PLANT STEROLS - A NEUTRON-DIFFRACTION STUDY OF SITOSTEROL AND STIGMASTEROL IN SOYBEAN PHOSPHATIDYLCHOLINE MEMBRANES

Neutron scattering experiments have been performed on oriented Soybean phosphatidylcholine (SPC) bilayers, containing sitosterol or stigmast erol, two major sterols of plant plasma membranes. Sitosterol and stig masterol were either protonated or deuterated on position C-25 of the lateral chain. Incorporation of sitosterol leads to an increase of the hydrophobic thickness of SPC bilayers of 1.2 and 2 Angstrom when pres ent, at 16 and 30 mol%, respectively. On the other hand, no change was observed when stigmasterol is present in the bilayer at its maximal s olubility of 16 mol%. These results are in agreement with the fact tha t sitosterol is more efficient than stigmasterol to order acyl chains of SPC, as already shown with other biophysical techniques. In order t o get more insight into the behavior of the lateral chains of the two sterols, the proton-deuterium contrast method was used in order to loc ate the H-2(25) atoms of the two sterols. For sitosterol, this atom wa s found close to the center of the bilayer at +/-(1.6 +/- 0.2 Angstrom ), with a width, nu = 2.5 +/- 0.5 Angstrom. For stigmasterol, the diff erence profile could be fitted in two different ways: either two possi ble locations are found at +/-(2.3 +/- 0.2 Angstrom) and +/-(10 +/- 0. 2 Angstrom) with the same width, nu = 2.5 +/- 0.5 Angstrom or only one broad distribution at +/-(6.1 +/- 0.3 Angstrom), nu = 8.5 +/- 0.7 Ang strom. The results are discussed in terms of difference of dynamics fo r the lateral chain of the two sterols. (C) 1998 Elsevier Science B.V. All rights reserved.