Abstract
The absorbance change at 518 nm (ΔA518) was measured in response to single-turnover actinic laser flashes under varied conditions. This response is usually attributed to an electrochromic effect on endogenous pigments. Particularly in carefully prepared pea chloroplasts, a slowly rising portion of ΔA518 was consistently observed. The substances DBMIB, DCMU and ADP all removed the slow component in different ways. It is argued that this part of the P518 response is due to the electrogenic effect of protons released from the oxidation of plastoquinone. On this basis, it is suggested that P518 responds approximately equally to protons released during the oxidation of water and of plastoquinone, but with different time courses. ADP seems to switch the path of protons from plastoquinone to a route in which they do not induce a transthylakoid field.
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