Energy transfer and trapping in the photosystem I core antenna. A temperature study

Abstract

The fluorescence decay kinetics of the photosystem I-only mutant strain of Chlamydomonas reinhardtii, A4d, are used to study energy transfer and structural organization in photosystem I (PSI). Time-resolved measurements over a wide temperature range (36-295 K) have been made both on cells containing approximately 65 core chl a/P700 and an additional 60-70 chl a + b from LHC proteins and on PSI particles containing 40-50 chl a/P700. In each case, the fluorescence decay kinetics is dominated by a short component, tau 1 which is largely attributed to the lifetime of the excitations in the core complex. The results are discussed in terms of simulations of the temperature dependence of tau 1 in model systems. Spectral inhomogeneity and the temperature dependence of the spectral lineshapes are included explicitly in the simulations. Various kinds of antenna arrangements are modeled with and without the inclusion of pigments with lower absorption energies than the trap (red pigments). We conclude that funnel arrangements are not consistent with our measurements. A random model that includes one or two red pigments placed close to the trap shows temperature and wavelength dependence similar to that observed experimentally. A comparison of the temperature dependence of tau 1 for cells and PSI particles is included.

Energy transfer and trapping in photosystem I reaction centers from cyanobacteria.

“…This model is consistent with the rather complex observed wavelength dependence of fluorescence decay in PSI at low temperatures (9). It was also concluded, based on the simulation, that red pigments are necessary to interpret the temperature and wavelength dependence of fluorescence decay at low temperatures (8,9). This conclusion is supported by the direct observation of red fluorescence (the fluorescence peaks to the red of 700 nm) of PSI at room temperature (10).…”

“…To take account of inhomogeneous broadening and multiple spectral types in PSI, a multicolor random model with a few pigments that absorb above 700 nm (red pigments) was proposed (8). This model is consistent with the rather complex observed wavelength dependence of fluorescence decay in PSI at low temperatures (9). It was also concluded, based on the simulation, that red pigments are necessary to interpret the temperature and wavelength dependence of fluorescence decay at low temperatures (8,9).…”

“…One of the fundamental issues regarding PSI is whether there exist red Chls around the reaction centers (8,9,23). Temperature dependence of energy transfer in PSI can be modeled successfully only when a few red pigments are included (8,9).…”

“…In time-resolved fluorescence experiments on photosystem II (PSII)-deficient C. reinhardtii mutants, the kinetic complexity of the decays depended on the number of peripheral Chls associated with the core antenna (9). The shortest decay component arises from excitation trapping in the PSI core.…”

Characterization of Low-Energy Chlorophylls in the PSI-LHCI Supercomplex from Chlamydomonas reinhardtii. A Site-Selective Fluorescence Study

“…44 We may speculate that the second pool, at 701 nm, correspond to one to two red Chls located close to the RC as reported for C. reinhardtii. 28,29 The sharp character of the 697-and 701-nm bands may indicate that the mechanism of their red shift is different from the C700 pool: for example that they originate from monomeric Chls which are tuned by the protein environment. 11 13 and Gobets and van Grondelle.…”