Spectral Hole Burning: Methods and Applications to Photosynthesis

Abstract

Spectral hole burning Spectroscopy (SHB) is a high resolution technique that has been fruitfully applied to the study of photosynthetic protein complexes as well as electronic transitions in amorphous systems. It overcomes the limitations from inhomogeneous broadening and provides an improvement in spectral resolution of 2-3 orders of magnitude. The experimental aspects of SHB, including an apparatus for the hole burning studies at high pressures are discussed in considerable detail. For several reaction center and antenna complexes, the dependence of hole profiles on the burn wavelength has been used to identify the site inhomogeneous and homogeneous contributions to the-region absorption profiles. It has also been used to determine the excited state linear electron-phonon coupling parameters. Such data is crucial for understanding the energy and electron transfer dynamics. For example, the invariance of zero phonon hole width of P870* on burn wavelength strongly suggests that the recently observed non-single exponential decay kinetics in primary charge separation are not due to a distribution of values for electronic coupling matrix elements. The observation of nonphotochemical hole burning (NPHB) for reaction center of Rhodopseudomonos viridis has led to the identification for the first time, of all six states (including the special pair upper dimer component) of a bacterial RC. Recent results of hole burning in FMO antenna complex at high pressures have identified the need for modifying existing theories of pressure dependence of hole burned spectra. Absorption spectra for reaction center of Rhodopseudomonas viridis at high pressures are also presented. Novel constant fluence hole burning Spectroscopy used in the identification of BChl 870 and BChl 896 bands in the antenna complex of Rhodobacter sphaeroides as well as the lowest energy state of LHC II complex of Photosystem II is dismissed. Abbreviations: BChl-Bacteriochlorophyll; Chl-Chlorophyll; CP47-A 47 kDa core antenna complex of Photosystem II; FMO complex-Fenna-Matthews-Olson complex of green sulfur bacteria; LH 1, LH 2-Antenna complexes of purple bacteria; LHC II-Antenna complex of plants; NPHB