How the formation process influences the structure of BChl c aggregates

The change of absorption spectra has been measured during the drying process of (3¹ R)bacteriochlorophyll (BChl) c from diethyl ether, dichloromethane (CH₂Cl₂) and carbon tetrachloride (CCl₄) solutions. Absorption maxima of the Q_y(0–0) transition of BChl c appear at 659 nm in diethyl ether, 680 nm in CH₂Cl₂ and 710 nm in CCl₄. All these peaks are red-shifted to about 740 nm with formation of solid high aggregates when the solutions are completely dried. Fourier transform infrared spectra of the three solid aggregates are almost identical. However, magnetic circular dichroism and circular dichroism spectra are different and can be explained in terms of variations in stacking size of the aggregates and molecular arrangement of BChl c. Small-angle X-ray diffraction has been observed only for the aggregates treated with CH₂Cl₂, and the same sample gave rise to highly resolved cross polarization/magic angle spinning ¹³C nuclear magnetic resonance spectrum. The results suggest that molecular ordering of the solid-state BChl c aggregates is highly dependent on the formation process which is largely determined by the solvent used.