Monomeric bacteriochlorophylls BA and BB in photosynthetic reaction centers from Rhodobacter sphaeroides R26 were exchanged with (132-hydroxy-) bacteriochlorophylls containing a 3-vinyl- or 3-(α-hydroxyethyl)-substituent instead of the 3-acetyl group. The corresponding binding sites must be tolerant to the introduction of the polar residue at C-132 and modifications of the 3-acetyl group. According to HPLC analysis, the exchange with both pigments amounts to ≦ 50% of the total BChl contained in the complex, corresponding to ≦ 100% of the monomeric BChl aBA,B. The absorption spectra show significant changes in the Qx and Qy-region of the monomeric bacteriochlorophylls. By contrast, the absorption of the primary donor (P870) and reversible photobleaching is retained. The circular dichroism is also unchanged in the 870 nm region. The positive cd band located at around 800 nm in native reaction centers, shifts with the (blue-shifted) Qy absorption(s) of BA and/or BB, whereas the position of the negative one remains nearly unaffected. The data indicate that the latter is the upper excitonic band of the primary donor, and that there is little interaction of the monomeric BA/BB with the primary donor.
