A Theory of Dispersive Kinetics in the Energy Transfer of Antenna Complexes

Abstract

The inherent glasslike structural heterogeneity of photosynthetic antenna protein complexes is now known to result in a distribution of values for any given donor-acceptor energy gap. The width of the distribution is sufficiently broad to raise the possibility that the kinetics of D-A energy transfer might be dispersive and strongly dependent on temperature especially since the chlorophyll optical transitions are characterized by weak electron-phonon coupling. An approximate theory is presented that allows for computationally simple analysis of this problem. It is based on the familiar nonadiabatric energy-transfer theory and Condon approximation and, thus, is applicable to conventional Forster transfer involving localized phonons. The case of delocalized phonons is also treated. Calculations are presented for a model D-A system which, based on hole-burning data, can be considered to be realistic and typical of many complexes. The results reveal pronounced and temperature-dependent dispersion in the kinetics. The effect of pure electronic dephasing on the calculated results is considered. The theory is not applicable to the situation where excitonic level splittings are large relative to the pure dephasing frequencies of the levels, i.e., where one has to go beyond the Condon approximation. Nevertheless, the theory can be easily extended to cover this strong coupling case which can be expected to be important for many photosynthetic complexes at sufficiently low temperatures. The possibility that strong coupling might lead to a diminuation in the degree of dispersive kinetics is considered.

Nonphotochemical Hole-Burning Studies of Energy Transfer Dynamics in Antenna Complexes of Photosynthetic Bacteria

“…Such a temperature dependence has recently been reported for the lowest energy Q,-state of the photosystem I1 reaction center [36], the LHC I1 antenna complex of photosystem I1 [ 191, and the lowest exciton level of the E850 molecules in the LH2 antenna complex of purple bacteria [37]. However, the hole widths for h, = 826.8 nm are not solely determined by the TLS.…”

Hole burning with pressure and electric field: A window on the electronic structure and energy transfer dynamics of bacterial antenna complexes

“…An exciting advance in photosynthesis research was the recently determined X-ray structure of LH2 from Rps. acidophila (strain 10050) at 2.5 A resolution [25,26]. The structure shown in Figure 1.4 has nine pairs of transmembrane spanning a, P-polypeptides arranged with a C9 symmetry.…”

“…Both a and p helix axes run nearly parallel to the membrane normal, 2' and 15" respectively. A protomer, the smallest repeating unit of the LH2 structure, is defined as a radially related a, p pair and the associated pigments, which includes three BChl a and two carotenoid molecules (rhodopin glucoside, see Figure 1.1 (d) for its structure) [25].…”

“…molecules is quite different with the former in a relatively polar region and the latter in a hydrophobic region [25]. Besides, B800, a-B850 and p-BS50 (the prefixes a-and p-label the polypeptides to which the B850 molecules are bound) distort to different extent and in different ways, which can cause Qy-energy shift of up to several tens of nanometers [36,37].…”

“…Of particular relevance to this dissertation is the mechanism for B800 3 B850 transfer in the LH2 complex which has been extensively studied by ultrafast pump-probe and hole burning spectroscopies [7- The three types of energy transfer mechanisms mentioned above are discussed separately in the remaining part of this chapter. In addition, a modified Forster theory for energy transfer developed by Small and coworkers [25] which is based on the Fermi-Golden rule is also presented. This theory has been successfully applied to explain the resilience of B800 3 B850 energy transfer rates to pressure and temperature [13,26] as well as to species and mutations which changes the B800-B850 energy gap by several hundred wavenumbers [27,28].…”