Autocatalyzed self-aggregation of (3(1)R)-[Et,Et]Bacteriochlorophyll c(F) molecules in nonpolar solvents. Analysis of the kinetics

The present work presents a detailed kinetic investigation of the autocatal ytic aggregation of (3(1)R)-[Et,Et]-BChl c(F) (BChl c) which is the main ba cteriochlorophyll contained in the light-harvesting antennae of Chlorobium tepidum. Upon dilution with n-hexane, solutions of BChl c in dichloromethan e form the same large aggregates as are encountered in the light-harvesting antennae. These aggregates are characterized by a strongly red-shifted abs orption maximum at 739 nm and a tubular structure. The rate: of formation o f the 739-nm maximum increases dramatically with BChl c concentration, The time dependence includes an induction period followed by an exponential bui ldup and thus results in an overall sigmoidal growth of the aggregate in ti me. It is evidence in favor of an autocatalytic self-assembly process start ing from spontaneously formed critical nuclei, rather than the instant and direct aggregation hitherto assumed for BChl c. The empirical equation is p resented that describes the buildup of the aggregates in the exponential ph ase 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 aggregate s (assuming that the tubuli have formed by closure of curved sheets) and on the experimental finding that the 739-nm aggregates have an architecture d ifferent from that uf the majority of the 680- and 705-nm oligomers that pr evail in CH2Cl2 reproduces this empirical equation. The equation does not d iscriminate between four formally conceivable two-dimensional patterns of a rrangement of the BChl c molecules in the sheets, including the one that is strongly favored by already existing experimental and computational eviden ce. 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.