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 initial electron tranfer in reaction centers from Rhodobacter sphaeroides R26 was studied by a subplcosecond transient pump-probe technique. The measured kinetics at various wavelengths were analyzed and compared with several mechanisms for electron tra er. An unambiguous determination of the initial electron transfer mechanism in reaction centers cannot be made by studying the anion absorption region (6404690 nm), due to the spectral congestion in this region. However, correlations between the stimulated emission decay of the excited state of the special pair, P*, at 926 nm and bleaching of the bacteriopheophytin Qx absorption at 545 mu suggest that the electron transfer at 283 K is dominated by a two-step sequential mechanism, whereas one-step superexchange and the two-step sequential mechanism have about equal contributions at 22 K.The highly efficient primary charge separation in photosynthesis occurs in a membrane-bound protein complex called the reaction center (RC). The x-ray crystal structure of RCs from Rhodopseudomonas (Rps.) viridis and Rhodobacter (Rb.) sphaeroides has been determined and reveals a pseudo C2 symmetry (1-3). In the RC, two strongly interacting bacteriochlorophylls form the special pair, denoted P. Along each side of the C2 axis there is a monomeric bacteriochlorophyll (BA and BB) adjacent to the special pair, followed by a bacteriopheophytin (HA and HB) and then by a quinone molecule (QA and QB). There is also a single non-heme iron on the C2 axis between the two quinones.Electron transfer could occur along either branch related by the C2 symmetry, yet it occurs only down one branch (branch A) from P* to HA in about 3 ps at room temperature. One unresolved question concerns the role of BA in the initial charge separation process between P* and HA. Early picosecond and femtosecond measurements on RCs from Rb. sphaeroides and Rps. viridis at room temperature and at 10 K (4-9) did not detect any intermediate state involving B-, and the decay of P* and the bleaching of HA had the same time constant. These observations indicated that the initial electron transfer from P*BAHA to P+BAH-occurred either by a single-step superexchange mechanism or by a two-step hopping process with the second step much faster than the first step. The one-step superexchange mechanism has been supported by measurements by Kirmaier and Holten (10,11). Using an electric field to perturb the electron transfer, Lockhart et al. (12) concluded that (at 77 K) the two-step mechanism was unlikely. The same conclusion was reached by Ogrodnik et al. (13) (19) suggests that for reasonable parameter values, the electron transfer is dominated by the two-step sequential mechanism at room temperature, while the contribution of the one-step superexchange mechanism becomes important at low temperature.In this paper, we address the mechanism of the initial electron transfer by using kinetic data for Rb. sphaeroides R26 RCs obtained at several wavelengths where the various species involved absorb. The analysis of our da...

We present simulations of energy transfer in reaction centers of the purple bacteria Rhodopseudomonas viridis and Rhodobacter sphaeroides. Calculations using a weak coupling model based on Förster's theory give transfer rates that are at least an order of magnitude slower than the results from recent femtosecond transient absorption studies. The discrepancy between experimental and theoretical results is discussed in terms of the various assumptions upon which the model is based.

Global gene expression was analyzed in Saccharomyces cerevisiae T2 cells grown in the presence of hardwood spent sulphite liquor (HW SSL) and each of the three main inhibitors in HW SSL, acetic acid, hydroxymethyfurfural (HMF) and furfural, using a S. cerevisiae DNA oligonucleotide microarray. The objective was to compare the gene expression profiles of T2 cells in response to the individual inhibitors against that elicited in response to HW SSL. Acetic acid mainly affected the expression of genes related to the uptake systems of the yeast as well as energy generation and metabolism. Furfural and HMF mainly affected the transcription of genes involved in the redox balance of the cell. On the other hand, the effect of HW SSL on S. cerevisiae T2 cells was distinct and considerably more diverse as compared to the effect of individual inhibitors found in lignocellulosic hydrolysates. This is not surprising as HW SSL contains a complex mixture of inhibitors which may act synergistically. HW SSL elicited significant changes in expression of genes involved in diverse and multiple effects on several aspects of the cellular structure and function. A notable response to HW SSL was decreased expression of the ribosomal protein genes in T2 cells. In addition, HW SSL decreased the expression of genes functioning in the synthesis and transport of proteins as well as metabolism of carbohydrates, lipids, vitamins and vacuolar proteins. Furthermore, the expression of genes involved in multidrug resistance, iron transport and pheromone response was increased, suggesting that T2 cells grown in the presence of HW SSL may have activated pheromone response and/or activated pleiotropic drug response. Some of the largest changes in gene expression were observed in the presence of HW SSL and the affected genes are involved in mating, iron transport, stress response and phospholipid metabolism. A total of 59 out of the 400 genes differentially expressed in the presence of HW SSL, acetic acid, HMF and furfural, belonged to the category of poorly characterized genes. The results indicate that transcriptional responses to individual lignocellulosic inhibitors gave a different picture and may not be representative of how the cells would respond to the presence of all the inhibitors in lignocellulosic hydrolysates such as HW SSL.