[Photopotential during the photochemical reaction of chlorophyll with duroquinone]

Under the illumination of ethanol solutions of chlorophyll "alpha" and "beta" in the presence of duroquinone positive photopotential (PP) appears, the sign of which is not changed with an increase of pH. However, if duroquinone is used, which had been illuminated with white light in the absence of air, in ethanol, which results in an increase of concentration of its reduced forms, at neutral weakly alkaline pH the PP sign gets negative. This fact supports the assumption that the change of PP sign at the change of pH when parabenzoquinone is used as an oxidizer results from the change of the direction of electron transfer during photoreaction. During photoreaction of pheophytin incapable of photooxidation with duroquinone and disposed to photoreduction, PP at all pH is negative.