M; [Fe(CN);-] = 5·0x 10-'M; and [Os(VIII)} = 3'90 x 10-tM for glutamic acid,glutamine and asparatic acid and [Os(VIII)] = 2·80 x 10-IMfor leucine}103[Fe(CN):-1 10' ko (mole Iitre? min-1)M --------------------------~--Leucine1·0 1·893·0 2·405·0 2·757·0 2·948·0 2'5010·0 2·40(tl/2) of the reactions dropped sharply and the plots?f [Fe(CN)~-] .versus Iltl/2 were linear with positiveintercepts (FIg. 4). At high [ferrocyanide] where[Fe(CN)g-] versus time plots were linear, ko valuesfirst levelled off and then showed a slight de-creasing trend (Table 2).Effect of varying ionic strength and COHo]- Additionof NaCl04 showed a negligible effect on the reactionrates up to 0·4211 NaCl04• while a tenfold increasein [alkali] showed about 60% increase in ko valuesof leucine, glutamic r.cid and glutamine andabout 4% increase in the case of asparatic acidat 35°.Effect Of varying temperature - The kinetics weremeasu~ed .at 35°, 40°, 45° and 50°. The energiesof activation calculated from the Arrhenius plotswere 9'21, 8·06, 8·29 and 13·15 kcal mole"! forleucine, glutamic acid. asparatic acid and glutaminerespectively.DiscussionThe 03(VIII)-catalysed oxidation of ec-aminoacids by Ierricyanide in the presence of Ierrocyanidefollows complex kinetics. The zero order depen-dence in ferricyanide clearly suggests its involvementin the fast step. The strong c italytic influence ofIerrocyanide at its low concentration also suggeststhe possible interaction between amino acids andterrocyanide. Under the experimental conditionthe initial rates of oxidation are very small incomparison to those in the presence of ferrocya-nide.Fe(II) complexes of amino acids are reportedin the literature- and in some cases" Fe(CN):- com-plexes of amino acids are also mentioned. In thepres en t case the more favourable path for theoxidation of amino acids seems to involve a primaryreaction between the amino acids and Fe(CN)~- togive a complex. As the oxidation does not proceedin the absence of Os(VIII) even in the presence offerrocyanide, the amino acid-Ierrocyanide complexformed is oxidized by Os(VIII) in a slow step.Hence, a mechanism as shown in Scheme 1 may beproposed.The proposed AA-Fe(LN)~- complex has alreadybeen characterized spectrophotometrically in ourk,AA + Fe(CN):- ~ complex (X) fastk_,k,(X) + Os(VIII) -+ Os(VI) + Fe(CN):- + oxidationproducts slow (3)k,'Os(VI) + 2F.,(CN): -+ Os(VIII) + 2Fe(CN):- ... fast ... (4)... (2)Scheme 1earlier studies'. Similar results were obtained forth e amino acids studied presently.It is mentioned in the Iiterature+! that aminoacids form complexes with Os (VIII) which are sub-sequently degraded to keto acids, ammonia andOs(VI). The liberation of ammonia is a func-tion of tl;.e rate .of oxi?ation. It is also reportedthat at high [ammo acid] Os(VI) is further reducedto Os(IV) by a fresh molecule of amino acid. Thereactions may be represented as shown in Scheme 2.As the concentration of Os(VIII) present in thereaction mixture is ""' 1O-a times to that of aminoac~d. t~e reactio? in Scheme 2 represents nearly 0'1 %OXIdatIOn of ammo acids. However, in the presenceof f~rricyaride a f~~t interaction between Os (VI)species and Fe(CN)6- takes place to regenerateOs(VIIIJ7.Applying the steady state conditions with respectto (X), (YJ and Os(VI) in steps 2-7, the rate laweq.uation for the disappearance of Fe(CNn- is ob-tamed as:d[Fe(CN)l-] _ ks[AA][Os(VIII)][Fe(CN)~-P--~---- ks[AA]+ka[Fe(CN)t]Z{k4ks + klkz[Fe(CN)t]}k~ +k. k-lwhere k_l~k2[Os(VIII)] has been taken as a suitableapproximation.Further step (4) is very