PRIMARY QUANTUM CONVERSION: EPR EVIDENCE 



339 



has not been possible to observe either the presence or absence of a 

 resolved peak in the action spectrum due to the secondary absorption 

 peak at X = 8000 A in i?. rubrum. It is felt that this is due to difficul- 

 ties in experimental design, Androes etal. (11) have demonstrated that 

 self- absorption effects may distort the action spectra when too high a 

 sample concentration is used. Published action spectra have also suf- 

 fered distortion from the use of light intensities capable of saturating 

 the photoresponse, A better defined maximum at 8800 A in the action 

 spectrum has been obtained in R. rubnim by reducing the light inten- 

 sity an order of magnitude below that used by Androes (=^10^^ 



Rhodospirillum rubrum chromatophores 



lo-io'^ Q/sec, Temp. = 300*' K 



ACTION SPECTRUM 



I 



r^ — ABSORPTION I 



/T 



^ % 



7000 8000 9000 10,000 



WAVELENGTH (A) 



Fig. 4. Action spectrum of chromatophores from R. riihvum, taken at 

 constant quanta/sec-cm2 incident on the sample. The absorption spectrum 

 is shown in dotted lines for comparison. 



quanta/sec- cm2), and is shown in Fig. 4. Distortions still mask pos- 

 sible contributions by the secondary peak. Further reduction of light 

 intensity and/or concentration reduces the magnitude of the signal to 

 nearly that of the noise observed in our spectrometer. Thus we face 

 the need for a significant increase of the signal-to-noise ratio, which 

 will require extensive instrumentation. In accordance with the above 

 observation, Weaver (15) has recently published an action spectrum of 

 Chlamydomonas reinhardi in which distortion was reduced by extrapo- 

 lation of the light-induced signal to low light intensities. 



