Monomers, dimers, and tetramers of 4-n-propyl-5-ethyl farnesyl bacteriochlorophyll c in dichloromethane and carbon tetrachloride

Abstract

Absorption (ABS) and circular dichroism (CD) spectra were recorded for 6 concentrations (2.0-290 μM) of bacteriochlorophyll (BChl) c in each solvent. Monomer spectra were obtained by adding methanol (1:200) to each sample. The monomer showed an ABS peak and a CD trough at 664 nm in CH2Cl2 (ABS peak at 665 nm in CCl4). Dimer-plus-monomer spectra were obtained by subtracting high concentration (e.g., 290 μM) spectra appropriately scaled from lower concentration (e.g., 26 μM) spectra. Pure dimer spectra were then obtained by subtracting monomer spectra appropriately scaled from dimer-plus-monomer spectra. The dimer showed an ABS peak at 679 nm in both CH2Cl2 and CCl4 and a CD trough at ca. 670 nm in CH2Cl2. The optical properties of the dimer do not agree with the model for bacteriochlorophyllide d [Smith KM, Bobe FW, Goff DA and Abraham RJ (1986) J Am Chem Soc 108: 1111-1120]. Higher aggregate spectra were obtained by subtracting appropriately scaled monomer and dimer spectra from high concentration (e.g., 290 μM) spectra. The aggregate showed ABS shoulders at ca. 636 and 678 nm with a peak at 702 nm in CH2Cl2 and at 708 nm in CCl4; the CD spectrum in either solvent showed peaks at 638 and 679 nm with troughs at 658 and ca. 710 nm. These spectra are consistent with an excitonic interaction between 4 chromophores in the aggregate. Each of the 12 original ABS spectra was deconvoluted in terms of the appropriate monomer, dimer and aggregate spectra, and the concentrations of each component were determined. Plots of log aggregate concentration vs. log dimer concentration lay on or near a line of slope 1.9 for CH2Cl2 and on or near a line of slope 2.1 for CCl4. The aggregate was thus shown to be a tetramer. The theoretical relationship between dimers and monomers (slope 2.0) was not observed in all cases.