FLUORESCENCE FROM MAJOR AND MINOR BACTERIOCHLOROPHYLL COMPONENTS IN VIVO*

Abstract— In the photosynthetic bacteria Chromatium, Rhoahpirillum rubrum, and Rhodopseudomonus spheroides the fluorescence of bacteriochlorophyll is probably free of contamination by a “fast” component of delayed emission, judging from the characteristics of the delayed light measured 3 msec after excitation. In Rps. spheroides the pigment P870, associated with photochemical reaction centers, is non-fluorescent in its photochemically active state. Fluorescence of P870 can be induced by either of two agencies that suppress its photochemical activity: exposure to Na₂S₂O₄ and (in a dry chromatophore film) dessication. The yield of fluorescence from the major (light harvesting) component of bacteriochlorophyll in vivo is brought to a common maximum value by conditions that suppress the photochemical activity of P870. In addition to dessication and exposure to Na₂S₂O₄ these conditions include saturating illumination and exposure to K₃Fe(CN)₆. Of these four treatments only the last two bleach the long wave absorption band of P870. These experiments support the following assertions: (1) P870 traps singlet excitation energy absorbed by the light harvesting BChl; the trapping function of P870 depends on its ability to initiate and participate in photochemistry. (2) Both dessication and exposure to Na₂S₂O₄ suppress the photochemical activity of P870 by blocking an event that proceeds directly from the excited singlet state in P870. (3) The fluoresecence of BChl in vivo is emitted almost entirely by a major (light harvesting) component.