Beck, Mihály Tibor

Görög, Sándor

English

Beck Mihály Tibor

Görög Sándor

University of Szeged

MUTUAL CATALYSIS IN SYSTEM CoUD-CLYCYLGLYCINE-ASCORBIC ACID-OXYGEN By M. T. BECK and S. GÖRÖG Institute of Inorganic and Analytical Shemistry. The University, Szeged Many papers deal with the phenomena observed on the oxygenation of different Co(II)- amino acid and peptide complexes. It was observed that the oxygenation takes place in two steps. The first step is a reversible uptake of molecular oxygen followed by an irreversible process. Tlie most thoroughly ", studied systems are Co(ü)--histidine and Co(II)~glycylglycine [l^. [2]. In tlie earlier investigations it was supposed that the valence of central ion does not ciiangq in a Co(II)~histidine system either in a reversible or an irreversible reaction. Tanford and his coworkers' paper does not deal with this problem dcsfiaiteiy. investigation aimed to study the catalytic effect of these compleses on autoxidatioa reactions. Co(II)-glycyiglycine was chofsei. as model, since -according to earlier findings - in the case of Co(ID-histidine complex disturbing by processes have io be taken irto consideration First of all we examined the validity of tlie earlier statement tliat the oxidation state of central ion does not change during even the irreversible reaction. The simplest way for tlie solution of this problem is the comparative study of the propsrtias of Co(III) complexes and the products of irreversible oxidation [3]. á n e i v method to prepare Co( III)-glycylglycine complex was elaborated. The essGnca of this method is that the complex is prepared from hexammine Co(ÍÍI) chloride and gly.c/íglycine in the presence of active carbon as catalyst. The aiasonidm salt was obtained with 90£per ceti yield, the free acid with 30 per cent yield. The properties of complex obtained are the same as that of tlie product of irreversible oxidation of Co(II) glycylglycine. The anion character of tlie complex is proved by its electrophoretic behaviour and that the compound can be prepared in the form of ammonium salt. It may be expected thai the complex formed during the reversible oxigenation can catalyse tlie autoxidatjon reactions since tlie attachement of oxygen into the complex molecule causes the weakening of the 0-0 bond. Really, our experiments showed that the Co(II) glycylglycine complex is an effective catalyst of different autoxiclati Oil procsssss :iU [oxidation of ascorbic acid. The gasvolumetric kinetic experiments weire carried out in a Csűrös apparatus [l-J . Experiments carried out at different pH showed that tlie velocity of catalytic reaction and Hie amount oxygen consumed increase with increasing pIKFig. 1). 63 Hl? 0 Fig. 1. Catalyzed oxygen unlike of ascorbic acid at different pH, (Oxygen consumtion of blanks are sdbstracted.) C a s c -.4.10-2; C C o = 4.10-3-; tsrfogat 25 ml. t flli-n *<« Fig. 2. Change of absorbancy during the oxygenation of Co/II/-gycylglycine complex innthe presence and abs^jice of ascorbic acid. 'Co to-3, ^ e / l ; C 10 a s c - mole/1', pi I filter S 52 8 ; cuvette 3̂ 5 cm i 1,6 U o,« 11 u Co, : PiSC - J'-IS --1:1 t miu t o ¿ ^ c l Fig. 3. Effect of f8 on the formation of intermediate. mole/1 pH • 7,5 cuvette 5 cm i wavelength 35o mp. 64 Neither the Co(II) a or t'.\<s product of irreversible oxidation (Co(III) glycylgly-cine complex) can catalyse the auloxidation of ascorbic acid,' consequently, the oxygen carrying Go(Il) glycylglycine complex is responsible for th® catalysis observed. Below w© shall present a direct @vid®nce of this. During these experiments an unexpected phenomenon was observed. Namely, in the presence of Co(II)glycylglycine complex not only the autoxida-tion of ascorbic acid is accelerated but the ascorbic acid also increases the velocity of oxygenation of Co(II)glycylglycine complex. This latter process can be easily followed by spectrophoiometric measurements. The absorbancy of CoGDglycylgiyciiie is negligibly small in comparison to the absorbancy of both the oxygen carrying and the irreversible complexfs. Simultansous determi-nation of thase two compounds is possible because the brown intermediate has a selective absorbancy in the near ultraviolet while at 520 nyi the absorbancy both of the complexss is UT same. So th© measurement at 520 mu gives the sum