THE CHEMICAL PROCESSES IN PLANT RESPIRATION lOQ 



of this type of coupling are oxidations in the presence of ferrous 

 ions as e.g. the oxidation of tartaric acid by hydrogen peroxide 

 in the presence of ferrous salts, as described by Fenton.^ Tar- 

 taric acid is attacked only very slowly by hydrogen peroxide. 

 After the addition of a trace of ferrous sulphate there follows 

 a rapid oxidation of tartaric acid to di-hydroxy-mahc acid. 

 Manchot and Wilhelms- have found that in the case of such 

 processes there occurs a reduction of hydrogen peroxide by the 

 formation of secondary iron peroxide, FeoOo-^ The iron perox- 

 ide then oxidises various acceptors which are unaffected by 

 hydrogen peroxide because the oxidation potential of iron perox- 

 ide is much higher than that of hydrogen peroxide. If we pic- 

 ture the oxidation of tartaric acid by atmospheric oxygen by 

 means of coupled reactions, an increase of the potential must 

 take place twice; the first time from molecular oxygen to hydro- 

 gen peroxide, then from hydrogen peroxide to iron peroxide. 

 Tartaric acid is finally attacked by means of the last union of 

 oxygen : 



(I) H + H + 0.2 = HoO, 



(II) 3H0O. + FeoOo = Fe.Os + 3H2O 

 (IIIiFeoQs -f 2COOH— CHOH— CHOH— COOH = FeoOa + 



2COOH— COH=COH— COOH -f 2H0O. 



It is evident that iron peroxide contains two atoms of acti- 

 vated oxygen and that it is reduced to ferric oxide after this 

 oxygen is spHt off. It is worthy of note that ferric oxide is only 

 very slowly oxidised by hydrogen peroxide. The iron peroxide 

 does not regenerate itself with a sufficient velocity and so does 

 not represent a catalyst but an inductor. 



According to Bach and Chodaf* physiological oxidation in 

 plant cells is accomplished according to the same scheme which 



'Fenton. Jour. chem. soc. (London) 65: 899. i895. [Or the experiments by Ray 

 (J. Gen. Physiol. 5: 469, 611. 623. 1922-23) with unsaturated organic acids or plant 

 tissues.] 



2 Manchot und Wilhelms. Ber. d. chem. Gas. 34: 2479. I90i. 



3 According to Engler and Weissberg (Krit. Studien uber die Vorgange der Autoxydation. 



0\ /O 



P. 104) the structural formula of this peroxide is ; ^Fe— O— Fe<^^. It corresponds to the 



hydrate Fe(0H)2— O — OH which contains i atom of activated oxygen. 



•Bach. Compt. rend. 124: 954- i897; Ber. d. chem. Ges. 41: 216. 1908; Bach und 

 Chodat. /6iV. 35:2466, 3943. 1902; 36: 601. 1903:37:36. 1904; Arch, des SCI. phy- 

 siques at nat. de Geneve 17: 477- 1904; and others. 



