SPECTROSCOPIC CHARACTERIZATION OF THE REACTION CENTER OF PHOTOSYSTEM-II USING POLARIZED-LIGHT - EVIDENCE FOR BETA-CAROTENE EXCITONS IN PSI-II REACTION CENTERS

The absorption and linear dichroism spectra of the isolated D1/D2/cytochrome b-559 Photosystem II reaction centre from pea (Pisum sativum) measured at 77 K are shown to depend on the concentration of the detergent Triton X-100 (TX-100) in the supporting buffer. This implies that the PS II reaction centre adopts different conformations in different concentrations of this detergent. In the case of the absorption due to beta-carotene, additional transitions are observed when the reaction centre is suspended in a medium containing low levels (0.02% TX-100) when compared with higher concentrations (0.20%) of TX-100. Differences are also seen in the lowest energy Q(y)(0-0) band. Suspension of the PS II reaction centre in higher concentrations of TX-100 results in a slight decrease (7%) in the dipole strength of one or more of long wavelength transitions relative to that measured in 0.01% TX-100, resulting in a large shift of the absorption maximum from 677.5 nm to 672 nm. The linear dichroism spectrum of the PS II reaction centre complex measured in a buffer containing 0.02% (w/v) TX-100 indicates the presence of one spectral form of beta-carotene. When measured in a buffer containing 0.02% (w/v) TX-100, the linear dichroism spectrum has more transitions in the beta-carotene absorption region. We discuss the possibility that the differences are due to excitonic interactions between molecules of beta-carotene which lie close together at lower detergent concentrations. The effect of the detergent TX-100 on the conformation of the PS II reaction centre is discussed in relation to the number of beta-carotene molecules in the preparation.