Autocatalyzed Self-Aggregation of (3R)-[Et,Et]Bacteriochlorophyll c_F Molecules in Nonpolar Solvents. Analysis of the Kinetics

Abstract

The present work presents a detailed kinetic investigation of the autocatalytic aggregation of (31 R)-[Et,Et]BChl c F (BChl c) which is the main bacteriochlorophyll contained in the light-harvesting antennae of Chlorobium tepidum. Upon dilution with n-hexane, solutions of BChl c in dichloromethane form the same large aggregates as are encountered in the light-harvesting antennae. These aggregates are characterized by a strongly red-shifted absorption maximum at 739 nm and a tubular structure. The rate of formation of the 739-nm maximum increases dramatically with BChl c concentration. The time dependence includes an induction period followed by an exponential buildup and thus results in an overall sigmoidal growth of the aggregate in time. It is evidence in favor of an autocatalytic self-assembly process starting from spontaneously formed critical nuclei, rather than the instant and direct aggregation hitherto assumed for BChl c. The empirical equation is presented that describes the buildup of the aggregates in the exponential phase as a function of time and of the overall BChl c concentration. Kinetic modeling based on a two-dimensional sheet structure of the 739 nm aggregates (assuming that the tubuli have formed by closure of curved sheets) and on the experimental finding that the 739-nm aggregates have an architecture different from that of the majority of the 680- and 705-nm oligomers that prevail in CH2Cl2 reproduces this empirical equation. The equation does not discriminate between four formally conceivable two-dimensional patterns of arrangement of the BChl c molecules in the sheets, including the one that is strongly favored by already existing experimental and computational evidence. Comparison between the empirical and the kinetically modeled equations yields the size of the critical nuclei as ca. 14 BChl c molecules for three (including the one that is strongly favored) of the four possible patterns and as ca. 10 for the fourth one.

Structure of the light-harvesting bacteriochlorophyll c assembly in chlorosomes from Chlorobium limicola determined by solid-state NMR

“…The farnesyl chains disrupt the interaction in the x direction in Fig. 4G when BChl molecules are stacked with coordination bonds between Mg and C3 1 -OH as reported (9)(10)(11). Thus BChl c molecules would stabilize the planar structure.…”

“…The rod elements of Chlorobium limicola, the target of this work, are composed of BChl c. Light energy captured by BChl c in the rod elements is transferred to the reaction centers in the cytoplasmic membrane through BChl a in baseplate proteins (7). The BChl c molecule has two stereoisomers and homologues as shown in 1 -hydroxyl group and the Mg atom and a hydrogen bond between the coordinated 3 1 -hydroxyl and 13 1 -carbonyl groups (9)(10)(11).…”

“…1 A) are parallel to each other. Balaban et al (11,16) and van Rossum et al (17) interpreted the solid-state NMR data as monomer-based parallel-chain stacking. Mizoguchi et al (8) also proposed a monomer-based parallel-chain model on the basis of NMR investigations on BChl c aggregates in solution.…”

Solvation effect and aggregation of semipermanent spiro[indole-phenanthrolinoxazines] in CH3CN / H2O binary solvent

“…Fluff-like heterogeneous particles were formed and then precipitated. 15 Conditions for the observation of aggregation. Aggregation was observed on increasing both the spiroxazine concentration and the water content.…”

Atomic structure of the bacteriochlorophyll c assembly in intact chlorosomes from Chlorobium limicola determined by solid-state NMR

“…limicola, (Mizoguchi et al 2000), which is the subject of this review. A series of investigations have indicated that a coordination bond of the 3 1 -hydroxyl group in a BChl c molecule to the Mg atom in a neighboring one, and a hydrogen bond between the coordinated 3 1 -hydroxyl and 13 1 -carbonyl groups are involved in the BChl c assembly formation (Nozawa et al 1990;Hildebrandt et al 1994;Blankenship et al 1995;Balaban et al 2000;Mizoguchi et al 2000). The structure determination of the BChl c assembly in chlorosomes is a long-standing problem.…”