Dynamical Transitions at Low Temperatures in the Nearest Hydration Shell of Phospholipid Bilayers

For the so-called dynamical transition from harmonic to anharmonic (or diffusive) motions in biological systems, the presence of hydration water is important. To explain the molecular mechanism of this transition, the information on molecular motions in the nearest hydration shell would be helpful. In this work, to study molecular motions in the nearest hydration shell of spin-labeled model biological membranes, a pulsed version of electron paramagnetic resonance, electron spin echo envelope modulation (ESEEM) spectroscopy, is used. For hydration by deuterium water, the ²H ESEEM frequency spectra resemble the solid-state ²H NMR line shape that is widely used for structural and dynamical studies. Two types of model membranes were investigated and compared: bilayers consisting of unsaturated lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and bilayers consisting of fully saturated lipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). The lipid chain packing for the POPC bilayer is known to be more defective than that for the DPPC bilayer. For both the POPC and the DPPC bilayers, the ²H ESEEM NMR-like spectra showed a sharp narrowing between 180 and 190 K. From the other side, in both bilayers at 188 K, an inflection was observed for the temperature dependence of molecular motions detected by the spin relaxation of spin labels in the bilayer interior. It was concluded that dynamical transition in the bilayer interior is accompanied by an onset of isotropic water molecular dynamics in the nearest hydration shell of the bilayer with a rate of ∼10⁵ s^-1. Also, the ²H ESEEM NMR-like spectra in the POPC bilayer showed slight changes above 100 K that could be ascribed to another dynamical transition resulting in the appearance of restricted orientational motion of water molecules. These data also are interrelated with spin relaxation of spin labels in the POPC bilayer interior and support the hypothesis ascribing the transition at 100 K to excessive lipid chain flexibility.