A METHOD FOR CALCULATING QUANTUM YIELDS FOR THE 

 FORMATION OF REACTION INTERMEDIATES 



Daniel Rubinstein 



The appearance or disappearance of absorption bands during 

 illumination of photo synthetic organisms has been observed by many 

 investigators beginning with Duysens (1). Whether or not these absorp- 

 tions bands are due to compounds in the main path of photosynthesis 

 could be best determined by quantum yield measurements. Photo- 

 synthesis in monochromatic red light has been observed to have a 

 quantum requirement of eight (±2) quanta per molecule of oxygen 

 evolved ^^\ A much larger quantum requirement for the appearance of 

 an absorption band would suggest that this band is not due to a compound 

 in the mainstream of photosynthesis. 



Quantimi yields have usually been estimated by examining the rate 

 of increase in absorbance, and dividing this rate by that of the light 

 absorption. The increase in absorbance is then converted into increase 

 in concentration (moles per liter) by assuming a plausible value for the 

 extinction coefficient. In some cases, such as the appearance of a band 

 at 420 mn in Porphvridium (3) or Chromatium (4) attributable to a 

 cytochrome, the extinction coefficient is known from measurements in 

 vitro; but for nearly all other difference bands, whose molecular origin 

 is unknown, a guess must be made. In such cases, it is usually 

 assumed that the extinction coefficient of the unknown compound is 

 approximately the same as that of chlorophyll (or a similar organic 

 pigment). 



Many plots of absorption changes as a function of exciting light 

 intensity have been published (4-10). in general, these have been 

 plotted to determine whether several absorption bands belong to the 

 same pigment, or whether an absorption change at a certain wave- 

 length should be attributed to more than one pigment(6) . If the plots for 

 changes at different wavelengths are identical, they may be due to the 

 same pigment. If a "light curve" (as these plots are designated) 

 appears to have inflections, more than one pigment may be involved. 

 Usually, no further information is sought from these plots. A more 

 detailed analysis shows, however that additional information maybe 

 deduced from them. Specifically, the shape of the light curve may 



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