Chlorophyll fluorescence characteristics of system I chlorophyll a-protein complex and system II particles at room and liquid nitrogen temperatures

1. Emission spectra of a system I chlorophyll (Chi) a-protein complex (SI Chl-P) and system II particles, prepared by the method of Dietrich and Thornber (25), and by the method of Huzisige et al. (24), respectively, were measured at room and liquid nitrogen temperatures to characterize the emission bands originating from system I and system II. 2. Room temperature and 77°K spectra clearly show that the F695 (690-697 nm) fluorescence band originates from both photosystems. In SI Chl-P the F695 band was observed both at room and at liquid nitrogen temperatures. 3. At 77°K, the Chi a fluorescence at 685 nm is nearly as intense as that at 720 nm (long-wavelength band) in dilute samples of SI Chl-P. Reabsorption of 685 nm fluorescence has distorted considerably the shape of emission spectra of system I published thus far. In dilute samples of system II, the F695 is as (or more), intense as F685, and the F735 is drastically decreased. 4. Additionally, it is reported here that in Cyanidium caldarium, studied to compare the in vivo system with isolated SI Chl-P and system II preparations, the 695 nm band is present upon excitation in both system I and system II; the ratio of the long-wavelength fluorescence (F735) to the short-wavelength fluorescence (F685) is much higher than those in the purified preparations. Conceivably, the high values, obtained in the dilute samples of algae, are due to the reabsorption of the fluorescence from the shortwavelength form of Chi a in the chloroplast in vivo. Furthermore, in this alga the phycocyanin fluorescence band is split with maxima at 655 (phycocyanin) and 665 nm (allophycocyanin) at 77°K. At room temperature, however, the allophycocyanin fluorescence predominates having a peak at about 670 nm. The relative increase in phycocyanin fluorescence at 77°K may be due to a decrease in the energy transfer from it to allophycocyanin in agreement with slow Forster type transfer.