A long-wave absorbing form of chlorophyll a responsible for the "red drop" in fluorescence at 298 degrees K and the F723 band at 77 degrees K.

When Chlorella cells are ruptured at pH 4.6 by sonication in air, its absorption spectrum can be best explained if one assumes that a long-wave chlorophyll a form (Chl a 693 ) is preferentially destroyed. Using these preparations, and comparing them with the algal suspension and the sonicates prepared at pH 7.8 under argon, we make the following conclusions: (a) The red drop beginning at about 675-68o nm in the action spectrum* of fluorescence at 298 °K must be due to the presence of a non(or weakly) fluorescent form of chlorophyll a. We suggest that this form is Chl a 693. The red drop is absent in the aerobic sonicates. (b) The red drop in fluorescence in whole algal cells is not due to any errors in absorption measurements; this drop is clearly present in the anaerobic sonicates. (c) The emission band at 723 nm, discovered by S. S. BRODY in whole Chlorella cells at 77 °K, may be due to increased fluorescence efficiency of Chl a 693 at low temperature; the F723 band is absent in aerobic sonicates.