Resonance Raman and Surface-Enhanced Resonance Raman Spectra of LH2 Antenna Complex from Rhodobacter sphaeroides and Ectothiorhodospira sp. Excited in the Qx and Qy Transitions†

Abstract

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.

Reference database of Raman spectra of biological molecules

“…by using surface enhanced Raman spectroscopy (SERS) or resonance Raman (RR) spectroscopy. 13,14 This is an interesting approach when only one or a limited number of biomolecules are of interest. However, when an overview of the entire cell composition is needed, one has to rely to general Raman spectroscopy.…”

Photosynthetic Bacterial Light-Harvesting Antenna Complexes Adsorbed on Silica Nanoparticles Revealed by Silica Shell-Isolated Au Nanoparticle-Enhanced Raman Spectroscopy

“…sphaeroides, although its crystal structure has not been available yet, the electron micrography (EM) and atomic force micrography (AFM) images of its 2D crystal show that it also exists as a nonamer. It contains sphaeroidene as the only carotenoid molecule when grown photosynthetically under anaerobiosis . By use of surface-enhanced resonance Raman scattering (SERRS), it has been demonstrated that the sphaeroidene in LH2 from Rba.…”

“…It contains sphaeroidene as the only carotenoid molecule when grown photosynthetically under anaerobiosis. 21 By use of surface-enhanced resonance Raman scattering (SERRS), it has been demonstrated that the sphaeroidene in LH2 from Rba. sphaeroides is preferentially located near the membrane's cytoplamic side, 15,22 same as those in LH2 from Rps.…”

Towards quantification of vibronic coupling in photosynthetic antenna complexes

“…We estimate the dipole strength |⃗ µ a 1 | 2 , of state |a 1 ⟩, and |⃗ µ b 0 | 2 , of state |b 0 ⟩, to be 0.04 and 0.09, respectively. 40,43 The pair of transition dipoles, ⃗ µ a 0 and ⃗ µ a 1 , belong to the same potential energy surface and are parallel to each other within the Condon approximation. For simplicity, the direction of ⃗ µ b 0 is set at 45…”

“…sphaeriodes reveal vibrational modes with frequency similar to the gap between B850 and B850* states. 40 We posit that such a vibrational mode on the B850 state can interact with B850* states though non-adiabatic coupling and give rise to NBO exciton-states shown in green, though the resonance Raman data do not directly measure this quantity. The extent of non-adiabatic coupling governs coherence properties and, as we will show, gives accessible explanation to the difference in the coherence lifetime observed with canonical 2D and coherence-specific polarization sequence.…”