Mechanism of Chlorophyll Action in Photosynthesis

Abstract

THE use of tritiated water affords a sensitive means of answering at least one question about early photosynthetic events : Do any of the hydrogen atoms of the chlorophyll molecule intervene in the electron transport chain between water and oxidant ? It has already been established¹ that all the hydrogen–carbon bonds of the chlorophyll molecule are sufficiently stable to prevent hydrogen exchange in neutral organic solvents in the dark; a tracer experiment is therefore feasible. At least one current school of thought maintains that the participation of hydrogen of the chlorophyll molecule in a chemical reaction is a requisite step in the electron transfer which originates in photoactivated chlorophyll². Experiments in which Chlorella or Scenedesmus cells were illuminated in tritiated water have led us to the conclusion that some hydrogen of chlorophyll is activated during photosynthesis. A similar light-dependent and heat-sensitive activation of chlorophyll a and bacteriochlorophyll has been observed in a variety of cell-free preparations. The position in the chlorophyll molecule which is photochemically labilized is tentatively concluded to be C₁₀.