Electron spin resonance and photoreaction of Mn(II) in lettuce chloroplasts.

Electron spin resonance (esr) of lettuce chloroplasts yields three types of signals: (i) a broad (~900 G) signal around g = 2.22 (apparently due to Cu²⁺ complexes); (ii) an Mn²⁺ spectrum around g = 2.003 consisting of six hyperfine lines (A = 94.5 G) of ~30 G width; and (iii) a sharp signal at g = 2.00 due to photosignals I and II. The present work is concerned with the Mn²⁺ signal and its relation to the photosynthetic process. Intensity measurements were performed by comparing the intensities of the Mn²⁺ signals of two identical chloroplast preparations, one of which was slightly acidified. The integrated intensity of the signal in the normal preparation was approximately one-fourth of that in the acidified sample, suggesting that only the?$\text{1}\text{2}\text{?}\text{1}\text{2}$ fine structure band is observed in untreated chloroplasts. This indicates that the manganese in the chloroplasts is bound in an asymmetric environment, apparently in protein complexes. The Mn²⁺ signal is light sensitive, decreasing on illumination and reappearing in the dark. Typical values for the half-lives of the light and dark processes in normal chloroplasts are 0.25 and 2.1s, respectively. The effect is interpreted in terms of the photooxidation of Mn²⁺ to higher oxidation states which are invisible to esr spectroscopy. In order to determine whether this process is related to photosynthesis the effect of certain reagents and treatments that are known to affect the photosynthetic system was studied. It was found that the oxygen evolution inhibitors 3-(3,4 dichlorophenyl)-1,1-dimethylurea (DCMU) and carbonylcyanide-p-trifluoromethoxyphenylhydrazone (FCCP) as well as the electron donors, phenylenediamine and sodium ascorbate, reduce or completely eliminate the light effect on the Mn²⁺ signal. Heat treatment and Tris washing caused deceleration of both the light and dark reactions. These effects indicate that the photooxidation of the Mn²⁺ is related to the photosynthetic cycle, the most probable site being the water splitting apparatus of photosystem II.