Structural Transformation Among the Aggregate Forms of Bacteriochlorophyll c as Determined by Electronic-Absorption and NMR Spectroscopies: Dependence on the Stereoisomeric Configuration and on the Bulkiness of the 8-C Side Chain ‡

Abstract

Transformation among the aggregate forms of bacteriochlorophyll (BChl) c characterized by the wavelength of the Qy absorption, i.e. the dimer (B675), B705, B720 and B745, was traced by electronic-absorption spectroscopy for each of the isomers including R[E,E], R[P,E], R[I,E], S[P,E] and S[I,E] suspended in the mixtures of methylene chloride and n-hexane. A combination of NMR spectroscopy determining the structural motifs and calculation of the shift of the Qy absorption reflecting the long-range transition dipole–transition dipole interactions among the macrocycles in the entire aggregate structures proposed the following models: B705d (B705d′), a linear array of straight (inclined) columns consisting of a pair of the piggyback dimers; B720d and B745d, an assembly of two and five shifted-inclined columns consisting of more than six piggyback dimers; and B720m and B745m, an assembly of one and two parallel stepwise stacking of ∼30 monomers. Calculations of the steric energies rationalized two different pathways of transformations: the dimer → B705d (B705d′) → B720d → B745d for the R isomers; and the monomer → (B720m) → B745m for the S isomers. Addition of S[I,E] seems to trigger the B745d → B745m transformation of the R isomers.