Lipid Librations at the Interface with the Na,K-ATPase

Evidence for capture of spin-labeled ibuprofen drug molecules by lipid rafts in model membranes

Lipid rafts are lipid-cholesterol nanostructures thought to exist in cell membranes, which are characterized by higher ordering compared to their surroundings. Ibuprofen and other non-steroidal anti-inflammatory drugs (NSAIDs) have a high affinity for phospholipid membranes and can alter their structure and biological properties. Here we use electron paramagnetic resonance (EPR) in its pulsed electron spin echo (ESE) version to study spin-labeled ibuprofen (ibuprofen-SL) in a raft-mimicking bilayer, which consists of an equimolar mixture of the phospholipids dioleoyl-glycero-phosphocholine (DOPC) and dipalmitoyl-glycero-phosphocholine (DPPC), with cholesterol added in various proportions. ESE decays are sensitive to the presence of low-temperature small-angle orientational motions of molecules - stochastic molecular librations. The data obtained show that in the presence of lipid rafts the temperature dependence of the spin relaxation rate induced by this motion reaches a plateau. This behavior is characteristic of non-cooperative motion of a surface-bound molecule. Based on this analogy, the data obtained were interpreted as evidence that ibuprofen-SL molecules are adsorbed on the raft boundaries.

Peptide-membrane binding is not enough to explain bioactivity: A case study

Membrane-active peptides are a promising class of antimicrobial and anticancer therapeutics. For this reason, their molecular mechanisms of action are currently actively investigated. By exploiting Electron Paramagnetic Resonance, we study the membrane interaction of two spin-labeled analogs of the antimicrobial and cytotoxic peptide trichogin GA IV (Tri), with opposite bioactivity: Tri(Api⁸), able to selectively kill cancer cells, and Tri(Leu⁴), which is completely nontoxic. In our attempt to determine the molecular basis of their different biological activity, we investigate peptide impact on the lateral organization of lipid membranes, peptide localization and oligomerization, in the zwitter-ionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) model membrane We show that, despite their divergent bioactivity, both peptide analogs (i) are membrane-bound, (ii) display a weak tendency to oligomerization, and (iii) do not induce significant lipid rearrangement. Conversely, literature data show that the parent peptide trichogin, which is cytotoxic without any selectivity, is strongly prone to dimerization and affects the reorganization of POPC membranes. Its dimers are involved in the rotation around the peptide helix, as observed at cryogenic temperatures in the millisecond timescale. Since this latter behavior is not observed for the inactive Tri(Leu⁴), we propose that for short-length peptides as trichogin oligomerization and molecular motions are crucial for bioactivity, and membrane binding alone is not enough to predict or explain it. We envisage that small changes in the peptide sequence that affect only their ability to oligomerize, or their molecular motions inside the membrane, can tune the peptide activity on membranes of different compositions.

Librational Dynamics of Spin-Labeled Membranes at Cryogenic Temperatures From Echo-Detected ED-EPR Spectra

Methods of electron spin echo of pulse electron paramagnetic resonance (EPR) spectroscopy are increasingly employed to investigate biophysical properties of nitroxide-labeled biosystems at cryogenic temperatures. Two-pulse echo-detected ED-spectra have proven to be valuable tools to describe the librational dynamics in the low-temperature phases of both lipids and proteins in membranes. The motional parameter, α2τC, given by the product of the mean-square angular amplitude, α2, and the rotational correlation time, τC, of the motion, is readily determined from the nitroxide ED-spectra as well as from the W-relaxation rate curves. An independent evaluation of α2 is obtained from the motionally averaged ¹⁴N-hyperfine splitting separation in the continuous wave cw-EPR spectra. Finally, the rotational correlation time τC can be estimated by combining ED- and cw-EPR data. In this mini-review, results on the librational dynamics in model and natural membranes are illustrated.

