Low-Temperature Dynamical Transition in Lipid Bilayers Detected by Spin-Label ESE Spectroscopy

Evidence for an Ordering Transition near 120 K in an Intrinsically Disordered Protein, Casein

“…Then, Figure 2 shows that relaxation at the middle of the high-field component (spectral position 2) is faster than that at the two outer shoulders of the spectrum. As the shoulders refer to the so-called canonical orientation of the magnetic tensor to the external magnetic field, being therefore less sensitive to reorientations, this behavior is typical for spin-relaxation induced by low-amplitude molecular librations [ 15 , 16 , 17 ]. Note also that, as echo decays are only due to relaxation processes in the spin system, the motion is to be stochastic.…”

“…The pure contribution of librational motion may be obtained at X-band EPR from the ratio of ESE decays at two field positions shown in Figure 2 which is expected to depend exponentially on the time delay τ (Equation (1)) [ 15 , 16 , 17 ]: where Δ W may be termed as an anisotropic relaxation rate. The representative examples of the decays E 1 (2τ) and E 2 (2τ) and their ratios are shown in Figure S3 of Supplementary Materials , which indeed demonstrates exponential decay.…”

“…These librations are a general property of molecular solids and highly viscous liquids. ESE-detected librations in molecular systems are determined by the nearest surrounding of a molecule [ 16 , 17 ]. Therefore, this approach may serve as a tool to study molecular packing in the molecular systems.…”

“…Using this approach, molecular packing was studied in gel-phase biological membranes (lipid bilayers) [ 16 , 17 , 18 , 19 ] and in glassy ionic liquids [ 20 , 21 , 22 , 23 ]. Further, interaction of free fatty acids with proteins [ 24 ] and interaction of sugars with biological membranes [ 25 ] was investigated.…”

“…Further, interaction of free fatty acids with proteins [ 24 ] and interaction of sugars with biological membranes [ 25 ] was investigated. Stochastic molecular librations are sensitive to a so-called dynamical transition in proteins and in other biological systems [ 16 , 19 , 26 , 27 , 28 ], which is known occur between 170 and 220 K from numerous neutron-scattering experiments and MD simulations to—see e.g., [ 29 , 30 , 31 ] for recent references. Note however that the nature of this transition is still heavily debated.…”