PHOTOPHYSICS OF PHYCOERYTHROCYANINS FROM THE CYANOBACTERIUM WESTIELLOPSIS-PROLIFICA STUDIED BY TIME-RESOLVED FLUORESCENCE AND COHERENT ANTI-STOKES-RAMAN SCATTERING SPECTROSCOPY
S. Schneider et al., PHOTOPHYSICS OF PHYCOERYTHROCYANINS FROM THE CYANOBACTERIUM WESTIELLOPSIS-PROLIFICA STUDIED BY TIME-RESOLVED FLUORESCENCE AND COHERENT ANTI-STOKES-RAMAN SCATTERING SPECTROSCOPY, Journal of photochemistry and photobiology.B, Biology, 26(1), 1994, pp. 75-85
Three building blocks of the antenna complexes of the cyanobacterium W
estiellopsis prolifica were studied: PEC(X), which is similar to the a
lpha-subunit of phycoerythrocyanin (PEC), trimers of PEC and monomers
derived from these by deaggregation with KSCN. The fit of the fluoresc
ence decay curve of PEC(X) requires at least four exponentials, althou
gh it supposedly contains only one chromophore. The coherent anti-Stok
es Raman scattering (CARS) spectra indicate that the heterogeneity obs
erved is due to geometrical isomers, which are in part generated by ph
otoinduced processes. A similar heterogeneity in chromophore structure
and properties is also found in the monomers, where four exponentials
are needed to fit the fluorescence decay curve, As in trimers, there
is a long-lived, low-amplitude component, which can be assigned to imp
urities and/or oxidation products. The energy transfer time between th
e two phyocyanobilin chromophores in the beta-subunit is about 500 ps;
the lifetime of the fluorescing beta-chromophore is 1.5 ns. The phyco
violobilin chromophore in the alpha-subunit adopts different geometrie
s characterized by fluorescence liftetimes of about 240 and 800 ps. No
evidence was found for energy transfer between the alpha-chromophore
and the beta-chromophores. This energy transfer occurs in trimers on a
time scale of less than 20 ps; the energy transfer time between the t
wo different types of beta-chromophore is about 250 ps and the lifetim
e of the terminal emitter is about 1.5 ns. The excited state kinetics
are therefore similar to those of PEC trimers from Mastigocladus lamin
osus, as are the CARS spectra, indicating a similar chromophore-protei
n arrangement. In comparison with phycocyanin, the ordering of the exc
ited states of chromophores beta 84 and beta 155 may be changed. Altho
ugh PEC trimers of Westiellopsis prolifica show almost as good a photo
stability as trimers of Mastigocladus laminosus, monomers are so photo
labile that no CARS spectra could be recorded.