Journal of Physical Chemistry B, Vol.112, No.42, 13285-13291, 2008
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-steafic acid in 1-Palmitoyl-2-oleoyl-sit-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 similar to 120 K for 5-DOXYL-stearic acid and down to similar 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, similar to 0.1-1 degrees, 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 similar to 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 similar to 100 K.