Zinc methyl 13(1)-deoxopyropheophorbide a, which lacked the 13(1)-oxo group in the exo five-membered E-ring, exhibited faster demetalation kinetics than zinc methyl pyropheophorbide a under acidic conditions, indicating that the 13-keto group of chlorophyllous pigments was responsible for resistance against removal of the central metal from the chlorin macrocycle.