Low-temperature (4-77°k) Spectroscopy of Anacystis; Temperature Dependence of Energy Transfer Efficiency

From the absorption spectra, emission and excitation spectra of chlorophyll a (Chl a) and phycocyanin fluorescence measured (from 4 to 77 °K) at different wavelengths, we confirmed that a large portion of Chl a (Chl a 670, Chl a 678 and Chl a 7o5),.in Anacystis, is in System I, and that System II is composed mainly of phycocyanin and of some Chl a 670 and Chl a 678. Our data also confirmed the following correspondence between absorption and fluorescence bands: phycocyanin (with bands at 580, 625 and 634-637 nm) and allophycocyanin (approx. 650 nm) are responsible for the broad fluorescence band at 653-655 nm and Chl a 670 for F68o (when Chl a 678 is preferentially extracted). (Date in ref. 20 show that Chl a 678 is responsible for F685.) The ratio of f715 to f685 + f695 was greater in System I than in System II. The efficiency of energy transfer from Chl a 670 to Chl a 678, that approaches ioo % even at 4 ° K, was found to be independent of temperature. However, transfer efficiency from phycocyanin to Chl a was temperature dependent; it was lower at 4 than at 77 ° K. The quantum yield of Chl a fluorescence (~t excitation ---435 nm) was higher (by a factor of 4) at 4 than at 77 ° K. The estimated quantum yield of fluorescence in Anacystis at 4°K is only 6 % (). excitation = 560 nm) and approx. 3 % (4 excitation = 436 nm).