Circular and magnetic circular dichroism studies of bacteriochlorophyll c aggregates: T-shaped and antiparallel dimers

Circular dichroism (CD) and magnetic circular dichroism (MCD) spectra have been measured on (3¹R)-bacteriochlorophyll (BChl) c aggregates in organic solvents. BChl c exists as a monomer in acetone with a Q_y(0-0) transition at 661 nm. In a mixed solvent system of methanol and dichloromethane (CH₃OH/CH₂Cl₂ = 1/4000), another Q_y(0-0) transition appeared at 680 nm with a nondispersion-type CD signal. The ratio of MCD intensity to its dipole strength (B/D) for the Q_y(0-0) transition is half the value of its monomer species, indicating a dimerization of BChl c. The apparent inconsistency between the CD and MCD results has been reconciled by assuming the formation of BChl c dimer with a T-shaped conformation. In neat CCl₄, a dispersion-type CD signal was observed for the Q_y(0-0) exciton components at 680 and 710 nm. MCD spectra of the exciton-type Q_y(0-0) components suggest that two BChl c molecules are parallel with their macrocycle planes as judged by the mixing effect between the excited states of Q_x(0-0) and Q_y(0-0). The CD and MCD results are consistent with the fact that a BChl c dimer with an antiparallel conformation is dominant in the neat CCl₄ as reported previously. Corresponding Q_x(0-0) transitions were determined around 650 nm by the simultaneous deconvolution of absorption, CD, and MCD spectra. It is demonstrated that the antiparallel BChl c dimer shows two exciton components for both Q_x(0-0) and Q_y(0-0) transitions. Finally, we evaluated solvent dependence of the stability of the BChl c dimer by two-dimensional exchange spectroscopy (EXSY) experiments.