A new compound called MCPNA was studied in spinach chloroplasts. It acts as an electron acceptor in the photosystem 1 of isolated thylakoids. MCPNA inhibits oxygen production in the presence of certain molecules but not others. It works best at concentrations between 3 µM and 100 µM. At higher concentrations, it can inhibit another part of the photosystem. MCPNA does not affect the process of turning light into energy. When exposed to light, thylakoids can transfer electrons to MCPNA, which explains how it affects chloroplasts.