Effects of acid pH and urea on the spectral properties of the LHII antenna complex from the photosynthetic bacterium Ectothiorhodospira sp.

Abstract

The aim of this study was to investigate the spectral modifications of the LHII antenna complex from the purple bacterium Ectothiorhodospira sp. upon acid pH titration both in the presence and absence of urea. A blue shift specifically and reversibly affected the B850 band around pH 5.5±6.0 suggesting that a histidine residue most probably participated in the in vivo absorption red shifting mechanism. This transition was observed in the presence and absence of urea. Under strong chaotropic conditions, a second transition occurred around pH 2.0, affecting the B800 band irreversibly and the B850 reversibly. Under these conditions a blue shift from 856 to 842 nm occurred and a new and strong circular dichroism signal from the new 842 nm band was observed. Reverting to the original experimental conditions induced a red shift of the B850 band up to 856 nm but the circular dichroism signal remained mostly unaffected. Under the same experimental conditions, i.e. pH 2.1 in the presence of urea, part of the B800 band was irreversibly destroyed with concomitant appearance of a band around 770 nm due to monomeric bacteriochlorophyll from the disrupted B800. Furthermore, Gaussian deconvolution and second derivative of the reverted spectra at pH 8.0 after strong-acid treatment indicated that the new B850 band was actually composed of two bands centered at 843 and 858 nm. We ascribed the 858 nm band to bacteriochlorophylls that underwent reversible spectral shift and the 843 nm band to oligomeric bacteriopheophytin formed from a part of the B850 bacteriochlorophyll. This new oligomer would be responsible for the observed strong and mostly conservative circular dichroism signal. The presence of bacteriopheophytin in the reverted samples was definitively demonstrated by HPLC pigment analysis. The pheophytinization process progressed as the pH decreased below 2.1, and at a certain point (i.e. pH 1.5) all bacteriochlorophylls, including those from the B800 band, became converted to oligomeric bacteriopheophytin, as shown by the presence of a single absorption band around 843 nm and by the appearance of a single main elution peak in the HPLC chromatogram which corresponded to bacteriopheophytin.Keywords: bacteria; chromatography; HPLC; photosynthesis; pigment; spectroscopy.In photosynthesis, light is absorbed by the antenna complexes and the resulting excitation energy funneled toward the reaction center where the primary charge separation takes place [1]. Three types of antenna complex can be isolated from the photosynthetic bacteria. The antenna complex I (LHI) or B880 [2,3] is intimately associated with the reaction center and is present in all photosynthetic bacteria in a constant ratio with respect to the reaction center. The antenna complex II (LHII) or B800±850 [4±8] is located more peripherally [9] and its concentration can vary with growth conditions. Finally, the antenna complex III (LHIII) or B800±820 [10,11] is also arranged peripherally and corresponds to a modification of the LHII complex. All the antenna complexes a...

Probing the binding site of 800‐nm bacteriochlorophyll in the membrane‐linked LH2 protein of Rhodobacter capsulatus by local unfolding and chemical modification

“…A similar but irreversible shift of the 800‐nm band induced by acid/chaotropic conditions of the buffer was recently reported in the isolated LH2 of Ectothiorhodospira sp. [27,28].…”

The study of photo-induced ultrafast dynamics in light-harvesting complex LH2 of purple bacteria

“…Interestingly, although the degradation of both B800 and B850 Q y bands takes place at practically the same potential, the B800 band bleaches faster than the B850 band [52], which indicates that the ring structure of the monomeric BChl-B800 is more incompact than the dimeric BChl-B850. The observed concomitant slight red shift of B850 band (from 850 nm to 855 nm) is likely attributed to the changes of BChl-protein and BChl-BChl interaction, which arise from the minor changes in distance between the pigment molecules [70][71][72] and/or the changes in hydrogen bonding between the protein and the pigment [73,74] upon the electrochemical oxidation. Moreover, the loss of BChl-B800 caused by electrochemical oxidation may also bring the BChl-B850 band to a slight red shift, as is similar to the observation upon selective removal of the BChl-B800 from the native LH2 [11,75].…”

Alkaline Denaturation of the Light-Harvesting Complex II from the Purple Bacterium Ectothiorhodospira Sp.: Kinetic Evidence of the Existence of the 780 nm Upper Exciton Component of the B850 Bacteriochlorophylls

“…These experimental conditions were chosen on the basis of previous observations that suggested a strong stability of this LHII complex in various chaotropic media. For instance, 8.9 M urea had no significant effect on the NIR spectral properties of the complex at normal pH (40). Furthermore, high concentrations of urea were necessary to induce denaturation of this antenna complex at pH as low as 2.1-…”