Influence of hydration on segmental chain librations and dynamical transition in lipid bilayers

Continuous wave electron paramagnetic resonance spectroscopy of chain-labeled phospholipids is used to investigate the effects of hydration on the librational oscillations and the dynamical transition of phospholipid membranes in the low-temperature range 120-270 K. Bilayers of dipalmitoylphostatidiycholine (DPPC) spin-labeled at the first acyl chain segments and at the methyl ends and prepared at full, low, and very low hydration are considered. The segmental mean-square angular amplitudes of librations, 〈α²〉, are larger in the bilayer interior than at the polar/apolar interface and larger in the fully and low hydrated than in the very low hydrated membranes. For chain segments at the beginning of the hydrocarbon region, 〈α²〉-values are markedly restricted and temperature independent in DPPC with the lowest water content, whereas they increase with temperature in the low and fully hydrated bilayers, particularly at the highest temperatures. For chain segments at the chain termini, the librational amplitudes increase progressively, first slowly and then more rapidly with temperature in bilayers at any level of hydration. From the temperature dependence of the mean-square librational amplitude, the dynamical transition is detected around 240 K at the polar/apolar interface in fully and low hydrated DPPC and at around 225 K at the inner hydrocarbon region for bilayers at any hydration condition. At the dynamical transition the bilayers cross low energy barriers of activation energy in the range 10-20 kJ/mol. The results highlight biophysical properties of DPPC bilayers at low-temperature and provide evidence of the effects of the hydration on the dynamical transition in bilayers.

Probing Small-Angle Molecular Motions with EPR Spectroscopy: Dynamical Transition and Molecular Packing in Disordered Solids

Disordered molecular solids present a rather broad class of substances of different origin—amorphous polymers, materials for photonics and optoelectronics, amorphous pharmaceutics, simple molecular glass formers, and others. Frozen biological media in many respects also may be referred to this class. Theoretical description of dynamics and structure of disordered solids still does not exist, and only some phenomenological models can be developed to explain results of particular experiments. Among different experimental approaches, electron paramagnetic resonance (EPR) applied to spin probes and labels also can deliver useful information. EPR allows probing small-angle orientational molecular motions (molecular librations), which intrinsically are inherent to all molecular solids. EPR is employed in its conventional continuous wave (CW) and pulsed—electron spin echo (ESE)—versions. CW EPR spectra are sensitive to dynamical librations of molecules while ESE probes stochastic molecular librations. In this review, different manifestations of small-angle motions in EPR of spin probes and labels are discussed. It is shown that CW-EPR-detected dynamical librations provide information on dynamical transition in these media, similar to that explored with neutron scattering, and ESE-detected stochastic librations allow elucidating some features of nanoscale molecular packing. The possible EPR applications are analyzed for gel-phase lipid bilayers, for biological membranes interacting with proteins, peptides and cryoprotectants, for supercooled ionic liquids (ILs) and supercooled deep eutectic solvents (DESs), for globular proteins and intrinsically disordered proteins (IDPs), and for some other molecular solids.

Structural properties of supercooled deep eutectic solvents: choline chloride–thiourea compared to reline

Deep eutectic solvents (DESs) are eutectic mixtures of hydrogen bond acceptors and hydrogen bond donors which melt at much lower temperatures than the individual components. DESs attract growing interest because...

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

Intrinsically disordered proteins (IDPs) are proteins that possess large unstructured regions. Their importance is increasingly recognized in biology but their characterization remains a challenging task. We employed field swept Electron Spin Echoes in pulsed EPR to investigate low-temperature stochastic molecular librations in a spin-labeled IDP, casein (the main protein of milk). For comparison, a spin-labeled globular protein, hen egg white lysozyme, is also investigated. For casein these motions were found to start at 100 K while for lysozyme only above 130 K, which was ascribed to a denser and more ordered molecular packing in lysozyme. However, above 120 K, the motions in casein were found to depend on temperature much slower than those in lysozyme. This abrupt change in casein was assigned to an ordering transition in which peptide residues rearrange making the molecular packing more rigid and/or more cohesive. The found features of molecular motions in these two proteins turned out to be very similar to those known for gel-phase lipid bilayers composed of conformationally ordered and conformationally disordered lipids. This analogy with a simpler molecular system may appear helpful for elucidation properties of molecular packing in IDPs.