fast in comparison to step(7). and therefore taking k3[Fe(CN)~-]2~k6[AA] therate Eq. (8) reduces to Eq. (9)._ d[Fe~N)~-] = [AA][Os(VIII)]~ k!k5_ + ~lk2[Fe(CNH·] 1Lk4+ks k-l JThe above rate law (Eq. 9) predicts that the rateof oxidation would be first order each in [AA] and[Os(VIII)], and zero order in [ferricyanide]. Theef£ect of ferrocyanide on the rate would be propor-tional to {constant +iYe(CN)t-]} as was observedexperiment~lly (Fig. 4, where lit. a: ko).The fall in order of the reaction in amino acidsmay be explained as follows: since step (4) is veryfast Os (VI) formed during the oxidation of aminoaci~ is ~mmedia!ely converted into Os(VIII) byIerricyanide. It 1S only at very high [amino acid]... (8)... (9)711INDIAN J. CHEll., VOL. 15A, AUGUST 1977that some of Os(VI) is reduced to Os(IV) via theslow step (7). Evidently this will result in a de-crease in [Os(VIII)J.which in turn will decrease therate of oxidation. However, in such a case theplots of l/ko versus l/[AA] should be linear withintercept according to rate law (8) (Fig. 2).Thus the fall in ko value at high [amino acid]has been interpreted in terms of the fact that theterm k.[AA] is not negligible in comparison toka[Fe(CN):-J1in Eq. (8). Consequently, at largerconcentrations of amino acid a reaction independentof ferricyanide is not warranted. In fact, at suchhigh amino acid concentrations where the rate ofdisappearance of ferricyanide would become in-dependent of [aminoacid], a second order dependencein ferricyanide should be obtained. Therefore, formoderately large concentrations of the amino acidsthe order in ferricyanide would be between 0 and 2.Inverse [Fe(CN)tJ versus time plots (Fig. ~) athigh amino acid concentrations are almost linearsuggesting a secondorder dependencein ferricyanide.It is further observed that in the case of oxidationof glutamine which shows minimum variation fromthe first order behaviour (Fig. 2), the order inferricyanide at higher [glutamine] is minimum (.-.1).It therefore, appears that the order in amino acidd~creases from 1 to 0 while that for ferricyanideincreases from 0 to 2 with a large increase in [amino712acid]. This is in good [agreement with the ratelaw (8).The dependence on large ferrocyanide concen-tration seems to be more complicated than shownby Eq. (8). The fall in ko values (Table 2) at highferrocyanide concentrations may be possibly dueto the back conversion of Os(VIII) to Os(VI)through the reverse of step (4). As ko is propor-tional to [Os(VIIl)], a substantial increase in[ferrocyanide} when accompanied by a decreasein [Os(VIII)), may result in a decrease in kevalue.AcknowledgementThe authors are thankful to the UGC,NewDelhi,for providing financial assistance.References1. UPADHYAY,S. K. &: AGRAWAL,M. C., Indian j. Chem.,15A (1977), 416.2. FEIGL, F., Spot tests in organic analysis (Elsevier, London),1960, 236, 382.3. NYILASI, J. & ORSOS,P., Acta chim. Acad. Sci. Hung.,75 (1973), 405.4. NVILASI, J. & ORSOS, P., Magyar Kem Foly.,78 (1972),407.5. ALBERT, A., Biochem. j., 47 (1950), 531.6. MALIK, W. U. & ASLAM,M., Indi_ j. Chem., 8 (1970),736.7. SINGH, N. P., SINGH, V. N. & SINGH, M. P., Aust. j,Chem., 21 (1968). 2913.

Kinetics of Os(VIII)-catalysed Alkaline Hexacyanoferrate(III) Oxidation of Some ɑ-Amino Acids in Presence of Excess of Ferrocyanide

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Kinetics of Os(VIII)-catalysed Alkaline Hexacyanoferrate(III) Oxidation of Some ɑ-Amino Acids in Presence of Excess of Ferrocyanide

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