Picosecond fluorescence from photosynthetic systems in vivo

Wavelength-Dependent Optical Response of Single Photosynthetic Antenna Complexes from Siphonous Green Alga Codium fragile

The siphonaxanthin-siphonein-Chl-a/b-protein (SCP) complex from the siphonous green alga Codium fragile is the major light-harvesting complex (LHC) of these alga and is highly homologous to that of green plants (trimeric pigment-protein complex, LHCII). Interestingly, we find remarkable differences in the spectral response from individual SCP complexes when excited at 561 and 639 nm. While excitation in the green spectral range reproduces the common LHCII-like emission features for most of the complexes, excitation in the red spectral range yields a red-shifted emission and a significant reduction of the fluorescence decay time. We hypothesize that the difference in spectral response of SCP to light in the green and red spectral ranges can be associated with the adaption of the algae to their natural habitat under water, where sudden intensity changes are diminished, and excess light features a red-enhanced spectrum that comes at tidal timings.

Ultrafast excited state dynamics in the monomeric and trimeric photosystem I core complex of Spirulina platensis probed by two-dimensional electronic spectroscopy

Photosystem I (PSI), a naturally occurring supercomplex composed of a core part and a light-harvesting antenna, plays an essential role in the photosynthetic electron transfer chain. Evolutionary adaptation dictates a large variability in the type, number, arrangement, and absorption of the Chlorophylls (Chls) responsible for the early steps of light-harvesting and charge separation. For example, the specific location of long-wavelength Chls (referred to as red forms) in the cyanobacterial core has been intensively investigated, but the assignment of the chromophores involved is still controversial. The most red-shifted Chl a form has been observed in the trimer of the PSI core of the cyanobacterium Spirulina platensis, with an absorption centered at ∼740 nm. Here, we apply two-dimensional electronic spectroscopy to study photoexcitation dynamics in isolated trimers and monomers of the PSI core of S. platensis. By means of global analysis, we resolve and compare direct downhill and uphill excitation energy transfer (EET) processes between the bulk Chls and the red forms, observing significant differences between the monomer (lacking the most far red Chl form at 740 nm) and the trimer, with the ultrafast EET component accelerated by five times, from 500 to 100 fs, in the latter. Our findings highlight the complexity of EET dynamics occurring over a broad range of time constants and their sensitivity to energy distribution and arrangement of the cofactors involved. The comparison of monomeric and trimeric forms, differing both in the antenna dimension and in the extent of red forms, enables us to extract significant information regarding PSI functionality.

Difference in light use strategy in red alga between Griffithsia pacifica and Porphyridium purpureum

Phycobilisomes (PBSs) are the largest light-harvesting antenna in red algae, and feature high efficiency and rate of energy transfer even in a dim environment. To understand the influence of light on the energy transfer in PBSs, two red algae Griffithsia pacifica and Porphyridium purpureum living in different light environment were selected for this research. The energy transfer dynamics in PBSs of the two red algae were studied in time-resolved fluorescence spectroscopy in sub-picosecond resolution. The energy transfer pathways and the related transfer rates were uncovered by deconvolution of the fluorescence decay curve. Four time-components, i.e., 8 ps, 94 ps, 970 ps, and 2288 ps were recognized in the energy transfer in PBSs of G. pacifica, and 10 ps, 74 ps, 817 ps and 1292 ps in P. purpureum. In addition, comparison in energy transfer dynamics between the two red algae revealed that the energy transfer was clearly affected by lighting environment. The findings help us to understand the energy transfer mechanisms of red algae for adaptation to a natural low light environment.

Energy transfer in a model of the photosynthetic unit

A simple model of the photosynthetic unit has been constructed and used for simulated Förster-type energy migration, fluorescence and intersystem crossing, in order to gain insight into the conditions that influence both the form and the lifetime of the fluorescence decay in vivo. The model consists of a two-dimensional random lattice with one central trap. The simulation was done by means of repetitive Monte Carlo-type computations. The results obtained show that the form of the decay curve changes from exponential to non-exponential, as the chlorophyll concentration (molecules/nm2) is increased. The fluorescence lifetimes (tau 1/e) were also found to decrease substantially with only slight increases inc concentration. At a concentration comparable to that of chlorophyll in the chloroplast, both the form of the fluorescence decay and the lifetime are in fair agreement with experiment in vivo. The reasons for non-exponentially of the decay as well as the properties of energy migration are discussed. Preliminary work involving the dependence of trapping rate on donor concentration is also presented.

Fluorescence of light-harvesting chlorophyll a/b-protein complexes: implications for the photosynthetic unit

To the extent that extracted light-harvesting chlorophyll proteins (LHCPs) retain the chlorophyll configuration which they had in vivo, information on the optical properties of LHCPs is useful for an assessment of the transfer process of the primary excitation energy in photosynthesis. Within this context we report and discuss the implication of three kinds of data on spinach chloroplast LHCP. First, an analysis of the spectroscopic dependence of absorption, polarization and circular dichroism (reported recently by R.L.V.) suggests a model affording the possibility of easy chlorophyll a intercomplex transfer with chlorophyll b groups acting as local antitraps. Second, the ratio of LHCP emission and absorption probabilities obeys the Stepanov relation over a relatively wide range, an observation which suggests rapid Chl b-Chl a excitation equilibration. Finally, an LHCP absolute fluorescence yield as great as 10% has been measured, which provides a possible upper limit for the yield of the antenna fluorescence.

Transfer and trapping of excitation energy in photosystem II

The fluorescence yield of chlorophyll a of system II in spinach chloroplasts as a function of the fraction q- of reaction centres in the weakly trapping state PQ-, with reduced acceptor Q-, and reduced primary donor chlorophyll, P, of the reaction centre, is described by the function phi = a/(1 - pq-), a and p being constants (Van Gorkom et al. 1978); P was estimated to be 0.74. By special treatment and additions it was ascertained that the donor complex (S-states, see below) was in the reduced state. Three models of pigment systems have been considered: separate units; units with a boundary limiting energy transfer; and the matrix or pigment bed model, which was found to describe the experimental data. The following supplementary assumptions were made: ktf greater than kt greater than k't greater than 0. The rate constant ktf is that for electronic excitation transfer from a chlorophyll a molecule (or reaction-centre chlorophyll) to the surrounding chlorophyll molecules; kt and k't are rate constants for trapping at the reaction centres in the state PQ and PQ-, respectively. From this model and additional data such as fluorescence yield in vivo and in vitro, kt was estimated to be 4 X 10(11) S-1 and k't = 7.1 X 10(10) S-1; ktf greater than 10(12) S-1. In dark-adapted Chlorella, a series of curves respresenting changes in fluorescence yield as a function of time in a succession of six 16 microseconds xenon flashes spaced at 3 s crossed at one point. It is concluded from this and other observations that in the states S2 and S3 (with two or three oxidizing equivalents in the donor complex of system II) a certain fraction of the reaction centres occurs in a special conformational state. In this state electron transfer and, possibly, energy transfer to P+ are appreciably decreased.