Electron spin echo detection of stochastic molecular librations: Non-cooperative motions on solid surface

In frozen biological media and molecular glasses only restricted motions exist; because of the weakness and disorder of intermolecular bonds these motions may have stochastic nature. Electron spin echo (ESE) spectroscopy of spin-labeled molecules allows detecting their restricted stochastic rotations (stochastic molecular librations). As in molecular disordered media motions may be highly cooperative, it would be desirable to investigate their spectroscopic manifestation also in the systems where cooperative effects would be certainly ruled out. In this work, ESE of spin-labeled molecules adsorbed on inorganic SiO₂ surface was investigated in a wide temperature range. The rate of motion-induced spin relaxation was found to become measurable above 130 K, increasing with temperature and attaining then a saturating behavior with a well-defined maximum near 250 K. For two types of molecules differing remarkably in their size and polarity (a small highly-polar nitroxide radical and a large spin-labeled peptide), quite similar results were obtained. This saturating behavior was quantitatively reproduced in simulations within a simple model of jump between two close orientations. Comparison with experiment allowed estimate that at 250 K the correlation time of the motion τ_c is of the order of several tens of nanoseconds and the angle α between two orientations is around 0.02 rad. As the found saturating behavior is a property of individual motions, for any other molecular system an excess of the spin relaxation rate above the maximum found here for adsorbed molecules may be ascribed to cooperative motions. Comparison with literature data on molecular systems of different origin has shown that effects of cooperativity indeed are present and, moreover, may be very essential.

Lipid chain mobility and packing in DOPC bilayers at cryogenic temperatures

Low-temperature molecular mobility and packing in biological tissues are important for their survival upon cryopreservation. Electron paramagnetic resonance (EPR) in its pulsed version of electron spin echo (ESE) allows studying stochastic librations of spin-labeled molecules, the type of motion which dominates at low temperatures. These librations are characterized by the parameter <α²>τ_c where <α²> is the mean squared angular amplitude and τ_c is the correlation time for the motion. This parameter is known to be larger for higher temperature and for looser intermolecular structure. In this work, ESE data for the bilayers comprised of doubly-unsaturated DOPC (dioleoyl-glycero-phosphocholine) lipids and mono-unsaturated POPC (palmitoyl-oleoyl-glycero-phosphocholine) lipids with spin-labeled stearic acids added were obtained in the temperature range between 80 and 210 K; the results were compared also with the previously obtained data for fully-saturated DPPC (dipalmitoyl-glycero-phosphocholine) lipid bilayers [J. Phys. Chem. B2014, 118, 12,478-12,485; Appl. Magn. Reson. 2018, 49, 1369-1383]. It turned out that for DOPC bilayers the <α²>τ_c values are of intermediate magnitude between those for POPC and DPPC bilayers, which implies an intermediate density of lipid packing. A possible explanation of this result could be rearrangement at cryogenic temperatures of the DOPC lipid tails, with their terminal segments folding cooperatively. This interpretation is also in agreement with the known thermodynamic properties of gel-fluid transition for DOPC bilayer.

Unsaturated lipid bilayers at cryogenic temperature: librational dynamics of chain-labeled lipids from pulsed and CW-EPR

Fully hydrated bilayers of monounsaturated palmitoyloleoylphosphatidylcholine (POPC) and diunsaturated dioleoylphosphatidylcholine (DOPC) lipids have low main phase transition temperatures (271 K for POPC and 253 K for DOPC). Two-pulse echo detected spectra, combined with continuous wave electron paramagnetic resonance spectroscopy, are employed to study the low-temperature lamellar phases of the POPC and DOPC unsaturated bilayers that are usually studied in the fluid state. Phosphatidylcholine spin-labeled at C-5 and C-16 carbon atom positions along the acyl chain were used and the temperature varied over the range 77-270 K. Segmental chain librational oscillations of small amplitude and with correlation time in the subnanosecond to nanosecond range are found in both membranes. The mean-square angular amplitude, α2, of librations increases with temperature, is larger close to the bilayer midplane than close to the first acyl chain segments, and is larger in diunsaturated than in monounsaturated bilayers. In the inner hydrocarbon region of both lipid matrices, α2 increases first slowly and linearly with temperature and then more rapidly, and a dynamical transition is detected in the range 190-210 K. Compared to dipalmitoylphosphatidylcholine bilayers of fully saturated symmetric chain lipids, the presence of double bonds in the acyl chain enhances the intensity of librational motion which is characterized by larger angular variations at the terminal methyl ends. These findings highlight biophysical properties of unsaturated bilayers in the frozen state, including a detailed characterization of segmental chain dynamics and the evidence of a dynamical transition that appears to be a generic feature in hydrated macromolecular systems. These results can also be relevant in regulating membrane physical properties and function at higher physiological temperatures.

