Primitive and Present-day Photobiological Processes 621 



energy, and is followed by recombination in case (a) and either recombination 

 or attack on some other molecule, resulting in increased molecular size, in case {b). 

 If we rule out the possibility of aromatic ions, since we suppose ourselves 

 dealing with the pre-aromatic stage of synthesis, molecules containing — SH 

 groups and — S — S — bonds are the most plausible choices for ion and radical 

 formation respectively. Sulphur compounds could also very reasonably provide 

 the traps required for the nondissociative step mentioned previously. We there- 

 fore look very carefully at the role of sulphur in present-day biology. 



EVIDENCE FROM EXISTING BIOLOGICAL PROCESSES 



The great importance of such sulphur-containing molecules as coenzyme A, 

 glutathione and SH-enzymes is well known. The problem that confronts us is 

 how to distinguish a primitive process from one of later development. The 

 most reasonable approach appears to be: 



Look for common features in natural processes still intimately connected with 

 a hght absorption or emission step. 



Observe how organisms react to excessive amounts of radiation. 



The processes which have so far been examined are set down below. 



Photosynthesis 



The generality of the photosynthetic process makes it clear that whereas it is 

 not a primary process (the photosynthetic pigments themselves being more 

 complex than the molecules being considered here, and anyway unable to survive 

 irradiation with 2000 Â hght), it does belong far back in the evolutionary se- 

 quence. It is therefore reassuring to find 6 : 8-dithio-octanoic acid postulated 

 by Calvin and his co-workers [3] as the most likely electron-acceptor of the 

 photoelectrons formed on irradiation of the chlorophyll grana. 



CHo COOH CH2 COOH 



/ \ / / \ / 



CH CH(CHo)4 CH2 CH(CH2)4 



11^ II 



S S +2e'^S- S- 



If Calvin's postulated mechanism is correct, the thioctic acid occupies a key 

 position linking the photosynthesis with the Krebs cycle. This seems quite in 

 keeping with the idea that sulphur occupied a key rôle in primitive bioenergetics. 



The cytochrome system is not directly concerned with a photo-process, but 

 in view of the structural similarity between the haem and chlorophyll molecules 

 the confirmation of Theorell and his co-workers [4] that the haem is attached to 

 its protein by two cysteine residues provides an interesting connection between 

 the two systems. 



Vision 



Wald & Brown [5] found that the photobleaching of rhodopsin (the protein- 

 retinene compound responsible for rod vision) hberated free SH-groups and 

 that the resynthesis of rhodopsin from retinene and opsin could be inhibited if 



