Absorption and circular dichroism spectroscopy and modeling of various light-harvesting 2 complexes from purple bacteria

Abstract

The absorption and Circular Dichroism (CD) spectra of LH2 complexes from various purple bacteria have been measured and modeled. Based on the lineshapes of the spectra we can sort the LH2 complexes in two distinguished groups: `acidophila¿ -like (type 1) and `molischianum¿ -like (type 2). Some of the measured and modeled complexes (except of Rhodopseudomonas (Rps.) acidophila and Rhodospirilum (Rsp.) molischianum) are: HL (High-Light) Rps. palustris, HL Rps. cryptolactis and Chromatium (Cm.) vinosum (type 1 complexes), as well as Rhodobacter (Rb.) sphaeroides strains G1C and 2.4.1, and Rubrivivax gelatinosus (type 2 complexes). Although the ringsize can vary experiments are consistent with the fact that all LH2 complexes are ringlike structures with high similarities between them. Starting from the known structures of Rp. acidophila and Rs. molischianum we can model the absorption and CD spectra of several species by just slightly varying some key parameters like the interaction strength, the energy difference of the a- and ß- bound B850 BChl¿s, the orientation of the B850 and B800 BChl¿s, and the (in)-homogeneous broadening. For example, the signs of the CD bands and their relative magnitude depend strongly on the orientation of the pigments and the excitonic interaction energy, while the red shift of the B850 CD lines with respect to the absorption maximum can be reproduced by varying the energy difference of the a- and ß- bound B850 BChl¿s and taking into account interactions between all the pigments of the ring.