Heterogeneity of thylakoid membranes studied by EPR spin probe

A lipophilic nitroxyl radical, 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl 1-adamantylacetate, has been applied to EPR spin probe study of chloroplasts and subchloroplast fragments of different types. The latter originate from grana and the grana core regions. The binding of the spin probe to the membranes was revealed by specific changes in a shape of the EPR spectra. A share ef membrane-bound spin probe was different for chloroplasts and subchloroplast fragments, as well as its rotational correlation time and apparent enthalpy and entropy activation of nitroxide rotational motion. The binding of the spin probe induced a significant decrease in the amount of the oxidized P700 and changes in the kinetics of its light oxidation and dark recovery. This suggests that one of the sites of nitroxyl radical binding is the nearest surrounding of the pigment-protein complexes of Photosystem I (PSI). Distinctions in mobility of spin probe immobilized by chloroplasts and their fragments can be caused by the different environment of the PSI complexes located in various regions of thylakoid membranes.