Dynamic Exchange Properties of the Antiparallel Bacteriochlorophyll c Dimers

The dynamic exchange behavior of the (3¹R)-type bacteriochlorophyll (BChl) c dimer with an antiparallel piggy-back conformation has been investigated by two-dimensional nuclear magnetic resonance exchange spectroscopy (EXSY). The exchange rate between two BChl c molecules in the antiparallel dimer was evaluated from the integrated intensities of cross-peaks due to chemical exchange in the EXSY spectra, and its values were found to dramatically increase with the rise of solvent polarity. The temperature dependence of the exchange rates can be well expressed by the Arrhenius equation, the parameters of which show the correlation of the fast exchange rate with the stability of a transition state in the exchange process. Thermodynamic analysis was also applied to investigate the substituent effect on the exchange rate of the antiparallel dimer. The increase of the exchange rate upon addition of substituents at the peripheral 8-position was mainly attributed to the rise in frequency factor rather than activation energy. A high exchange of the (3¹R)-type BChl c dimer by a small amount of (3¹S)-type BChl c also resulted from the significant increase of the frequency factor. The substituent effects demonstrate that the antiparallel dimer formed by the most abundant BChl c homologue in the native light-harvesting entity, called chlorosome, can be highly exchanged by minor BChl c components. The results suggest that minor BChl c homologues in the chlorosome could bring about a phase transition of the most abundant BChl c from the stable dimer to a large aggregate.