Fluorescence lifetimes of oriented green bacteria cells, cell fragments and oriented bacteriochlorophyll c molecules

The decays of the polarized components of fluorescence (excited at 410 nm by a 70 fs laser flash) of whole bacteria Prosthecochloris aestuarii cells, their fragments and isolated bacteriochlorophyll c molecules located in isotropic and stretched polymer films have been measured and analysed using exponential components which in the literature data are assigned to monomers, small aggregates and oligomers of bacteriochlorophyll c. The amplitudes of the lifetime components depend only slightly on the light polarization but they change strongly with a change in the spectral region of fluorescence observation and the type of sample. Two spectral regions of fluorescence emission have been observed: at 680 nm (bacteriochlorophyll c and bacteriopheophytin monomers) and at 750 nm (bacteriochlorophyll c oligomers). The longest decay component (from 5.6 to 7.4 ns for various samples and light polarizations) is similar to that observed for monomer bacteriochlorophyll c in various solutions. The middle component (1.1-2.7 ns) is probably due to small bacteriochlorophyll c aggregates, whereas the shortest one (ranging from 0.001 to 0.37 ns depending on the type of sample, polarization of light, etc.) is related to bacteriochlorophyll c oligomers. Some components exhibit negative amplitudes, showing that the emission is generated by excitation transfer from other pigment forms. The amplitude of oligomer component emission is low in bacteria and bacteria fragments in stretched film, whereas it is high for artificial oligomers. This is because in organisms the excitation is efficiently transferred to bacteriochlorophyll a complexes emitting in a longer-wavelength range, whereas in the model system oligomers of bacteriochlorophyll c are the final accepters of excitation. In bacteria the stretching of a polymer matrix not only orients the whole object but also, to some extent, perturbs the natural arrangement of pigment and the disaggregated part of oligomers to monomers and smaller aggregates. Therefore in stretched polymer we observe the middle component of decay. The intensities of various polarized components of emission measured for the same sample are different. The coefficients of the anisotropy of emission a short time (about 100 fs) after excitation and averaged over the whole decay time for anisotropic and isotropic samples have been calculated. In some cases a strong depolarization of fluorescence during the decay time is observed; in others the anisotropy only changes slowly in this time. The results are compared with literature data for lifetimes and anisotropy of fluorescence of bacteriochlorophyll c in organisms and in artificial aggregates.