ESR and NMR Studies on the Effects of Magnesium Ion on Chloroplast Manganese

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Abstract
The addition of 1–50 mM MgCl, to “low salt” thylakoid membranes causes a monotonic decrease in the transverse, aqueous proton relaxation rate (R2 = 1/T2) and an increase in the 6‐line ESR spectrum characteristic of free Mn(II) but does not affect the oxygen evolution rate. These results suggest that magnesium ion at low concentrations replaces Mn(II) from a very loosely bound pool which is not involved in oxygen evolution. The decrease of R2 is probably due mainly to the smaller molar relaxivity of free compared with bound Mn(II). R2 can be affected by conformational and structural changes involving the Mn(II) as well as by the amount of Mn(II) and the nature of its binding. However, MgCl2 causes the same decrease in R2 for trypsin treated thylakoids, which do not undergo the gross structural changes, e.g. grana stacking, that it produces in untreated membranes. R2 measurements of isolated light harvesting complex preparations indicate the presence of bound Mn in it; neutron activation analysis of these samples shows that they have about one‐third of the functional manganese bound in thylakoid membranes—this may be the tightly bound pool of Mn, but the results are not conclusive.