Excitonic Energy Level of Homo- and Hetero-Dimers and Their Composition in the Native Water-Soluble Chlorophyll Protein from Lepidium Virginicum

The native water-soluble chlorophyll protein, CP663, extracted from Lepidium virginicum, consists of four subunits, each with one chlorophyll (Chl), and the four Chls form two dimers in an orthogonal arrangement. Accordingly, CP663 is considered to have three dimers, Chl a-Chl a (AA), Chl a-Chl b (AB), and Chl b-Chl b (BB), in a certain composition and to provide six excitonic transitions because each dimer gives rise to low (L)- and high (H)-energy transitions. To investigate the excitonic transitions and the dimeric composition, the absorption and circular dichroism spectra of CP663 were measured and the electronic transitions of the dimers were calculated by Zerner's intermediate neglect of differential overlap method. On the basis of the experimental and calculated results, the Qy-absorption band was deconvoluted by the Gaussian fitting method. The resulting six Gaussian components were assigned to the individual transitions and the excitonic transition energy was found to increase in the order AA(L), AB(L), AA(H), BBL, AB(H), and BBH. From the spectral areas of the components, the composition of the dimers was calculated as AA:AB:BB = 0.52:0.34:0.14, indicating that CP663 accommodates the homodimers in preference to the heterodimer.