1610 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



Azide. An inhibition of the Hill reaction by sodium azide was ob- 

 served by French, Holt, Powell and Anson (1946), Macdowall (1949) (c/. 

 Table 35. XI), and Arnon and Whatley (1949), but not by Hill and Scaris- 

 brick (1940) and Aronoff (1946). 



Clendenning and Gorham (1950) found that the effect of azide is dif- 

 ferent with quinone and with Hill's mixture as oxidants. Wessels (1954) 

 noted that NaNs reacts with quinone. Low concentrations (<6.10~' 

 mole/1.) of azide caused only partial inhibition of oxygen liberation with 

 quinone as oxidant. The strong inhibition, observed by Arnon and What- 

 ley with quinone (50% inhibition at 6 X 10~^ mole/1.) must have been due 

 to a reaction of the latter with the azide. With 2,6-dichlorophenol-indo- 

 phenol (which does not react with NaNs) Wessels found only partial inhibi- 

 tion by 6 X 10~^ mole/1. NaNs, and complete inhibition by 6 X 10"^ mole/ 

 1., in approximate agreement with Macdowall's data. 



Hydroxylamine. Hill and Scarisbrick (1940) reported that the Hill 

 reaction is unaffected not only by cyanide but also by hydroxylamine — a 

 result which seemed to disagree with the conclusion, reached in chapt. 12 

 (Vol. I, page 311) that hydroxylamine probably is a specific poison for the 

 oxygen-liberating enzymatic system in photosynthesis. However, Hill's 

 observations were confirmed by Aronoff (1946), who found no effect of 

 hydroxylamine on photoreduction of quinone by fragmented chloroplasts 

 from spinach leaves. 



French, Holt, Powell, and Anson (1946), on the other hand, noted an 

 inhibition of the photoreduction of Hill's mixture by hydroxylamine; and 

 Macdowall (1949, cf. Table 35.XI) found that 3 X 10 -Mf hydroxylamine 

 inhibits the oxygen liberation by chloroplasts, with phenol-indophenol as 

 oxidant, by 50%. The effect was about the same in strong and in weak light 

 (see Vol. I, page 312 for similar observations in photosjnithesis) . Arnon 

 and Whatley (1949) also found hydroxylamine to be a strong inhibitor for 

 the photoreduction of quinone by chloroplasts. 



Clendenning and Gorham (1950) found that 10"^ mole/1. NH2OH in- 

 hibited the photoreduction of quinone but not of Hill's mixture. Wes- 

 sels (1954) noted that, similarly to azide, hydroxylamine reacts with 

 quinone (and also with 2,6-dichlorophenol-indophenol, probably reducing 

 the dye to a leucodye). To be able to observe the true effect of NH2OH on 

 Hill's reaction, Wessels used toluylene blue, whose E'q is too high for it to be 

 reduced by hydroxylamine. He found a strong, albeit not complete, in- 

 hibition by 6 X 10~^ mole/1. NH2OH; the surmise that hydroxylamine 

 must inhibit the Hill reaction was thus confirmed. The controversial re- 

 sults obtained by other observers who used stronger oxidants, which react 

 directly with NH2OH, must have been due to differences in the relative 

 amounts of the reactants (perhaps also in temperature and schedule of the 

 experiments). 



