Modified reaction centers from Rhodobacter sphaeroides R26

Abstract

Monomeric bacteriochlorophylls BA and Ba in photosynthetic reaction centers from Rhodobacter sphueroides R26 were exchanged with (132-hydroxy-)bacteriochlorophylls containing a 3-vinyl-or 3-(a-hydroxyethyl)-substituent instead of the 3-acetyl group. The corresponding binding sites must be tolerant to the introduction of the polar residue at C-13* andmodifications of the 3-acetyl group. According to HPLC analysis, the exchange with both pigments amounts to S$ 50% of the total BChl contained in the complex, corresponding to 6 100% of the monomeric BChl aBA,a. The absorption spectra show significant changes in the Qx and Qv-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 Ba, 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/Bs with the primary donor.

Primary electron transfer kinetics in bacterial reaction centers with modified bacteriochlorophylls at the monomeric sites BA,B.

“…If the vinyl group itself leads to structural changes, they should show up not only in reaction dynamics but also in the absorption, circular dichroism, or ENDOR spectra of P. All observations seem to exclude structural changes of P: (i) The Q(P) absorption band of the special pair P is unchanged between native RCs and [3-vinyl]RCs. (ii) Circular dichroism (23) and ENDOR experiments (unpublished data) show no significant changes at A > 840 nm. There is also some evidence for the absence of a structural change at the binding sites from the fact that Bchl-a and [3-vinyl]Bchl-a show the same wavelength shift upon transferring the pigments from diethylether into RCs (23).…”

“…(ii) Circular dichroism (23) and ENDOR experiments (unpublished data) show no significant changes at A > 840 nm. There is also some evidence for the absence of a structural change at the binding sites from the fact that Bchl-a and [3-vinyl]Bchl-a show the same wavelength shift upon transferring the pigments from diethylether into RCs (23). As a consequence, we may exclude-at first approximation-structural changes as a cause of the deceleration of the primary ET.…”

Formation of charge separated state P+Q−A and triplet state 3P at low temperature in Rhodobacter sphaeroides (R‐26) reaction centers in which bacteriopheophytin a is replaced by plant pheophytin a

“…A promising approach to clarify this relationship is an introduction of controlled changes to the structure and the observation of consequent alterations of the RC properties. Recently a new procedure has been worked out [4][5][6], employing the chemical exchange of endogeneous pigments with those in solution. In our previous work [7,8] we described room-temperature spectroscopic and photochemical properties of Rb.…”

Stereoselectivity of Pigment Exchange with 132‐Hydroxylated Tetrapyrroles in Reaction Centers of Rhodobacter Sphaeroides R26

“…The role of the reactive 2-oxoester system and the relatively labile asymetric center for the function in electron transfer or assembly of the RC pigment-protein complexes is uncertain (Struck et al, 1990; Correspondence to H. Scheer, Botanisches Institut, Universitat Fax: f 4 9 89 1782274. Ahhreviations.…”

“…67, D-80638 Miinchen, Germany Scheer and Struck, 1993;Bylina et al, 1988;Mdntele, 1993). The reactive 2-oxoester system may represent an electron source or reservoir during the charge separation process (Struck, 1990). It has been deduced from the X-ray structure that BPhe in HA is principally able to enolize (partially) via protonation of the 13' carbonyl group by the near-by Glu-L104 (Michel et al, 1986;El-Kabbani et al, 1991 ;Ermler et al, 1994).…”