On the Low-Temperature Onset of Molecular Flexibility in Lipid Bilayers Seen by Raman Scattering

Abstract

For dipalmitoylphosphatidylcholine (DPPC) lipid/water bilayers, a detailed temperature dependence of the Raman scattering spectra at the spectral range of the CH 2-stretching modes was investigated. Below 150 K the ratio of intensities of the 2880 cm (-1) antisymmetric vibration line and the 2850 cm (-1) symmetric one was found to be nearly temperature-independent. Between 150 and 230 K it decreases slightly as temperature increases; and above 230 K it decreases remarkably. This decrease is accompanied with broadening of the antisymmetric line, from 4.2 cm (-1) at 100 K to 5.7 cm (-1) at 296 K. According to literature, the decrease of the antisymmetric line may be interpreted in two ways: (i) the appearance of a static conformational disorder (or of a disorder fluctuating at the time scale larger than picoseconds) and (ii) relaxation at the ps time scale, which is induced by coupling with temperature-activated librational-torsional motion of the lipid chain. Both these interpretations imply that obtained data evidence the appearance of molecular flexibility of lipids around approximately 200 K. The observed effect is to be compared with low-temperature dynamical transition found in disordered media with neutron scattering, Mossbauer absorption, molecular dynamics simulations and other techniques. This transition implies that with temperature increase harmonic atomic motions are transformed to large-amplitude anharmonic (or stochastic) ones. The characteristic times of these motions lay at the ps time scale. The closeness of the temperature of the transition and of the time scale of motions with those found in this work by Raman scattering for lipid bilayers supports the dynamic nature of the 2880 cm (-1) antisymmetric vibration line decrease (i.e., that it is induced by coupling with libration-torsion). To prove that the observed onset of flexibility is a property of a disordered state, Langmuir-Blodgett films of behenic acid were studied. These films contain, like lipids, long CH 2-tails, but, in opposite to bilayers, they have a well-ordered crystalline-like structure. The relative intensity of the antisymmetric/symmetric CH 2-stretching lines was found in these films to be temperature-independent in the whole temperature range studied, between 60 and 296 K.

Low-Temperature Dynamical and Structural Properties of Saturated and Monounsaturated Phospholipid Bilayers Revealed by Raman and Spin-Label EPR Spectroscopy

“…The data for the DPPC samples are consistent with results from previous studies. 11,12 The discontinuous break of the temperature dependencies near 314 K for DPPC indicates the gel−fluid transition (see, e.g., ref 13). In Figure 1a, it is observed that the I aCH /I sCH -1 ratio is temperature independent below ∼150 K and sharply decreases as temperature increases above ∼230 K. The behavior of the I aCH /I sCH -1 ratio correlates with the dynamical transition 11 and can be interpreted as a change in interchain interactions with temperature.…”

“…11,12 The discontinuous break of the temperature dependencies near 314 K for DPPC indicates the gel−fluid transition (see, e.g., ref 13). In Figure 1a, it is observed that the I aCH /I sCH -1 ratio is temperature independent below ∼150 K and sharply decreases as temperature increases above ∼230 K. The behavior of the I aCH /I sCH -1 ratio correlates with the dynamical transition 11 and can be interpreted as a change in interchain interactions with temperature. For the Int CC /Int CN ratio of the DPPC data (see Figure 1b), temperature dependence becomes noticeable only above ∼150 K, and a sharp decrease in temperature dependence is seen as the temperature is increased above ∼230 K. This is the manifestation of the change in conformational states of the nonpolar chains and can be described as a decrease in the alltrans states upon an increase in temperature.…”

“…11,12 In studies using phospholipids with saturated acyl chains (1,2-dipalmitoyl-snglycero-3-phosphocholine (DPPC)), it was shown that Raman bands corresponding to C−H and C−C vibrations of nonpolar chains are sensitive to the dynamical transition. It is known that the intensity of the 2880 cm −1 antisymmetric CH 2 -stretching vibration line is sensitive to intermolecular packing interactions.…”

Interaction of the cationic peptide bactenecin with mixed phospholipid monolayers at the air–water interface

“…There also exists a certain decrease (from 1.46 to 1.35) in R and a decrease in S (from 0.81 to 0.90) when the peptide is included in the monolayer probably as a result of strong electrostatic interactions between the peptide and the anionic lipid [79]. This indicates an increase in the chain rotational disorder which may involve a librational-tor- sional motion of the lipidic chains to accommodate the peptide which results in a sort of distorted structures (and subsequent increase of gauche conformations) [80] and, thus, in a ''more fluid'' monolayer, in agreement with the lower values of the compressional modulus values previously obtained.…”

Lipid droplet phase transition in freezing cat embryos and oocytes probed by Raman spectroscopy

“…Moreover, resonance Raman spectroscopy revealed changes of cytochrome redox state in freezing cells (45). The capability of Raman spectroscopy to investigate lipid phase state is proven by studies of frozen cells (2,46) and model lipid systems (47,48).…”