Spin label saturation transfer ESR studies of protein-lipid interactions in Photosystem II-enriched membranes

Saturation transfer ESR has been used to study the dynamic behaviour of iipids in the appressed regions of thylakoid membranes from pea seedlings. Four different phospnoand galacto-lipid spin labels (phosphatidylcholine labelled ~t the 12 or 14 C-atom positions of the m-2 chain, phosphatidylglycerol labelled .~t the 14-1msition nf !he sn2 chaiii, and monogalaetosyldiacylglycerol labelled at the 12-no¢itq_'e~ o, ¢ the m-2 chain) were ,~sed to probe the lipid environment in ?hot~ystem H-e~dched membranes prepared by detergent extraction. The ESR spectra show that the majarity of the lipid in these preparations is strongly motionally restricted. Values for the eifective rotational correlation times of the labelled chains were deduced from the lineheight ratios and integrals of the ~aturation transfer ESR spectra. The effective rotational correlation times were found to be in the i6 ~ s range, indicating a very low lipid chain mobility which correlates with the low lipid content of these preparations. Comparison of the effective rotational correlation times deduced from the different diagnostic regions of th.~ spectrum revealed little anisotropy in the chain mobility., indicating that the dominant motional mode was trans-gauche isomerization. The effective rotational correlation times deduced from the spectral integrals were similar to those deduced from the lineheight ratios, consistent with the absence of any appreciable fluid lipid component in these preparations. The results also indicate some selectivity, of interaction between the lipid species, with phosphatidylcholine exhibiting appreciably slower motion than either phosphatidylglycerol or monogalactosyldiac3 Iglycerol.