Interaction of photosynthetic pigments with various organic solvents 2: Application of magnetic circular dichroism to bacteriochlorophyll a and light-harvesting complex 1

Abstract

Magnetic circular dichroism (MCD) and absorption spectra of metal bacteriochlorin complexes have been measured on bacteriochlorophyll (BChl) a in various solvents and different forms of light-harvesting complexes 1 (LH1 complexes). In hydrophilic organic solvents, the MCD intensity of the Q y(0–0) transition of BChl a was sensitive to the wavelength of absorption maximum of Q x(0–0), and the ratio of MCD Q y(0–0) intensity to the dipole strength ( B/ D) was inversely proportional to the difference in energy between the Q x(0–0) and Q y(0–0). The similar correlation has been observed in metal chlorin derivatives as previously reported. The correlation depends on the coordination number of the Mg atom in BChl a and the molecules ligating to it. In a hydrophobic solvent such as carbon tetrachloride (CCl 4), however, the correlation did not hold because of the existence of aggregates. Hence, the correlation between the values of B/ D and the energy difference can be used to estimate the type and number of the molecules ligated to the Mg atom and to disclose the existence of aggregated pigments. We further apply the correlation to the LH 1 complex treated with n-octyl β- D-glucopyranoside.