Spin-labeling study of membranes in wheat embryo axes. I. Partitioning of doxyl stearates into the lipid domains

The interaction of lipid soluble spin labels with wheat embryo axes has bee n investigated to obtain insight into the structural organization of lipid domains in embryo cell membranes, using conventional electron paramagnetic resonance (EPR) and saturation transfer EPR (ST-EPR) spectroscopy. Stearic acid spin labels (n-SASL) and their methylated derivatives (n-MeSASL), labe lled at different positions of their doxyl group (n = 5, 12 and 16), wen us ed to probe the ordering and molecular mobility in different regions of the lipid moiety of axis cell membranes. The ordering and local polarity in re lation to the position of the doxyl group along the hydrocarbon chain of SA SL, determined over the temperature range from -50 to +20 degrees C, are ty pical for biological and model lipid membranes, but essentially differ from those in seed oil droplets. Positional profiles for ST-EPR spectra show th at the flexibility profile along the lipid hydrocarbon chain does exist eve n at lon temperatures, when most of the membrane lipids are in solid state (gel phase). The ordering of the SASL nitroxide radical In the membrane sur face region is essentially higher than that in the depth of the membrane. T he doxyl groups Of MeSASLs are less ordered (even at low temperatures) than those of the corresponding SASLs, indicating that the MeSASLs are located in the bulk of membrane lipids rather than in the protein boundary Lipids. The analysis of the profiles of EPR and ST-EPR spectral parameters allows u s to conclude that the vast majority of SASL and MeSASL molecules accumulat ed In embryo axes Is located in the cell membranes rather than in the inter ior of the oil bodies, The preferential partitioning of the doxyl stearates into membranes demonstrates the potential of the EPR spin-labelling techni que for the in situ study of membrane behavior in seeds of different hydrat ion levels. (C) 2000 Elsevier Science B.V. All rights reserved.