Resonance Raman Studies of Carotenoid Molecules Within Photosystem I Particles

ABSTRACT

In higher plants, carotenoid molecules play particularly important roles in harvesting light, stabilizing protein structures, regulating energy flow, and dissipating excess energy not required by the organism for photosynthesis. Low-temperature resonance Raman spectroscopy was used to study the spectral properties, binding sites and composition of major carotenoids in spinach Photosystem I particles. Photosystem I is a supercomplex of a reaction centre, core and light-harvesting complexes. Resonance Raman spectra of carotenoid molecules have four main groups of intense bands, which provide information on the conformation and configuration of these molecules. The most sensitive to their molecular configurations relaxed or distorted is v₄ band exhibiting an increase in both intensity and structure indicating out-of-plane distortion of the molecule. During prolonged exposure to high-light intensities various pigments in Photosystem I exhibited different susceptibility to photodestruction. Resonance Raman technique allowed us to recognize the type and conformation of photobleached carotenoid molecules and to reveal the involvement of these pigments in the photoprotection. Raman data revealed a nearly full photobleaching of the long-wavelength lutein molecules. The observed similar bleaching rate of the lutein molecules and the most red-shifted long-wavelength chlorophyll a, located in the antenna membrane protein Lcha4, suggested that these molecules are located closely. Our results showed that the photobleached antenna pigments and especially luteins and the most long-wavelength absorbing chlorophylls are involved in photoprotection of PSI core complex. We investigated the effect of histidine on the photobleaching of pigments in isolated particles of photosystem I. Our preliminary results showed that histidine reduces the photobleaching of antenna pigments and especially luteins and the most long-wavelength absorbing chlorophylls located in PSI antenna complex.

Keyword:

• Carotenoids
• Photosystem I
• Raman spectroscopy