Origin of optical activity in the purple bacterial photoreaction center

Journal Article · Tue Jul 18 04:00:00 EDT 1995 · Biochemistry (Eaton)

DOI:https://doi.org/10.1021/bi00028a016· OSTI ID:245281

Mar, T; Gingras, G ^[1]

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1. Universite de Montreal, Quebec (Canada)

The photoreaction center (RC) of purple bacteria contains four bacteriochlorophyll (Bph) and two bacteriopheophytin (Bph) molecules as prosthetic groups. Their optical activity, as measured by circular dichroism (CD) spectroscopy, is largely increased in situ as compared to organic solutions. The all-exciton hypothesis posits that this enhanced optical activity is entirely due to excitonic interactions between the electronic transitions of all six bacteriochlorin molecules. Using the simple exciton theory, this model predicts that the near-infrared CD spectra should be conservative. The fact that they are not, whether the special pair of Bch (SP) that constitutes the primary electron donor is reduced or oxidized, has been explained by hyperchromic effects. The present work tests this hypothesis by successively eliminating the absorption and, therefore, the optical activity of the Bphs and of the non-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 only the electronic transitions of SP and of one non-SP Bch also showed conservative CD spectra. These conservative CD spectra and their corresponding absorption spectra were simulated using simple exciton theory without assuming hyperchromic effects. Bleaching half of the 755-nm absorption band by phototrapping one of the two Bph molecules led to the complete disappearance of the corresponding CD band. This cannot be explained by the all-exciton hypothesis. These results suggest that the optical activity of the SP alone, or with one non-SP Bch, is due to excitonic interactions. They also suggest that the optical activity of the other three bacteriochlorins is due to other factors, such as pigment-protein interaction. 32 refs., 9 figs., 2 tabs.

• Sponsoring Organization: USDOE
• OSTI ID: 245281
• Journal Information: Biochemistry (Eaton), Journal Name: Biochemistry (Eaton) Journal Issue: 28 Vol. 34; ISSN 0006-2960; ISSN BICHAW
• Country of Publication: United States
• Language: English

Related Subjects

14 SOLAR ENERGY
55 BIOLOGY AND MEDICINE
BASIC STUDIES
ABSORPTION SPECTRA
BLEACHING
CHLOROPHYLL
DICHROISM
MOLECULAR STRUCTURE
OPTICAL ACTIVITY
PHOTOSYNTHETIC BACTERIA
PHOTOSYNTHETIC REACTION CENTERS
PIGMENTS