Singlet--singlet excitation annihilation measurements on the anntenna of Rhodospirillum rubrum between 300 and 4 K

• Published in: Biochimica et biophysica acta Vol. 976; no. 1; pp. 63 - 69
• Main Authors: Deinum, G, Aartsma, T.J, Grondelle, R. van, Amesz, J
• Format: Journal Article
• Language: English
• Published: 1989
• Subjects: bacteria; fluorescence emission spectroscopy; photosynthesis; temperature
• ISSN: 0006-3002; 1878-2434

By means of fluorescence measurements singlet-singlet excitation annihilation in the antenna of Rhodospirillum rubrum chromatophores was studied at temperatures between 300 and 4 K. Two fluorescence bands had to be assumed to explain the data at 4 K. These bands (F911 and F918) are attributed to emission from the main antenna pigment BChl 880 and from a minor spectral component BChl 896, respectively. The latter pigment is supposed to be associated with the reaction center (Van Grondelle, R., Bergstrom, H., Sundstrom, V., Van Dorssen, R.J., Vos, M. and Hunter, C.N. (1988) in Photosynthetic Light-Harvesting Systems (Scheer, H. and Schneider, S., eds.), pp. 519-530, Walter de Gruyter, Berlin). At 4 K 90% of the fluorescence originates from this pigment. Analysis of the data at 4 K indicated that BChl 896 is arranged in clusters of 21 +/- 9 BChls. Since there are presumably only 5 or 6 BChls 896 per reaction center, this indicates that several reaction centers are clustered. Annihilation measurements on the isolated B880 antenna complex indicate that, at 4 K, the spectral heterogeneity still exists and that the presence of BChl 896 is therefore not caused by interactions between the reaction center and antenna. The domain size of BChl 880 was estimated to be between 35 and 75 BChls in chromatophores at 4 K. A wavelength dependence of the annihilation, due to the presence of the two types of BChl, was observed between about 100 and 4 K. Measurements of fluorescence polarization suggest that this wavelength dependence may be caused by a reduction of the rate of energy transfer from BChl 896 to BChl 880 upon cooling. An increased annihilation between 100 and 300 K can, at least partly, be ascribed to an enhanced rate of energy transfer between individual BChls. This rate increases from an average value of (4 +/- 1) . 10(11) s-1 at 100 K to (1.5 +/- 0.5) . 10(12) s-1 at 300 K.