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 dependence of the proton relaxation rates (PRR) upon Mn concentration 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., ferricyanide) 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 intermediate 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 the charge accumulation (Cheniae, 1970; Olson, 1970;Renger, 1970;Earley, 1973). However, until recently there has been no direct experimental evidence to show that manganese changes its oxidation state during photosynthesis (Wydrzynski et al., 1976a,b).The current view is that manganese is complexed to some protein bound to the chloroplast membrane. However, attempts to isolate a manganese protein and to reconstitute oxygen evolution have failed (Cheniae and Martin, 1966; Lagoutte and Duraton, 1975;Henriques and Park, 1976). One possible exception is the stimulation of oxygen evolution by a low-molecular-weight manganese polypeptide isolated from Phormidium luridium (Tel-Or and Avron, 1974). In any event, if one wishes to study the role of manganese in oxygen evolution, it is necessary to use an experimental approach which responds to changes in situ of the manganese.The unpaired electrons of Mn(I1) can, in favorable instances, give rise to a detectable electron paramagnetic reso- Mn(II), the remainder being in a higher oxidation state@), probably Mn(II1). 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:s; and B = (0.9 f 0.09) X 1019. The oxygen-17 T1 and T2 data for suspensions before and after treatment with a detergent are consistent with the location of the manganese 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.nance (EPR') spectrum. However, so far there has been only one report of EPR spectra from Mn bound to chloroplasts (Siderer et al., 1977). Another possibility is that neighboring magnetic nuclei are perturbed enough by the large magnetic mom...

Molecular orbital study of photosynthetic water decomposition

“…This tendency will be strengthened by taking into account the abovestated facts, excluding possible involvement of Mn(IV). Thus, it is highly probable that Mn(II1) ion is utilized in the water-splitting reaction in agreement with experimental proposition [6]. All the above calculations were performed in the presence of proton-accepting site.…”

“…Here we assume the former possibility; as will be shown later, we found that no proton acceptor stronger than neutral water is necessary. Here, using the MO investigation, we aim to answer the following questions: [4,6].…”

Assessment of the requirement for aquaporins in the thylakoid membrane of plant chloroplasts to sustain photosynthetic water oxidation

“…[40][41][42][43]), to our knowledge, there is no measure of these two water permeability parameters for thylakoids. Nevertheless, the s a parameter for mean lifetime of water inside the thylakoid lumen has been measured using NMR by two independent laboratories [45,46] Table 2, volumes and areas from the literature [47,48], the apparent P 0 d for the chloroplast envelope and the erythrocyte plasma membrane were also calculated, and found to be only slightly lower than the experimentally determined P d values for the same type of membranes (Table 2). This indicates that calculation of P 0 d provides data that are reliable and useful for comparative studies in the absence of experimentally determined P d .…”

Initial stages of lichen hydration observed by proton magnetic relaxation

“…This occurs for membranes of photobiont where the reaction centre of photosystem II contains manganese ions affecting strongly the proton relaxation process in the range of 30 MHz (Blicharski et al, 1982 ;Khanna et al, 1983 ;Haran! czyk et al, 1986 ;Wydrzynski et al, 1978). Thus, the signal of the L " component, coming mostly from the water tightly bound on photobiont surface, is strongly reduced, whereas the loosely bound water fraction giving the L # signal is less (if at all affected by paramagnetic impurities dissolved in water volume.…”

“…between 0n10 and 0n23). This suggests that the proton spin-lattice relaxation at room temperature is dominated by the electron paramagnetism of endogenous centres (Wydrzynski et al, 1978) present in thallus. Thus, in this paper we focused on free induction decay characterized by the T # time constant (shorter than T " ) and which in this case gave more information.…”