Geometry for the Primary Electron Donor and the Bacteriopheophytin Acceptor in Rhodopseudomonas viridis Photosynthetic Reaction Centers

Abstract

The tetrapyrrole electron donors and acceptors (bacteriochlorophyll, BCh; bacteriopheophytin, BPh) within the bacterial photosynthetic reaction center (RC) are arranged with a specific geometry that permits rapid (picosecond time scale) electron tunneling to occur between them. Here we have measured the angle between the molecular planes of the bacteriochlorophyll dimer (primary donor), B(2), and the acceptor bacteriopheophytin, H, by analyzing the dichroism of the absorption change associated with H reduction, formed by photoselection with RCs of Rhodopseudomonas viridis. This angle between molecular planes is found to be 60 degrees +/- 2. This means that the ultrafast electron tunneling must occur between donors and acceptors that are fixed by the protein to have a noncoplanar alignment. Nearly perpendicular alignments have been determined for other electron tunneling complexes involving RCs. These geometries can be contrasted with models proposed for heme-heme electron transfer complexes, which have emphasized that mutually parallel orientations should permit the most kinetically facile transfers.

Excited States of the Photosynthetic Reaction Center of Rhodopseudomonas viridis: SAC−CI Study

“…Note, however, that the protein effect enlarges the difference. This assignment is consistent with the experimental result that an illumination of λ > 900 nm light to the PSRC bleached these peaks due to the photoreduction of the L-branch chromophores . Last, we note that the spectrum of Rb.…”

“…The calculated LD angles are 33.7° for H L and 28.6° for H M , which correspond well with the experimental angles around 40° for both, large negative values in the LD spectrum . The present result supports the energetic order of H L and H M proposed previously, and this order is considered to be due to the difference in molecular geometry between H L and H M since the gas phase result shown in Table shows the same order of the states as that in protein. Note, however, that the protein effect enlarges the difference.…”

Primary photochemistry of reaction centers from the photosynthetic purple bacteria

“…Low temperature linear dichroism measurements on trapped I-in Rps. viridis oriented whole cells and reaction centers [109,148], and 5K picosecond photodichroism measurements of state P+ I-in Rb. sphaeroides reaction centers [76], indicate that the broad [146].…”

Vibronic coupling to electron transfer and the structure of the R. Viridis reaction center

“…The relative orientations of the porphyrins toward each other was independently measured in R. viridis by Tiede et al [27]. They point out that the orientations contradict the suggestion, based on orbital overlap considerations, that the best arrangement for fast electron transfer would be with the planes of the porphyrins parallel.…”