ORIGIN OF OPTICAL-ACTIVITY IN THE PURPLE BACTERIAL PHOTOREACTION CENTER

The photoreaction center (RC) of purple bacteria contains four bacteri ochlorophyll (Bch) and two bacteriopheophytin (Bph) molecules as prost hetic groups. Their optical activity, as measured by circular dichrois m (CD) spectroscopy, is largely increased in situ as compared to organ ic solutions. The all-exciton hypothesis posits that this enhanced opt ical activity is entirely due to excitonic interactions between the el ectronic transitions of all six bacteriochlorin molecules. Using the s imple exciton theory, this model predicts that the near-infrared CD sp ectra should be conservative. The fact that they are not, whether the special pair of Bch (SP) that constitutes the primary electron donor i s reduced or oxidized, has been explained by hyperchromic effects. The present work tests this hypothesis by successively eliminating the ab sorption and, therefore, the optical activity of the Bphs and of the n on-special-pair (non-SP) Bchs. This was accomplished by trapping these pigments in their reduced state. RC preparations with the four non-SP bacteriochlorins trapped in their reduced state and, therefore, with an intact SP displayed conservative CD spectra. RC preparations with o nly the electronic transitions of SP and of one non-SP Bch also showed conservative CD spectra. These conservative CD spectra and their corr esponding absorption spectra were simulated using simple exciton theor y without assuming hyperchromic effects. Bleaching half of the 755-nm absorption band by phototrapping one of the two Bph molecules led to t he complete disappearance of the corresponding CD band. This cannot be explained by the all-exciton hypothesis. These results suggest that t he optical activity of the SP alone, or with one non-SP Bch, is due to excitonic interactions. They also suggest that the optical activity o f the other three bacteriochlorins is due to other factors, such as pi gment-protein interaction.