ИСТИНА

The main structural factors, determining the excitation energy transfer (EET) efficiency in the largest among all known photosynthetic antenna structures - the chlorosome of green bacteria - were studied. Picosecond fluorescence studies of BChl c exciton kinetics as a function of the chlorosome size in two strains from different families of the green bacteria (Chloroflexus aurantiacus and Chlorobium limicola) were undertaken. The different size of chlorosomal antennae (the different ratios of BChl c/BChl a) was adjusted by inhibiting the formation of BChl c by gabaculine. Using electron microscopic examination of micrographs of ultrathin sections of the cells, we discovered that in each culture (and even in the same cell) there are chlorosomes with different number of rods layers (the chlorosome thickness can vary up to 5 times!). It was shown that cells from each culture can be characterized by their own discrete distribution function F(p), were F(p) is the relative number of chlorosomes with p layers. It was found that the efficiency of EET from three-dimensional chlorosomal BChl c antenna to BChl a antenna of the baseplate is determined only by one chlorosome size, namely, by its height, i.e., by the number of rods layers. The measured BChl c excitation lifetimes show a quasi- linear dependence on the chlorosome size. The rate constant of fluorescence decay in the BChl c antenna (and, hence, the EET efficiency from the BChl c antenna to the baseplate) decreases with increasing the chlorosome size. That is why the measured values of BChl c fluorescence decay in whole cells exhibit lifetimes in a rather wide range, from 12 ps to 40 ps. Thus, the nature of the structure-function relationship in the light-harvesting antenna in two strains of green bacteria from different families was revealed.