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Effects of acid pH and urea on the spectral properties of the LHII antenna complex from the photosynthetic bacterium Ectothiorhodospira sp.
- Buche, A., Ramirez, J. M., Picorel, R.
- European journal of biochemistry 2000 v.267 no.11 pp. 3235-3243
- Ectothiorhodospira, absorption, bacteria, high performance liquid chromatography, histidine, pH, spectral analysis, titration, urea
- 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 pHâ5.5â6.0 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 pHâ2.0, affecting the B800 band irreversibly and the B850 reversibly. Under these conditions a blue shift from 856 to 842ânm occurred and a new and strong circular dichroism signal from the new 842ânm band was observed. Reverting to the original experimental conditions induced a red shift of the B850 band up to 856ânm but the circular dichroism signal remained mostly unaffected. Under the same experimental conditions, i.e. pHâ2.1 in the presence of urea, part of the B800 band was irreversibly destroyed with concomitant appearance of a band around 770ânm due to monomeric bacteriochlorophyll from the disrupted B800. Furthermore, Gaussian deconvolution and second derivative of the reverted spectra at pHâ8.0 after strongâacid treatment indicated that the new B850 band was actually composed of two bands centered at 843 and 858ânm. We ascribed the 858ânm band to bacteriochlorophylls that underwent reversible spectral shift and the 843ânm 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. pHâ1.5) all bacteriochlorophylls, including those from the B800 band, became converted to oligomeric bacteriopheophytin, as shown by the presence of a single absorption band around 843ânm and by the appearance of a single main elution peak in the HPLC chromatogram which corresponded to bacteriopheophytin.