412 BIRGIT VENNESLAND 



respectively, and indicate only the chemical identity of the initial reactants 

 and final products. 



Q + H.O ^H.Q + iOo (i) 



2Fe(CN)r+H20 ■ > 2Fe(CN)^-+2H+ + iOo (2) 



The possible mechanism of the Hill reaction can be represented 

 schematically in many ways [5]. A formulation which has been used 

 quite frequently is depicted in Scheme i, p. 413. At the top is a diagram 

 often employed to represent the oxidation-reduction reactions underlying 

 the Hill reaction. Photons, in the presence of chlorophyll, supply the 

 energy for a dismutation reaction which results in the formation of 

 a reductant XH, and an oxidant, YOH, from X and Y and water. The 

 reductant is available to reduce an added oxidant, the Hill reagent; 

 and the oxidant somehow excretes its oxygen in the form of O^. The 

 photon-requiring step, shown in equation i, is sometimes referred to as 

 the "splitting of water". X and Y are generally regarded as "built-in" 



hv 



Chi 



/ x,y\ 



/ \ 



XH YOH — ^ O, 



X+Y+HOH >XH+YOH (i) 



XH + Hill reagent > X + reduced Hill reagent (2) 



YOH >i02 + YH (3a) 



YH + Hill reagent > Y + reduced Hill reagent (3a') 



2YOH >J02 + 2Y + H,0 (3b) 



Scheme i 



components of the chloroplast. Since water is required to balance the 

 equation for the Hill reaction, it is convenient to bring it in at an early 

 stage. We should note, however, that it is impossible to denote how the 

 elements of water participate in the reaction unless we know the structural 

 formulae of all the reaction components. Equations 2 and 3 in Scheme i 

 show various steps in the process of reconverting XH and YOH to X and Y. 

 The regeneration of X is pictured as an oxidation of XH by the Hill oxidant 

 in equation 2. Two possible ways of regenerating Y are shown. One 

 alternative involves the formation of a second reductant, YH (equation 3a), 

 which reduces another molecule of oxidant (equation 3a'). The other 

 alternative shown is a dismutation between two molecules of YOH 

 (equation 3b). 



