Abstract:The resonance Raman spectrum of β-carotene in photosystem Ⅱ (PS Ⅱ )reaction center complex was characterized by four main bands, peaking at 1532 (νl), 1156 (ν2), 1010 (ν3) and 970 (ν4) cm -1, respectively, with several additional small Raman bands in the region between 1100 cm-1 and 1500 cm-1 It was suggested that β-carotene molecules of the reaction center complex were in all-trans configuration. The resonance Raman spectrum of an acetone extract from the reaction center complex also showed four main bands. The peak position of νl, ν3 and ν4 band shifted 5 cm-1 to the shorter wave number. The most dramatic changes were the reduction of the intensity of ν4. From the above results it was demon- strated that the conformation of β-carotene molecules in the PS Ⅰ reaction center was not the same as that of free β-carotene molecules in solution, but similar to that of carotenoid molecules in the photosynthetic bacterial reaction center, in other words, they are likely to be in a twisted conformation.