We have studied the effects of the detergent lauryl dimethylamine N-oxide and NaCl in the near infrared absorption spectra of the B800-850 antenna complex from Ectothiorhodospira sp. Strong spectral changes were induced on the BChl850 band by the lauryl dimethylamine N-oxide consisting of a blue shift, from 857 to 839-837 nm, and a hypochromism. No significant effects were detected on the BChl800 band in the same conditions. The changes were reversible after removing most of the detergent from the sample. Depending upon the detergent concentration in the solution, NaCl was also able to reverse the blueshift and increase the intensity of the 850 nm band close to the native values. Moreover, we have been able to separate both phenomena. Addition of 0.350 M NaCl after sample incubation with 0.15% (v/v) lauryl dimethylamine N-oxide for 30 min allowed a 9-10 nm redshift with no significant hyperchromism of the lowest energy band. We explained the overall effect of the detergent assuming that the lauryl dimethylamine N-oxide bound to the hydrophobic moiety of the complex and caused some protein conformational changes which affected the BChl850 domain without affecting that of the BChl800. The NaCl was able to circumvent these effects, most probably by acting directly on the BChl850 molecules or on the protein structure surrounding them.
Two dimensional crystals of LH2 light-harvesting complexes from Ectothiorhodospira sp. and Rhodobacter capsulatus investigated by electron microscopy Oling,
Well-resolved vibrational spectra of LH2 complex isolated from two photosynthetic bacteria, Rhodobacter sphaeroides and Ectothiorhodospira sp., were obtained using surface-enhanced resonance Raman scattering (SERRS) exciting into the Qx and the Qy transitions of bacteriochlorophyll a. High-quality SERRS spectra in the Qy region were accessible because the strong fluorescence background was quenched near the roughened Ag surface. A comparison of the spectra obtained with 590 nm and 752 nm excitation in the mid- and low-frequency regions revealed spectral differences between the two LH2 complexes as well as between the LH2 complexes and isolated bacteriochlorophyll a. Because peripheral modes of pigments contribute mainly to the low-frequency spectral region, frequencies and intensities of many vibrational bands in this region are affected by interactions with the protein. The results demonstrate that the microenvironment surrounding the pigments within the two LH2 complexes is somewhat different, despite the fact that the complexes exhibit similar electronic absorption spectra. These differences are most probably due to specific pigment-pigment and pigment-protein interactions within the LH2 complexes, and the approach might be useful for addressing subtle static and dynamic structural variances between pigment-protein complexes from different sources or in complexes altered chemically or genetically.
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