610 



A. A. KRASNOVSKIÏ 



absorbed by the pigment molecule is stored in the active photoproduas thus 

 formed; when the light is switched off, the reaction proceeds in the backward 

 direction, yielding the original pigment molecule. In this manner chlorophyll 

 and its analogues can play the part of a photocatalyst, thermodynamically raising 



Bocteriophaeophytin 



Protochlorophyll 



400 



Fig. I. Structure and absorption spectra of pigments before photoreduction 



and after the back-reaction in the dark. 



Spectra of photoreduced forms are shaded (after illumination in pyridine 



with ascorbic acid, in vacuo). 



the electron of the donor molecule 'uphill', which distinguishes the photo- 

 catalytic action of the pigments from the 'dark' catalytic action of cytochromes 

 which transfer electrons (in the thermodynamic sense) 'downhill'. 



The reaction of photoreduction is observed in all pigments with a structure 

 based upon a porphyrin- or azaporphyrin-ring system of conjugated double 



