Fluorescence bands and chlorophyll a forms

Fluorescence spectra were determined at temperatures between 20° and −196° for a number of photosynthetic organisms. Below −90° the single fluorescence maximum around 685 mμ was replaced by a system of three bands, at 686, 696 and 717–720 mμ in algal cells. Cooling usually resulted in a decrease of the 685-mμ band. In young cultures of blue-green and red algae the three bands were of about equal intensity; in old cultures and in green algae the 717-mμ band was dominant, while in the latter the 696-mμ bans was weak.

In green leaves and chloroplast also, three bands were present at low temperatures, at 686, 696 and 735–740 mμ. Here too, the 740-mμ band was by far the major one.

During cooling of both diluted and concentrated chlorophyll a solutions and chlorphyll adsorbed to filter paper, the height of the 677-mμ fluorescence band and the 730-mμ vibrational level were increased by a factor of about two, provided no increased reabsorption due to increased scattering could occur. In concentrated chlorophyll a solutions no extra bands could be detected.

The three fluorescence bands measured invivo at low temperatures are assumed to belong to three chlorophyll a forms: C_a 670-F 686; C_a 695-F 717 in algal cells. Apart from an increase in intrinsic fluorescence yield of C_a 695, the marked increase in 717-mμ fluorescence during cooling is suggested to be due to increased energy transfer from C_a 670 and C_a 680 to C_a 695 as a result of shrinkage, when the temperature is lowered.