Excitation energy pathways in the photosynthetic units of reaction center LM- and H-subunit deletion mutants of Rhodospirillum rubrum

Abstract

Light-induced reaction dynamics of isolated photosynthetic membranes obtained from wild-type (WT) and reaction center (RC)-subunit deletion strains SPUHK1 (an H-subunit deletion mutant) and SK Delta LM (an (L+M) deletion mutant) of the purple non-sulphur bacterium Rhodospirillum rubrum have been investigated by femtosecond transient absorption spectroscopy. Upon excitation of the spirilloxanthin (Spx) S(2) state at 546 nm, of the bacteriochlorophyll Soret band at 388 nm and probing spectral regions, which are characteristic for carotenoids, similar dynamics in the SPUHK1, SK Delta LM and WT strains could be observed. The excitation of Spx S(2) is followed by the simultaneous population of the lower singlet excited states S(1) and S* which decay with lifetimes of 1.4 and 5 ps, respectively for the mutants, and 1.4 and 4 ps, respectively, for the wild-type. The excitation of the BChl Soret band is followed by relaxation into BChl lower excited states which compete with excitation energy transfer BChl-to-Spx. The deexcitation pathway BChl(Soret) --> Spx(S(2)) --> Spx(S(1)) occurs with the same transition rate for all investigated samples (WT, SPUHK1 and SK Delta LM). The kinetic traces measured for the Spx S(1) --> S(N) transition display similar behaviour for all samples showing a positive signal which increases within the first 400 fs (i.e. the time needed for the excitation energy to reach the Spx S(1) excited state) and decays with a lifetime of about 1.5 ps. This suggests that the Spx excited state dynamics in the investigated complexes do not differ significantly. Moreover, a longer excited state lifetime of BChl for SPUHK1 in comparison to WT was observed, consistent with a photochemical quenching channel present in the presence of RC. For long delay times, photobleaching of the RC special pair and an electrochromic blue shift of the monomeric BChl a can be observed only for the WT but not for the mutants. The close similarity of the excited state decay processes of all strains indicates that the pigment geometry of the LH1 complex in native membranes is unaffected by the presence of an RC and allows us to draw a model representation of the WT, SK Delta LM and SPUHK1 PSU complexes.

Explaining the Temperature Dependence of Spirilloxanthin’s S* Signal by an Inhomogeneous Ground State Model

“…Depending on the chain length of the investigated carotenoid, the S* lifetime can be substantially longer or on the same time scale as S 1 . As first described for long-chain β-carotene homologues, S* has been interpreted as a separate electronic excited state, − an excited state isomer, a vibrationally hot ground state, populated by either Impulsive Stimulated Raman Scattering (ISRS) , or relaxation form S 1 , , the product of different ground state isomers, ,, or as the result of chemical impurities . We note that none of the proposed energy level models is able to explain all experimental findings in literature.…”

Lack of Excitation Energy Transfer from the Bacteriochlorophyll Soret Band to Carotenoids in Photosynthetic Complexes of Purple Bacteria

“…rubrum, based on transient absorption measurements with excitation at 388 nm. 6 We have previously investigated EET harvesting complexes (LH2) from wild-type (WT) cells of the purple bacterium Ectothiorhodospira (Ect.) haloalkaliphila.…”

“…Possible ways might be intramolecular conversion to BChl Q x or EET to S 2 of Cars. The latter possibility has been not much investigated, except for very few papers. − This is in part because intramolecular processes are generally considered to be faster than intermolecular ones (Kasha’s rule) …”

“…It is assumed that EET from the BChl Soret band to spirilloxanthin is possible in core complexes (LH1-RC) and LH1 of RC-less mutants from Rhodospirillum (Rsp.) rubrum , based on transient absorption measurements with excitation at 388 nm . We have previously investigated EET from the BChl Soret band to Cars in the peripheral light-harvesting complexes (LH2) from wild-type (WT) cells of the purple bacterium Ectothiorhodospira (Ect.)…”