174 INACTIVATION OF COMPLEMENT BY LIGHT 



III. 



But the progress of the Hght reaction is in turn affected by the 

 amount of Hght absorbed, and by the photochemical efficiency of the 

 absorbed hght. We may assume that the latter remains constant; 

 that is, that the same amount of light energy is needed at any stage 

 of the process to make a single molecule react. It is conceivable 

 that this might not be the case; but the very fact that the essential 

 reaction involved in the photoinactivation of com.plement seems to 

 follow the laws of a simple chemical reaction shows that such a 

 possibility is remote. 



It was to be feared, however, that the transparency of a sample 

 of serum would change during the course of photoinactivation. This 

 would have made it difficult or impossible to interpret the experiments. 

 If the transparency does remain constant it may do so for any of 

 several reasons. On the one hand the photolabile substance alone 

 may absorb the hght (photoproducts not absorbing) and be present 

 in sufficient concentration and depth to absorb all the light. In 

 this case the true order of the reaction is greater than the apparent 

 order by the number of photosensitive molecules participating. Thus 

 a bimolecular reaction appears monomolecular. It is obvious that as 

 such a reaction progresses and the photosensitive substance disappears, 

 the solution will ultimately transmit some light and the order of the 

 reaction can then no longer be calculated. On the other hand the 

 transparency may remain constant because a negligible portion of the 

 absorption is due to the photosensitive substance or because the 

 products of the light reaction have the same absorption coefficient 

 as the reacting substances. In either of these cases the intensity of 

 the hght impinging upon the photosensitive molecules is constant, 

 the amount of light absorbed may be assumed to vary as the con- 

 centration of absorbing molecules, i.e. Beer's Law,"* and, if the solution 

 is well stirred, the apparent order will be the same as the true order. 



It can be shown that a layer of complement of the depth and con- 

 centration used in determining the course of photoinactivation absorbs 

 practically all the effective light, and that the amount absorbed 

 remains constant during the reaction; i.e., that the photosensitive 



■^Beer, A., Ann. Phys. Chem., 1852, Ixxxvi, 78. 



