Abstract
Light-induced infrared difference spectroscopy (IR-DS) has been used, especially in the last decade, to investigate early photophysics, energy transfer and photoprotection mechanisms in isolated and membrane-bound light harvesting complexes (LHCs). The technique has the definite advantage to give information on how the pigments and the other constituents of the biological system (proteins, membranes, etc.) evolve during a given photoreaction. Different static and time-resolved approaches have been used. Compared to the application of IR-DS to photosynthetic Reaction Centers (RCs), however, IR-DS applied to LHCs is still in an almost pioneering age: very often sophisticated techniques (step-scan FTIR, ultrafast IR) or data analysis strategies (global analysis, target analysis, multivariate curve resolution) are needed. In addition, band assignment is usually more complicated than in RCs. The results obtained on the studied systems (chromatophores and RC-LHC supercomplexes from purple bacteria; Peridinin-Chlorophyll-a-Proteins from dinoflagellates; isolated LHCII from plants; thylakoids; Orange Carotenoid Protein from cyanobacteria) are summarized. A description of the different IR-DS techniques used is also provided, and the most stimulating perspectives are also described. Especially if used synergically with other biophysical techniques, light-induced IR-DS represents an important tool in the investigation of photophysical/photochemical reactions in LHCs and LHC-containing systems.
Highlights
Infrared difference spectroscopy (IR-DS) is a biophysical technique that can be helpful in the study of the mechanism of biochemical reactions [1,2]
Most investigations have concerned photosynthetic Reaction Centers (RCs) rather than light-harvesting complexes (LHCs). This is for two main reasons: (a) in RCs, real photo-induced chemistry is taking place, so that the capability of IR-DS to follow—in the same spectrum—the changes in several molecular moieties is a real asset to clarify the mechanism of reaction; and (b) in most—but not all—cases, photo-induced processes in light harvesting complexes (LHCs) are fast or extremely fast, so that peculiar techniques are necessary in order to get a IR difference spectrum
Most IR-DS studies rely on the use of Fourier Transform (FT) IR spectrometers
Summary
Infrared difference spectroscopy (IR-DS) is a biophysical technique that can be helpful in the study of the mechanism of biochemical reactions [1,2]. IR-DS is complementary to other biophysical techniques, which give information on specific molecular species This complementarity is evident in the case of Resonance Raman spectroscopy, where a vibrational spectrum of a chromophore inside a complex biological system can selectively be obtained. Most investigations have concerned photosynthetic Reaction Centers (RCs) rather than light-harvesting complexes (LHCs) This is for two main reasons: (a) in RCs, real photo-induced chemistry is taking place (electron transfer, proton transfer, etc.), so that the capability of IR-DS to follow—in the same spectrum—the changes in several molecular moieties is a real asset to clarify the mechanism of reaction; and (b) in most—but not all—cases, photo-induced processes in LHCs are fast or extremely fast, so that peculiar techniques are necessary in order to get a IR difference spectrum. This topic is, outside the domain of this review, which is focused on light-induced IR-DS studies on LHCs
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