Absorption Spectrum Changes in Chlorella and the 



Primary Process 



BERNARD L. STREHLER, Vniversity of Chicago {Pels Fund), 

 Visiting Investigator, Carnegie Institution of Washington, Stanford, 

 California, and V. H. LYNCH, Carnegie Institution of Washington, 



Stanford, California 



The primary ohemical reaction in photos3Tithesis is generally be- 

 lieved to lead to intermediates (both reducing compounds and pre- 

 cursors of molecular oxygen) which are capable of reacting back to 

 form excited chlorophyll. This reaction is presumabh^ responsible for 

 the weak chemiluminescence emitted by photosynthesizing tissues 

 following their illumination (1). 



If the primary oxidants and reductants in photosynthesis differ 

 from their dark precursors in color, it should be possible experi- 

 mentally to learn something of their nature through sensitive spec- 

 trophotometric measurements. The technique of measuring bio- 

 chemical kinetics through absorption spectrometry has been de- 

 veloped to a high degree by Chance (2) and in a parallel manner by 

 Duysens (3), Lundegardh (4), Witt (5), and French (6), each worker 

 using somewhat different techniques. 



During the summer of 1955, through the courtesy of Dr. C. S. 

 French, it became possible for us to use his sensitive derivative spec- 

 trophotometer to examine absorption spectrum changes in Chlorella 

 induced by illumination. Some years earlier, similar experiments were 

 attempted by the senior author at Oak Ridge. At that time changes in 

 transmission in the region 500 to 550 m^t were found, but since we 

 were looking for cytochrome bands at the time, and since the spectral 

 difference bands were quite broad, we rejected these early results as 

 probably due to some instrumental error or scattering change. 



Duysens several years ago described striking absorption changes 

 in Chlorella during illumination. The present studies confirm Duysens' 

 earlier work and in addition unco\-er a number of new difference spec- 

 trum bands hitherto unreported, as well as some unexpected kinetic 

 behaA-ior and a probable relationship of the spectral changes to the 

 process of photosjTithetic luminescence. These studies show that the 



89 



