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

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 molecule bound to some structure denser than the rest of the medium. Based on this analogy, the data obtained were interpreted as evidence that ibuprofen-SL molecules are adsorbed on the raft boundaries.

Characteristics of Phospholipid-Immunosuppressant-Antioxidant Mixed Langmuir-Blodgett Films

Hemocompatibility is one of the major criteria for the successful cardiovascular applicability of novel biomaterials. In this context, monolayers of certain biomolecules can be used to improve surface biocompatibility. To this end, biocoatings incorporating a phospholipid (1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC), an immunosuppressant (cyclosporine A, CsA), and an antioxidant material (lauryl gallate, LG) were fabricated by depositing Langmuir films onto gold or mica substrates using the Langmuir-Blodgett (LB) technique. These LB monolayers were thoroughly characterized by means of quartz crystal microbalance (QCM), atomic force microscopy (AFM), cyclic voltammetry (CV), and contact angle (CA) measurements. The obtained results indicate that the properties of these LB films are modulated by the monolayer composition. The presence of LG in the three-component systems (DOPC-CsA-LG) increases the molecular packing and the surface coverage of the substrate, which affects the wettability of the biocoating. From the different compositions studied here, we conclude that DOPC-CsA-LG monolayers with a DOPC/CsA ratio of 1:1 and LG molar fractions of 0.50 and 0.75 exhibit improved surface biocompatible characteristics. These results open up new perspectives on our knowledge and better understanding of phenomena at the biomaterial/host interface.

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.

Low-temperature librations and dynamical transition in proteins at differing hydration levels

Hydration of water affects the dynamics and in turn the activity of biomacromolecules. We investigated the dependence of the librational oscillations and the dynamical transition on the hydrating conditions of two globular proteins with different structure and size, namely β-lactoglobulin (βLG) and human serum albumin (HSA), by spin-label electron paramagnetic resonance (EPR) in the temperature range of 120–270 K. The proteins were spin-labeled with 5-maleimide spin-label on free cysteins and prepared in the lyophilized state, at low (h = 0.12) and full (h = 2) hydration levels in buffer. The angular amplitudes of librations are small and almost temperature independent for both lyophilized proteins. Therefore, in these samples, the librational dynamics is restricted and the dynamical transition is absent. In the small and compact beta-structured βLG, the angular librational amplitudes increase with temperature and hydrating conditions, whereas hydration-independent librational oscillations whose amplitudes rise with temperature are recorded in the large and flexible alpha-structured HSA. Both βLG and HSA at low and fully hydration levels undergo the dynamical transition at about 230 K. The overall results indicate that protein librational dynamics is activated at the low hydration level h = 0.12 and highlight biophysical properties that are common to other biosamples at cryogenic temperatures.

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.