Autocatalyzed self-aggregation of (31R)-[Et,Et]Bacteriochlorophyll cF molecules in nonpolar solvents.: Analysis of the kinetics

The present work presents a detailed kinetic investigation of the autocatalytic aggregation of (3(1)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 uf 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.