A reevaluation of the fluorescence of the core chlorophylls of Photosystem I

Surfactants induced changes in the 77 K fluorescence spectra of thylakoid membranes and of various Photosystem I (PS I) core chlorophyll-protein complexes isolated from higher plants and a thermophilic cyanobacterium. Triton X-100, sodium dodecyl sulfate and, to a much lesser extent, dodecyl maltoside shift the 735 nm fluorescence of PS I in higher plant thylakoids to much shorter wavelengths, while in the cyanobacterium at the same ratios of surfactant/chlorophyll, a shift of only a few nanometers occurs. A higher plant PS I core complex, having a chlorophyll P-700 of approx. 65, and a P-700-chlorophyll a-subunit I complex ( chlorophyll P-700 approx. 40) emit maximally in the 670-685 nm region in the presence of Triton or sodium dodecyl sulfate. However, when the surfactants associated with these complexes were exchanged for dodecyl maltoside, a reversible shift to 721-725 nm occurred in their emission maxima, but there was no change in their biochemical compositions. The unaltered wavelength maximum of the fluorescence of PS I core complex is likely to be close to 725 nm. Shifts of the emission spectrum caused by surfactants indicate the importance of controlling the type and amount of surfactant present to prevent an artifactual wavelength maximum being obtained for a complex. The data explain why different maxima have been reported for essentially identical PS I core chlorophyll-protein preparations, and indicate that the origin of PS I core complex fluorescence does not change as chlorophyll molecules are extracted from PS I core complex by Triton.