Nuclear Magnetic Relaxation by the Manganese in Aqueous Suspensions of Chloroplasts?

ABSTRACT: Proton and oxygen-17 N M R relaxation rate (T1-l and T2-l) data are presented for aqueous suspensions of dark-adapted chloroplasts. It is concluded from the depen-dence of the proton relaxation rates (PRR) upon Mn concen-tration that T1-l and T2-l are determined largely by the loosely bound Mn present in the chloroplast membranes. The frequency and temperature dependences of the PRR are characteristic of Mn(I1). The effects of oxidants (e.g., ferri-cyanide) and reductants (e.g., tetraphenylboron) on the PRR indicate that only about one-third to one-fourth of the loosely bound Mn is present in the dark-adapted chloroplasts as T h e involvement of manganese ions in photosynthesis was first suggested in the early studies of Pirson (1937). Much later, the site of its activity was assigned to the oxygen-evolving mechanism of photosystem I1 (Kessler et al., 1957; Spence and Possingham, 1961; Cheniae and Martin, 1966; Homann, 1968). Subsequently, Kok et al. (1 970) proposed a model for oxygen evolution in which a photochemically activated inter-mediate accumulates up to 4 oxidizing equiv before reacting with water to produce oxygen. Manganese can exist in several oxidation states and thus it could play a central role in th