Fast Stochastic Librations and Slow Rotations of Spin Labeled Stearic Acids in a Model Phospholipid Bilayer at Cryogenic Temperatures

The spin label DOXYL (4,4-dimethyl-oxazolidine-1-oxyl) is a nitroxyl ring that can be attached rigidly at specific C-atom positions in the stearic acid. 5-DOXYL-stearic acid and 16-DOXYL-stearic acid in 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayers were studied using electron spin echo (ESE) spectroscopy at low temperatures. The anisotropy of ESE decay across the electron paramagnetic resonance (EPR) spectrum evidence that these spin labels participate in orientational motions at temperatures down to ∼120 K for 5-DOXYL-stearic acid and down to ∼80 K for 16-DOXYL-stearic acid. Fast stochastic librations, with correlation time at the nanosecond time scale, manifest itself in a two-pulse ESE experiment. Stimulated three-pulse ESE experiment is sensitive to motions of ultrasmall amplitude, ∼ 0.1−1°, developing at the microsecond time scale. The stimulated ESE decays were found to depend on the product of the two time delays of the pulse sequence. This fact may be described within a simple model of slow inertial rotations developing within this small range of angles with a rate of ∼1 kHz. Both types of motion evidence the pronounced motional heterogeneity across the bilayer at cryogenic temperatures, with a remarkable increase of motion in the bilayer interior. The found low-temperature motions imply that hydrophobic parts of amphiphilic biomolecules may possess a noticeable mobility even at temperatures as low as ∼100 K.