119 



Eugene Pabinowitch 



photosynthesis in vivo remains inconclusive. We know from 

 the work of Krasnovsky, Evstigneev and coworkers, that, in 

 vitro , chlorophyll sensitizes energy-storing oxidation- 

 reductions, (such as that of riboflavin by ascorbic acid), 

 by itself undergoing reversible reduction. 



Measurement of difference spectra by Coleman (in our lab- 

 oratory), and by Kok at RIAS, suggested that in sufficiently 

 intense light, difference bands appear in the region of 

 chlorophyll absorption, in the red as well as in the blue, 

 indicative of reversible bleaching of chlorophyll, beginning 

 when saturation of photosynthesis sets in. However, neg- 

 ative difference bands appearing in the neighborhood of 

 680-690 mp, could be due to changes in fluorescence. 

 (Fluorescence caused by the modulated measuring light beam 

 could be made more intense by strong "background" of non- 

 modulated actinic light; it is known that the yield of 

 fluorescence in vivo is about doubled when cells pass from 

 the "light-limited" into the "light-saturated" state). 

 Coleman thought that he had disposed of these objections by 

 re-routing the light beam under a right angle to its orig- 

 inal direction, and decided that not more than 10'^ of the 

 difference band he observed at 680 m;i could be due to fluor- 

 escence effects. More recently, however, Mr. Rubinstein in 

 our laboratory found--by measurements in polarized light-- 

 that a much larger proportion, (perhaps, the whole) of the 

 negative difference bands observed at 680 mji, were in fact 

 due to fluorescence changes. Independently, similar con- 

 clusions were reached by Krasnovsky and coworkers. 



However, certain true changes in absorption spectra in 

 the chlorophyll a^ absorption region, do occur; they include 

 a "negative" band at 648 mp, and a "positive" one at 658 mu. 

 These bands were first observed by Strehler, and also noted 

 by Coleman. Recently, they were studied much more precisely 

 by Rubinstein in our laboratory. Their occurence suggests 

 that some reversible photochemical transformations do occur 

 in the chlorophyll system during photosynthesis; but whether 

 these are in the nature of oxidation-reductions (such as 

 would be expected if chlorophylls were to serve as a wayside 

 station in the transfer of electrons or TT-atoms in photo- 

 synthesis) remains uncertain. Kok found a difference band 

 at about 705 mji, which he attributed to reversible photo- 

 chemical transformation of a long-wave component of the 

 chlorophyll system directly associated with an enzymatic 

 center and serving as a final "sink" in the energy trans- 



