The effect of thioctic acid on the quantum efficiency of the Hill reaction.

If thioctic acid is an intermediate in photosynthesis closely related to the primary photochemical act as proposed by Calvin and Barltrop,I it should be possible to realize experimental conditions under which the concentration of thioctic acid limits the quantum efficiency of photosynthesis as well as the closely related Hill reaction. Conditions under which the amounts of thioctic acid and oxidant, quinone, were simultaneously limiting factors in the Hill reaction in continuous light have been described by Bradley and Calvin.2 A kinetic model consistent with this simultaneous limitation, the quantum conversion proposal of Calvin and Barltrop, and the photolysis experiments of Barltrop, Hayes, and Calvin' has been proposed by Bradley4 (Model T). This consisted essentially of three steps: (1) a light-capturing step to produce a relatively long-lived electronic excitation; (2) a conversion step in which the "excitation" is converted into some "chemical" form consisting of an oxidizing and a reducing agent; and (3) the reduction of quinone by the reducing agent (in this model, the dithiol of thioctic acid) and the liberation of oxygen from the oxidizing agent. Two sets of more or less specific reactions which conform to these requirements are given below: