Self Assembly of Bacterio Chlorophyll a and Bacteriopheophytin a in Micellar and Non-Micellar Aqueous Solutions; Application to the Pigment-Protein Organization in Light-Harvesting Complexes and Reaction Centers
While investigating the self assembly of photosynthetic pigments in aqueous solutions, it was found that very large aggregates maintained an equilibrium with monomers. This aggregation is cooperative and can be characterized by two equilibrium constants. The first equilibrium constant (Ka) describes dimer formation and the second one (Kb) describes the formation of larger oligomers. Ka and Kb for bacteriochlorophyll a (Bchla) are 1.5 × 103 M-1 and 2.2 × 106 M-1, respectively. Kb for bacteriopheophytine a (Bphea) is ∼109 M-1. The difference between the spectra of Bchl dimers and large oligomers is insignificant, indicating that the large oligomer consists of repeating dimers which are separated by ∼15Å from each other.
Self assembly of Bphea and Bchla has also been studied in aqueous solutions containing Triton X-100 (TX-100). To calculate the aggregation number and the corresponding equilibrium constant, we have developed a formalism which considers the pigment distribution in micelles of variable size. Application of the theory to Bchla and Bphea shows dimer formation in the micellar domain with Kd = 2.2 × 103 M-1 for Bchla in formamide/water (FW), Kd = 3.9 × 105 M-1 for Bphea in FW and Kd = 7.5 × 104 M-1 for Bphea in water. The photosynthetic pigments seem to have a major effect on the micelle size. In the pigment-free solutions the micelles contain ∼150 molecules of TX-100, while the addition of pigments alters this to 4000–40000 molecules, depending on the system.
Comparison with in vivo pigments suggests that the Bchla self assembly tunes the photosynthetic pigments to the prevailing light conditions and at the same time affects the organization of the polypeptide units within the intercytoplasmic membrane. This mechanism may explain the relationship between pigmentation and polypeptide assembly in light-harvesting complexes and reaction centers in photosynthesis.
KeywordsPhotosynthetic Pigment Photosynthetic Bacterium Photosynthetic Reaction Center Micelle Size Pigment Molecule
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