Investigations in the Photosynthetic Mechanism of 

 Purple Bacteria by Means of Sensitive Absorption 



Spectrophotometry 



L. N. M. DUYSENS,* Biophysical Research Group, Department of Physics, 

 University of Utrecht, Utrecht, Netherlands 



METHODS AND MATERIALS 



Apparatus. The apparatus used is described in another paper in 

 this volume. 



Bacieria-Rhodo spirillum ruhrum strain 1 was obtained through the 

 courtesy of Dr. C. B. van Niel, strain 4 from the Biophysical Re- 

 search Group, Utrecht. Both strains were grown for 1 or 2 days in 

 incandescent light, in stoppered test tubes completely filled with 

 1% Difco bactopeptone, strain 4 with the addition of V2% sodium 

 chloride. All experiments were done at room temperature: 18 to 

 24 °C. An aqueous extract of the bacteria was prepared in the 

 homogenizer according to French (c/. Milner et at. (1) ). 



The changes in absorption were measured in a 1-cm. Beckman 

 cell; the optical densities of the bacterial suspensions at 880 m^u 

 minus the optical densities at 960 m;u (to correct for scattering) were 

 about 1.0 as measured with a Beckman DU spectrophotometer. 

 These wavelengths were selected because the infrared maximum 

 of bacteriochlorophyll in intact cells is located at 880 m/^, while the 

 intrinsic absorption of the extracts is negligible at 960 m/x. 



CHANGE IN ABSORPTION AT ONE WAVELENGTH 



The time course of the changes in absorption appeared to depend 

 upon the suspension medium and the intensity of the exciting light. 

 Figure 1 shows the changes in optical density of a suspension of 

 Rhodo spirillum ruhrum strain 4 at 430 m/i in anaerobic peptone. For 

 intensities of the order of magnitude at which saturation of photo- 

 synthesis just occurs, the absorption decreases upon illumination 



* Future address: Biophysical Laboratory, Nieuvvsteeg, State University, 

 Leiden, Netherlands. 



164 



