It is shown that photodynamic hemolysis may occur at –79°C. if the erythrocytes are suspended in a solution containing 70 per cent glycerol which prevents hemolysis by freezing; but that there is no hemolysis under the same conditions at –210°C. At the higher temperature the viscosity of the solution is still low enough to permit appreciable movement of molecules, whereas at the lower temperature the molecules must be virtually immobile. The findings are compatible with the idea that the dye molecule acts in a cycle, bringing about successive oxidations by O2 molecules, as has been shown for photodynamic hemolysis at room temperature. The assumption of a combination between dye, O2, and substrate does not explain photosensitized hemolysis in the semi-solid state. The mechanism of photosensitized oxidation by O2 is discussed.

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