516 



Leo P. Vernon, Waldo S. Zaugg and Elwood Shaw 



response to light intensity. Whereas the FMN and vitamin K3 reactions satur- 

 ate at a moderate light intensity (20, 000 lux), the PMS supported reaction does 

 not saturate at light intensities of 50, 000 lux or at 154, 000 lux i^^). At these 

 high light intensities the photophosphorylation rates obtained with PMS are 

 high. This response indicates that PMS couples to the photochemical system 

 in a unique manner, and furthermore that the photochemical reaction is the 

 rate limiting step at all intensities employed in the PMS catalyzed photophos- 

 phorylation system. This is born out by the experiments of Hall and Arnon 

 who investigated the effect of ternperature upon various photophosphorylation 

 systems at low light intensity '^^'. Whereas vitamin K3 and FMN stimulated 

 photophosphorylation responded in the usual manner to temperature changes 

 from - 10 to 15° C, the PMS supported photophosphorylation did not change 

 through this temperature range. This response was to be expected, since the 

 photochemical reaction is limiting under these conditions. 



The present investigation shows that PMS does indeed react directly 

 with either the chloroplast or isolated chlorophyll a in the presence or 

 absence of detergents. Furthermore, this reaction can be coupled to the 

 reduction of quinones. The PMS photoreactions catalyzed by both chloro- 

 plasts and isolated chlorophyll respond in like manner to type of detergent 

 present and pH of the medium, suggesting a direct participation of the chloro- 

 phyll in the chloroplast when the intact chloroplast is used. This is consis- 

 tent with the unique response of PMS to light intensity in the photophosphory- 

 lation reactions listed above '^^' ^'^' and also agrees with the observation 

 made by the Marburg group (3) that when concentrations of reduced PMS 

 larger than 10'^ M are used, the PMS bypasses the cytochrome and reacts 

 directly with the chlorophyll a. 



It is apparent that the reaction described in these experiments is com- 

 plex. It is influenced by the nature of the detergent and the pH of the 

 medium, indicating it is sensitive to a change in the charge composition of 

 the medium. Although interesting speculations can be made concerning the 

 correlation of detergent type (anionic, neutral and cationic detergent) and 

 the reactions catalyzed, it would be premature to so extend the present data. 

 It is well known that the photochemical properties of dyes are markedly 

 changed by the environment in which they are found. This is particularly 

 true when the dyes are adsorbed on the surface of macromolecules as shown 

 by Oster and Bellin ^^^' ^^' and more recently by Kostenbauder and DeLuca, 

 who studied the ohotoactivity of riboflavin in the presence of sodium 

 decylsulfate ^^^'. 



The ability of quinones to act as electron acceptors in photochemical 

 reactions involving chlorophyll is well known. As an example of this type of 

 reaction can be cited the recent experiments of Tollen and Green ^^°' ^ ' , 

 who investigated electron transfer reactions between excited chlorophyll a 

 molecules and various quinones (including UQ^) dissolved in organic solvents. 

 These investigators showed that electron transfer to the quinone does occur, 

 and furthermore the structure of the quinone used as electron acceptor has a 



