Occurrence of Glass Transitions in Long-Chain Phosphatidylcholine Mesophases

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.

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.

Freezing of Aqueous Solutions and Chemical Stability of Amorphous Pharmaceuticals: Water Clusters Hypothesis

Molecular mobility has been traditionally invoked to explain physical and chemical stability of diverse pharmaceutical systems. Although the molecular mobility concept has been credited with creating a scientific basis for stabilization of amorphous pharmaceuticals and biopharmaceuticals, it has become increasingly clear that this approach represents only a partial description of the underlying fundamental principles. An additional mechanism is proposed herein to address 2 key questions: (1) the existence of unfrozen water (i.e., partial or complete freezing inhibition) in aqueous solutions at subzero temperatures and (2) the role of water in the chemical stability of amorphous pharmaceuticals. These apparently distant phenomena are linked via the concept of water clusters. In particular, freezing inhibition is associated with the confinement of water clusters in a solidified matrix of an amorphous solute, with nanoscaled water clusters being observed in aqueous glasses using wide-angle neutron scattering. The chemical instability is suggested to be directly related to the catalysis of proton transfer by water clusters, considering that proton transfer is the key elementary reaction in many chemical processes, including such common reactions as hydrolysis and deamidation.

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.

Crystalline mesophases: Structure, mobility, and pharmaceutical properties

Crystalline mesophases, which are commonly classified according to their translational, orientational, and conformational order as liquid crystals, plastic crystals, and conformationally disordered crystals, represent a common state of condensed matter. As an intermediate state between crystalline and amorphous materials, crystalline mesophases resemble amorphous materials in relation to their molecular mobility, with the glass transition being their common property, and at the same time possessing a certain degree of translational periodicity (with the exception of nematic phase), with corresponding narrow peaks in X-ray diffraction patterns. For example, plastic crystals, which can be formed both by near-spherical molecules and molecules of lower symmetry, such as planar or chain molecules, can have both extremely sharp X-ray diffraction lines and exhibit glass transition. Fundamentals of structural arrangements in mesophases are compared with several types of disorder in crystalline materials, as well as with short-range ordering in amorphous solids. Main features of the molecular mobility in crystalline mesophases are found to be generally similar to amorphous materials, although some important differences do exist, depending on a particular type of mobility modes involved in relaxation processes. In several case studies reviewed, chemical stability appears to follow the extent of disorder, with the stability of crystalline mesophase found to be intermediate between amorphous (least stable) and crystalline (most stable) materials. Finally, detection of crystalline mesophases during manufacturing of two different types of dosage forms is discussed.

Signal intensities in ¹H-¹³C CP and INEPT MAS NMR of liquid crystals

Spectral editing with CP and INEPT in (13)C MAS NMR enables identification of rigid and mobile molecular segments in concentrated assemblies of surfactants, lipids, and/or proteins. In order to get stricter definitions of the terms "rigid" and "mobile", as well as resolving some ambiguities in the interpretation of CP and INEPT data, we have developed a theoretical model for calculating the CP and INEPT intensities as a function of rotational correlation time τc and C-H bond order parameter SCH, taking the effects of MAS into account. According to the model, the range of τc can at typical experimental settings (5kHz MAS, 1ms ramped CP at 80-100kHz B1 fields) be divided into four regimes: fast (τc<1ns), fast-intermediate (τc≈0.1μs), intermediate (τc≈1μs), and slow (τc>0.1ms). In the fast regime, the CP and INEPT intensities are independent of τc, but strongly dependent on |SCH|, with a cross-over from dominating INEPT to dominating CP at |SCH|>0.1. In the intermediate regime, neither CP nor INEPT yield signal on account of fast T1ρ and T2 relaxation. In both the fast-intermediate and slow regimes, there is exclusively CP signal. The theoretical predictions are tested by experiments on the glass-forming surfactant n-octyl-β-d-maltoside, for which τc can be varied continuously in the nano- to millisecond range by changing the temperature and the hydration level. The atomistic details of the surfactant dynamics are investigated with MD simulations. Based on the theoretical model, we propose a procedure for calculating CP and INEPT intensities directly from MD simulation trajectories. While MD shows that there is a continuous gradient of τc from the surfactant polar headgroup towards the methyl group at the end of the hydrocarbon chain, analysis of the experimental CP and INEPT data indicates that this gradient gets steeper with decreasing temperature and hydration level, eventually spanning four orders of magnitude at completely dry conditions.

Interplay between hydration water and headgroup dynamics in lipid bilayers

In this study, the interplay between water and lipid dynamics has been investigated by broadband dielectric spectroscopy and modulated differential scanning calorimetry (MDSC). The multilamellar lipid bilayer system 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) has been studied over a broad temperature range at three different water contents: about 3, 6, and 9 water molecules per lipid molecule. The results from the dielectric relaxation measurements show that at temperatures <250 K the lipid headgroup rotation is described by a super-Arrhenius temperature dependence at the lowest hydration level and by the Arrhenius law at the highest hydration level. This difference in the temperature dependence of the lipid headgroup rotation can be explained by the increasing interaction between the headgroups with decreasing water content, which causes their rotational motion to be more cooperative in character. The main water relaxation shows an anomalous dependence on the water content in the supercooled and glassy regime. In contrast to the general behavior of interfacial water, the water dynamics is fastest in the driest sample and its temperature dependence is best described by a super-Arrhenius temperature dependence. The best explanation for this anomalous behavior is that the water relaxation becomes more determined by fast local lipid motions than by the intrinsic water dynamics at low water contents. In support for this interpretation is the finding that the relaxation time of the main water process is faster than that in most other host systems at temperatures below 180 K. Thus, the dielectric relaxation data show clearly the strong interplay between water and lipid dynamics; the water influences the lipid dynamics and vice versa. In the MDSC data, we observe a weak enthalpy relaxation at 203 K for the driest sample and at 179 K for the most hydrated sample, attributed to the freezing-in of the lipid headgroup rotation observed in the dielectric data, since this motion reaches a time scale of about 100 s at about the same temperatures.