Investigating the membrane orientation and transversal distribution of 17β-estradiol in lipid membranes by solid-state NMR

17β-Estradiol (E2) is a potent estrogen, which modulates many important cellular functions by binding to specific estrogen receptors located in the cell nucleus and also on the plasma membrane. We have studied the membrane interaction of E2 using a combination of solid-state NMR methods. 2H NMR results indicate that E2 does not cause a condensation effect of the surrounding phospholipids, which is contrary to the effects of cholesterol, and only very modest E2 induced alterations of the membrane structure were detected. 1H magic-angle spinning NMR showed well resolved signals from E2 as well as of POPC in the membrane-lipid layer. Two-dimensional NOESY spectra revealed intense cross-peaks between E2 and the membrane lipids indicating that E2 is stably inserted into the membrane. The determination of intermolecular cross-relaxation rates revealed that E2 is broadly distributed in the membrane with a maximum of the E2 distribution function in the upper chain region of the membrane. We conclude that E2 is highly dynamic in lipid membranes and may undergo rotations as it exhibits two polar hydroxyl groups on either side of the molecule.