196 B. CHANCE, M. BALTSCHEFFSKY, L. SMITH 



but in this case there then follows a slow decrease of optical density 

 which is attributed to oxidation of those pigments reduced by DPNH. 

 On cessation of illumination, the rapid effect is again completed be- 

 fore the slow one gets under way. The oxidation-reduction level then 

 returns very nearly to its initial value. 



Effect of light upon DPNH disappearance. In order to determine 

 whether the rate of DPNH disappearance is affected by infrared il- 

 lumination, we have repeated a portion of the experiment of Fig. 4 

 in Fig. 5 and have measured light absorption changes at 340 m^u. 

 Addition of DPNH causes an increase of optical density at 340 m/x, 

 recorded as an upward deflection. The utilization of DPNH produces a 

 downward deflection. Illumination causes such a very slight slacken- 

 ing of the rate of DPNH utilization that one may raise the question 

 as to whether DPNH participates in the light-induced reaction. 

 In addition, these effects on DPNH kinetics are irregular; in some 

 cases, an acceleration of DPNH disappearance was noted. 



INTERPRETATION 



Our current interpretation of the rapid increases of light absorp- 

 tion at 430 mfj, following infrared illumination of the cell extracts or 

 of the whole cells treated with phenyl mercuric acetate is as fol- 

 lows: the increased absorption corresponds (1) to the reduction of 

 a hemoprotein (£) or to the formation of an intermediate compound 

 of chlorophjdl, possibly an oxidation product as suggested by Duysens. 

 On the one hand, the peak in the region 430 to 434 m/x suggests hemo- 

 protein reduction (except for the broadness of the band) but, on the 

 other hand, the trough at 385 m/x suggests a disappearance of chloro- 

 phyll. 



There are a number of arguments against the simple explanation 

 that the effects at 430 to 434 m^ are caused by conversion of all the 

 chlorophyll into a new compound. The magnitude of the spectroscopic 

 change (about 0.025) corresponds to a very small fraction of the total 

 chlorophyll content (<1%, see Discussion), and is not increased with 

 stronger infrared illumination. Such a small extent of reaction is 

 inconsistent with the kinetics of the light reaction which is 4 to 9 

 times the speed of the dark reaction (see Fig. 3) and should correspond 

 to a large degree of completion of the reaction, i.e., conversion of all 

 the dark form of the 430 m^ pigment. Thus, if chlorophyll is this dark 

 form, only about 1% of the total cell chlorophyll can participate in 



