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In order to understand the organization of the PSI core antenna and to interpret results obtained from studies of the temperature and wavelength dependence of energy transfer and trapping in the PSI particles, we have constructed a model for PSI in which spectral heterogeneity is considered via a self-consistent approach based on Forster transport. The temperature dependence of the absorption and emission spectra of the individual Chl molecules in the protein matrix is calculated based on a model Hamiltonian which includes a phonon contribution. Time and wavelength resolved kinetics of PSI at different temperatures are investigated by means of two-dimensional lattice models. We conclude that wavelength-dependent fluorescence decay kinetics result only when two or more bottlenecks exist in the energy transfer and trapping process. A single trap or several pseudo-traps with spectrally identical environments do not lead to wavelength dependent decays. Simple funnel arrangements of the spectral types can be ruled out. At least one pigment with energy lower than the photochemical trap located close to the reaction center is required to produce the trends of the fluorescence lifetimes observed experimentally. The remainder of the core antenna is consistent with a random arrangement of spectral types.

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.

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17O electron nuclear double resonance (ENDOR) studies at X-band (9-GHz) and Q-band (35-GHz) microwave frequencies reveal that the [4Fe-4S]+ cluster of substrate-free aconitase [citrate (isocitrate) hydro-lyase, EC 4.2.1.3] binds solvent, HxO (x = 1, 2). Previous 17O ENDOR studies [Telser et al. (1986) J. Biol. Chem. 261, 4840-4846] had disclosed that Hx17O binds to the enzyme-substrate complex and also to complexes of enzyme with the substrate analogues trans-aconitate and nitroisocitrate (1-hydroxy-2-nitro-1,3-propanedicarboxylate). We have used 1H and 2H ENDOR to characterize these solvent species. We propose that the fourth ligand of Fea in substrate-free enzyme is a hydroxyl ion from the solvent; upon binding of substrate or substrate analogues at this Fea site, the solvent species becomes protonated to form a water molecule. Previous 17O and 13C ENDOR studies [Kennedy et al. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 8854-8858] showed that only a single carboxyl, at C-2 of the propane backbone of cis-aconitate or at C-1 of the inhibitor nitroisocitrate, coordinates to the cluster. Together, these results imply that enzyme-catalyzed interconversion of citrate and isocitrate does not involve displacement of an endogenous fourth ligand, but rather addition of the anionic carboxylate ligand and a change in protonation state of a solvent species bound to Fea. We further report the 17O hyperfine tensor parameters of the C-2 carboxyl oxygen of substrate bound to the cluster as determined by the field dependence of the 17O ENDOR signals. 17O ENDOR studies also show that the carboxyl group of the inhibitor trans-aconitate binds similarly to that of substrate.