314 CATALYST POISONS AND NARCOTICS CHAP. 12 



free oxygen {cf. Scheme 6.1). Apparently, the more sensitive of these 

 two reactions is the evolution of oxygen from the intermediate {O2}; 

 this is why ordinary photosynthesis is inhibited by very small quantities 

 of hydroxylamine. Larger quantities, however, also inhibit the reaction 

 by which the intermediate { O2 } is formed from the primary oxidation 

 products, {OH} or Z. This explains the protection of the adapted state 

 by hydroxylamine: in the presence of a sufficient quantity of this poison, 

 the primary oxidation products, which are not transformed by the 

 hydrogenase system, are prevented from being converted into the 

 "de-adapting" oxidants, {O2}, and instead disappear harmlessly, prob- 

 ably by back reactions with the primary reduction products, { H } or HX. 



The comparatively high concentrations of hydroxylamine used in 

 some of Gaffron's experiments made it possible that certain of the 

 observed affects could be due to compound formation with the carbonyl 

 groups in the metabolites, rather than to enzyme poisoning. However, 

 Gaffron (1944^) found that all effects caused by hydroxylamine can also 

 be obtained by means of comparatively small quantities of o-phenanthro- 

 line or phthiocol (2-Me-3-hydroxy-l,4-naphthaquinone), which have no 

 affinity for carbonyl groups; they are thus probably all due to specific 

 interactions with photosynthetic enzymes. Phthiocol is a compound 

 related to vitamin K, known to be present in green plants. 



Experiments with o-phenanthroline, as well as with phthiocol, have 

 confirmed an interesting observation (made earlier with hydroxylamine, 

 but considered uncertain because of the high concentration of the poison 

 which had to be used), that the rate of photoreduction in Scenedesmus 

 can be reduced by these poisons to one-half its usual value, but not any 

 further (Gaffron 19442). The quotient AH2/ACO2 remains equal to 2, so 

 that the over-all process still is that represented by equation (5.6), but 

 its quantum yield is only 1/16 (if the normal value was 1/8). Thus, 

 poisons of this type either block only one of two equally efficient channels 

 of photoreduction, or, more probably, block the normal path completely, 

 but leave open an alternative path which is half as efficient as the one 

 normally used. Gaffron suggested that this alternative mechanism may 

 be related to the mechanism of chemosynthesis in the same algae. It 

 was stated in chapter 9 (page 239) that the properties of hydrogen- 

 adapted algae indicate their capacity to use oxygen instead of the primary 

 photochemical oxidation product (Z or {OH}), and hydrogen instead of 

 the primary reduction product (HX or {H } ). Chemosynthesis of hydro- 

 gen bacteria was attributed on page 236 to a coupled reaction of hydrogen 

 with oxygen and carbon dioxide, which may be formulated as follows by 

 combining equations (6.11a) and (6.12): 



(12.3) 6 H,Ah ^ + j CO, } . { CH,0 } + H3O + 2 Ah 



