Nuclear magnetic relaxation by the manganese in aqueous suspensions of chloroplasts

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Abstract
Proton and O-17 NMR relaxation rate (T1-1 and T2-1) data are presented for aqueous suspensions of dark-adapted chloroplasts [Pisum sativum]. It is concluded from the dependence of the proton relaxation rates (PRR) upon Mn concentration that T1-1 and T2-1 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(II). The effects of oxidants (e.g., ferricyanide) and reductants (e.g., tetraphenylboron) on the PRR indicate that only about 1/3 to 1/4 of the loosely bound Mn is present in the dark-adapted chloroplasts as Mn(II), the remainder being in a higher oxidation state(s), probably Mn(III). The frequency dependence of the PRR for the chloroplast suspensions was fitted by a simplified form of the Solomon-Bloembergen-Morgan equations, and the following parameters were obtained: .tau.S = (1.1 .+-. 0.1) .times. 10-8 s; .tau.M = (2.2 .+-. 0.2) .times. 10-8 s and B = (0.9 .+-. 0.09) .times. 1019. The O-17 T1 and T2 data for suspensions before and after treatment with a detergent are consistent with the location of the Mn in the interior of the thylakoids. An analysis of the relaxation rates shows that the average lifetime of a water molecule inside a thylakoid is > 1 ms.

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