Vibrational techniques applied to photosynthesis: Resonance Raman and fluorescence line-narrowing

Abstract

Resonance Raman spectroscopy may yield precise information on the conformation of, and the interactions assumed by, the chromophores involved in the first steps of the photosynthetic process. Selectivity is achieved via resonance with the absorption transition of the chromophore of interest. Fluorescence line-narrowing spectroscopy is a complementary technique, in that it provides the same level of information (structure, conformation, interactions), but in this case for the emitting pigment(s) only (whether isolated or in an ensemble of interacting chromophores). The selectivity provided by these vibrational techniques allows for the analysis of pigment molecules not only when they are isolated in solvents, but also when embedded in soluble or membrane proteins and even, as shown recently, in vivo. They can be used, for instance, to relate the electronic properties of these pigment molecules to their structure and/or the physical properties of their environment. These techniques are even able to follow subtle changes in chromophore conformation associated with regulatory processes. After a short introduction to the physical principles that govern resonance Raman and fluorescence line-narrowing spectroscopies, the information content of the vibrational spectra of chlorophyll and carotenoid molecules is described in this article, together with the experiments which helped in determining which structural parameter(s) each vibrational band is sensitive to. A selection of applications is then presented, in order to illustrate how these techniques have been used in the field of photosynthesis, and what type of information has been obtained. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.

Concentration Effect on Quenching of Chlorophyll a Fluorescence by All-Trans-β-Carotene in Photosynthesis

“…Chlorophyll and carotenoids are both building blocks of the photosynthetic apparatus [ 1 , 2 , 3 , 4 , 5 , 6 ]. Chl-a can be found in higher plants, green algae, and some bacteria, and is considered the major light-harvesting pigment in these organisms [ 7 , 8 , 9 , 10 ]. Carotenoids are alkene molecules that contain π-electron conjugated bonds, and have many functions in organic life, such as energy transfer, photoprotection and interception of singlet oxygen [ 11 , 12 , 13 ].…”

Electron–Vibrational Coupling and Fluorescence Spectra of Tetra-, Penta-, and Hexacoordinated Chlorophylls c1 and c2

“…These techniques are very selective since they provide information only about emitting Chls present in complex enviroment. 4,[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Two recent fluorescence line-narrowing studies of isolated chlorin-type Chls in side, the calculated total intramolecular Huang-Rhys factor (0.86) was found to be three times larger than the experimentally determined one (0.29). This discrepancy was explained by taking into account that the major part of the total Huang-Rhys factor comes from the low-frequency modes which are damped in the condensed phase and thus do not contribute to the electron-vibrational coupling.…”

Probing environment fluctuations by two-dimensional electronic spectroscopy of molecular systems at temperatures below 5 K

“…Often, these are well distinguishable resonances above 200 cm −1 determined by Raman spectroscopy studies, ∆FLN and HB experiments. 31,66 At such frequencies, they are at rest in thermal equilibrium, and their dynamics is initiated by electronic transitions. Second type of nuclear dynamics is caused by fluctuating surrounding molecules or protein groups often referred as phonons.…”