of the complexes, while the msasuremsnt at 350 nyu gives the amount of the oxygen carrying complex only. Experiments were carried out partly with Pulfrich photometer in 3,5 cm cuvette by applying S 52 filter, partly with Beckman B spectrophotometer in 5 cm cuvette at 520 and 350 m/i Fig. 2 shows that the velocity of oxygenation of CoCID-glycylglyciae complex is greatly accelerated by ascorbic acid. The effect of the concentra-tion of ascoroic acid on the oxygenation is shown in Fig. 3, For sake of a better perspicuity th3 data obtained at 350 m^are plotted only. As can be seen from Fig. 3 the concentration of the oxygen carrying complex changes according to a maximum curv.?. in time. The rate of oxygen consumption first increases on increasing the concentration of ascorbic acid, over certain concentrations it becomes iadapandeat of ii. The absorbancy is, not reduced to zero even after a longer time because th® oxidation products of ascorbic acid also absorb at this wave .bugth. F'r, sY> V3 a? effect of pH on oxygenation. It is apparent that both the velocity of oxygenation and the maximum concentration of intermediate inc-rease monotonously on increasing the pH. Concerning TANFORD and his co-workers' /.¿vsshgations -- which are supported by our experiments - that the brown intsroiediab can be observed at higher pH vaflies only (over pH 8), further, according to the date of Fig, 3 and 4, in the presence of ascorbic acid the intermediate axil's for a longer tims at relatively low pH, one must suppose that either t!i3 ascorbic acid or its oxidation products is built into the complex an-:! hereby stabilises it Here we note that the complex forming effect -jf us oxidation products of ascorbic acid exhibits in the autoinhibition phenomenon in this system what is ©vidsnt from the curve _d of Fig. 4. In this expariarsnt tins reaction mixture contained beside fresh ascorbic acid an amount of oxidized aseoro.'o acid, no. In this case the curve lies considerably lower than without oxidized ascorbic acid. From the comparison of gasvolumetric (autoxidation of ascorbi£ acid) and the spectrophotometry (oxygenation of Co(II)-glycylglycine) measurements it can be seen that the velocity of autoxidation changes parallel to the concent-ration of brown intermediate and reduces to the velocity of non-catalyzed r®MCr 65 Fig. 4. o Effect of pH on the formation of intermediate. CQq [0 mole/1; C a s c iO"2 ; cuvette 5 cm', wavelength 350 mji; a.|T pl-I 6.9; b.) pl-l 7.5: &) pH V . t ; 4 ) pH 7.5; the solution contains i0"4 mole oxidized ascorbic acid, too . tion when the concentration af intermediate - the catalyst - is reduced to zero. The autoxidation of ascorbic acid and the oxygenation of Co(II) glycylglycine are not simple reactions themselves. The simultaneous investigation of these i makes almost impossible the quantitative treating og the data. Nevertheless the experimerts show that in the system Co( II)-glycylglycine-ascorbic »afcid-oxygen mutual catalysis or otherwise lire step-by-step activation of molecullr oxygen have to be taken into consideration From the molecular oxygen and ascorbic acid a labile adduct forms. This adduct can react with CodDglycylglycine in a fast reaction whereas the oxygen carrying complex and ascorbic acid form . The oxygen carrying complex having a higher activated oxygen oxidises the asdprbic acid with greater velocity than the molecular oxygen. It is' impossible to give a detailed mechanism, because of the mentioned disturbing by-processes. According to our preliminary experiments tire Co(II) glycylglycins complex can also catalyse tie autoxidation of sulphite and the phenomena observed in the case of ascorbic acid appear in this system too. Further experiments with this and other simple redox systems will be carried out. References [1] Hearon. J.Z., D. 3urk: J. Nat. Cancer Inst. J . 337 (1949). [2] Tanford. C.. D.C. Kirk. M.K. Chantooni: J. - vmsr.Chem. Soc. 76, 5325 (1954) [3] Beck. M.T.I Naturwiss. _45. 162 (1958). . [4] Csűrös, Z . SC. Zech. T. Pfliegel: M.T.A. KénuTud.Oszt. Közi. J , 4i (1952). 

Mutual catalysis in system Co(II)-glycylglycine-ascorbic acid-oxygen

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Mutual catalysis in system Co(II)-glycylglycine-ascorbic acid-oxygen

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