Differential scanning calorimetry of thermotropic phase transitions in vitaminylated lipids: aqueous dispersions of N-biotinyl phosphatidylethanolamines

Abstract

The thermotropic phase behavior of a homologous series of saturated diacyl phosphatidylethanolamines in which the headgroup is N-derivatized with biotin has been investigated by differential scanning calorimetry. In 1 M NaCl, derivatives with acyl chainlengths from C(12:0) to C(20:0) all exhibit sharp chain-melting phase transitions, which are reversible with a hysteresis of 1.5 degrees or less, except for the C(12:0) lipid which has a transition temperature below 0 degree C. The transition enthalpy and the transition entropy depend approximately linearly on the lipid chainlength, with incremental values per CH2 group that are very similar to those obtained for the corresponding underivatized phosphatidylethanolamines in aqueous dispersion. The chainlength-independent contribution to the transition enthalpy is significantly smaller than that for the underivatized phosphatidylethanolamines, and that for the transition entropy is much smaller; the latter suggesting that the N-biotinylated phosphatidylethanolamine headgroups are differently hydrated from those of the underivatized lipids. The gel-to-fluid phase transition temperatures of the N-biotinylated lipids are lower than those of the parent phosphatidylethanolamines, and their chainlength dependence conforms well with that predicted by assuming that the transition enthalpy and entropy are linearly dependent on chainlength. Although the chain-melting phase behavior is generally similar to that of the parent phosphatidylethanolamines, the gel phases (and the fluid phases in the case of chainlengths C(12:0) to C(16:0)) have a different lyotropic structure in the two cases, and this is reflected in the chainlength-independent contributions to the thermodynamic parameters. In the absence of salt, the thermotropic phase behavior of aqueous dispersions of the N-biotinyl phosphatidylethanolamines is considerably more complex. The transition temperatures are consistently lower than those in 1 M NaCI, but the transitions are broader, contain multiple peaks and exhibit a much larger hysteresis between heating and cooling scans. Additionally, the lipids with shorterchainlengths exhibit metastability in the absence of salt, converting from a micellar solution to a lamellar gel phase only after incubation at low temperature with freeze-thaw cycling.

Spin-Label Electron Spin Resonance Studies on the Dynamics of the Different Phases of N-Biotinylphosphatidylethanolamines

“…There is a large hysteresis between the heating and the cooling scans, with the transition occurring approximately 6-1 3 deg lower in temperature in the cooling scans. This hysteresis has been observed also by calorimetry (Swamy et al, 1994) and is a particular feature of the samples in the absence of salt, because the temperature dependences given in Figure 2 for samples in 1 M NaCl essentially are fully reversible. The transition (on heating) for each lipid in the absence of salt is found to occur at a lower temperature than that in the presence of salt (cf.…”

Structure and phase behavior of lipid suspensions containing phospholipids with covalently attached poly(ethylene glycol)

“…In terms of the latter two points, previous experiments have shown that either increasing the surface charge on PE bilayers by changing the pH or increasing the size of the PE head group by methylation decreases the phase transition temperature (Cevc, 1987). Moreover, biotinylated DSPE, which like PEG-lipids has a single negative charge and a more bulky head group, has a phase transition 20-30°C lower than that of DSPE, depending on the NaCl concentration in the buffer (Swamy et al, 1994).…”

Self-assembly, supramolecular organization, and phase transitions of a homologous series of N-acyl-l-alanines (n=8–20)

“…Since NaCl at 1.0 M concentration effectively screens the electrostatic repulsion [26][27][28], which leads to sharpening of the thermograms, we included 1 M NaCl in the buffer. When NAAs hydrated with PBS containing 1 M NaCl were subjected to differential scanning calorimetry, the transition temperatures were found to be lower as compared to those obtained for NAAs hydrated with water.…”