Dynamical transition in molecular glasses and proteins observed by spin relaxation of nitroxide spin probes and labels

In glassy substances and biological media, dynamical transitions are observed in neutron scattering that manifests itself as deviations of the translational mean-squared displacement, 〈x²〉, of hydrogen atoms from harmonic dynamics. In biological media, the deviation occurs at two temperature intervals, at ∼100-150 K and at ∼170-230 K, and it is attributed to the motion of methyl groups in the former case and to the transition from harmonic to anharmonic or diffusive motions in the latter case. In this work, electron spin echo (ESE) spectroscopy-a pulsed version of electron paramagnetic resonance-is applied to study the spin relaxation of nitroxide spin probes and labels introduced in molecular glass former o-terphenyl and in protein lysozyme. The anisotropic contribution to the rate of the two-pulse ESE decay, ΔW, is induced by spin relaxation appearing because of restricted orientational stochastic molecular motion; it is proportional to 〈α²〉τ_c, where 〈α²〉 is the mean-squared angle of reorientation of the nitroxide molecule around the equilibrium position and τ_c is the correlation time of reorientation. The ESE time window allows us to study motions with τ_c < 10^-7 s. For glassy o-terphenyl, the 〈α²〉τ_c temperature dependence shows a transition near 240 K, which is in agreement with the literature data on 〈x²〉. For spin probes of essentially different size, the obtained data were found to be close, which evidences that motion is cooperative, involving a nanocluster of several neighboring molecules. For the dry lysozyme, the 〈α²〉τ_c values below 260 K were found to linearly depend on the temperature in the same way as it was observed in neutron scattering for 〈x²〉. As spin relaxation is influenced only by stochastic motion, the harmonic motions seen in ESE must be overdamped. In the hydrated lysozyme, ESE data show transitions near 130 K for all nitroxides, near 160 K for the probe located in the hydration layer, and near 180 K for the label in the protein interior. For this system, the two latter transitions are not observed in neutron scattering. The ESE-detected transitions are suggested to be related with water dynamics in the nearest hydration shell: with water glass transition near 130 K and with the onset of overall water molecular reorientations near 180 K; the disagreement with neutron scattering is ascribed to the larger time window for ESE-detected motions.

Low-Temperature Dynamics of Chain-Labeled Lipids in Ester- and Ether-Linked Phosphatidylcholine Membranes

Continuous wave electron paramagnetic resonance spectroscopy and two-pulse echo detected spectra of chain-labeled lipids are used to study the dynamics of frozen lipid membranes over the temperature range 77-260 K. Bilayers of ester-linked dihexadecanoylphosphatidylcholine (DPPC) with noninterdigitated chains and ether-linked dihexadecyl phosphatidylcholine (DHPC) with interdigitated chains are considered. Rapid stochastic librations of small angular amplitude are found in both lipid matrices. In noninterdigitated DPPC bilayers, the mean-square angular amplitude, [Formula: see text], of the motion increases with temperature and it is larger close to the chain termini than close to the polar/apolar interface. In contrast, in interdigitated DHPC lamellae, [Formula: see text] is small and temperature and label-position independent at low temperature and increases steeply at high temperature. The rotational correlation time, τ_c, of librations lies in the subnanosecond range for DPPC and in the nanosecond range for DHPC. In all membrane samples, the temperature dependence of [Formula: see text] resembles that of the mean-square atomic displacement revealed by neutron scattering and a dynamical transition is detected in the range 210-240 K. The results highlight the librational oscillations and the glass-like behavior in bilayer and interdigitated lipid membranes.