Circadian rhythm in the kinetics of acid denaturation of cell membranes of Euglena gracilis.

The denaturation of Euglena gracilis in aggressive acids has been investigated by means of the chlorophyll oxidation. A comparative study in different halogen acetic acids revealed that the reaction kinetics are determined by a reaction between the acid anion and the outer membrane. The denaturation of cell membranes in trichloracetic acid obeys clear second-order kinetics with respect to the overall acid concentration and approaches first-order kinetics with respect to the proton concentration. Osmotic shrinkage immediately increases the acid permeability of the membrane; the increased permeability persists some time after reversal of the osmotic reaction. In nondividing cells which are entrained by a 12:12h light:dark rhythm a strong daily fluctuation is seen in the energy of activation of the denaturation ranging between 20 kcal/M early in the day and 34 kcal/M early at night. Under continuous bright light the fluctuation continuous for at least one further cycle. The susceptibility of the membrane fluctuates synchronously with the motoric activity of the cells but approximately 180° apart from the susceptibility of the circadian clock to phase shifting signals.