A theory of excitation energy transfer within the chlorosomal antennae of green bacteria has been developed for an exciton model of aggregation of bacteriochlorophyll (BChl) c (d or e). This model of six exciton-coupled BChl chains with low packing density, approximating that in vivo, and interchain distances of approximately 2 nm was generated to yield the key spectral features found in natural antennae, i.e., the exciton level structure revealed by spectral hole burning experiments and polarization of all the levels parallel to the long axis of the chlorosome. With picosecond fluorescence spectroscopy it was demonstrated that the theory explains the antenna-size-dependent kinetics of fluorescence decay in chlorosomal antenna, measured for intact cells of different cultures of the green bacterium C. aurantiacus, with different chlorosomal antenna size determined by electron microscopic examination of the ultrathin sections of the cells. The data suggest a possible mechanism of excitation energy transfer within the chlorosome that implies the formation of a cylindrical exciton, delocalized over a tubular aggregate of BChl c chains, and Forster-type transfer of such a cylindrical exciton between the nearest tubular BChl c aggregates as well as to BChl a of the baseplate.

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Green fluorescent proteins bearing the Y66H mutation exhibit strongly blue-shifted fluorescence excitation and emission spectra. However, these blue fluorescent proteins (BFPs) have lower quantum yields of fluorescence (Phi(f) approximately 0.20), which is believed to stem from the increased conformational freedom of the smaller chromophore. We demonstrate that suppression of chromophore mobility by increasing hydrostatic pressure or by decreasing temperature can enhance the fluorescence quantum yield of these proteins without significantly affecting their absorption properties or the shape of the fluorescence spectra. Analysis of the fluorescence lifetimes in the picosecond and nanosecond regimes reveals that the enhancement of the fluorescence quantum yield is due to the inhibition of fast quenching processes. Temperature-dependent fluorescence measurements reveal two barriers ( approximately 19 and 3 kJ/mol, respectively) for the transition into nonfluorescing states. These steps are probably linked with dissociation of the hydrogen bond between the chromophore and His148 or an intervening water molecule and to the barrier for chromophore twisting in the excited state, respectively. The chromophore's hydrogen-bond equilibrium at room temperature is dominated by entropic effects, while below approximately 200 K the balance is enthalpy-driven.

Spectral hole burning has been used to prove cxpcrimcnially the cxislcnce in natural antenna of one of the predicted structural optimizing factors -antenna pigment oligomcrization [J, Theor. Biol. I40 (1989) lG7]-ensuring high cllicicncy ofexcitation energy transfer from antenna toreaction center, This point has been examined for ~hc chlorosomal antenna of green bacterium Cllkurqf/~srrs aurunfiucus by hole burning in fluorescence excitation and emission spectra of intact cells at I .X K. The prrsistcnt hole spectra have been found tc be consistent with a strongly cxciton-couplcd bactcriochlorophyll c (BChl c) chromophorc sys~cm. The lowcat cxcilon state of BChl c oligomers has been directly detected and separated as the lowest energy inhomogcncously broadened bond (FWHM -90 cm-', position of maximum, at -752 nm) from the near-infrared BChl c band (FWHM -350 cm-'. poshion of maximum, at -742 nm) of I.8 K excitation spectrum.