Fluorescence temperature jump relaxations of dansylphosphatidylethanolamine in aqueous dispersions of dipalmitoylphosphatidylcholine during the gel to liquid-crystal transition

Phospholipid phase transitions: kinetics and structural mechanisms

A brief review is given on the principles and methods used to investigate structural phase transitions in phospholipid supramolecular structures. The conceptual differences of approaches close to and far from equilibrium are addressed, and the consequences in terms of the limits of interpretation for different types of methods, in particular referring to jump-relaxation and steady-state techniques, are surveyed. With the emphasis on connecting dynamic and structural information, the results obtained so far from different techniques are reviewed, and the open questions addressed. The more recent advances by millisecond time-resolved X-ray diffraction with synchrotron radiation and their main results obtained for transitions triggered by IR-laser temperature jumps are summarized. As a major novel aspect in the field, the necessity of considering martensitic, diffusionless transformation mechanisms and the occurrence of intermediate structures is highlighted.

Kinetics of the premelting (L beta'-P beta') and main transition (P beta'-L alpha) in hydrated dipalmitoylphosphatidylcholine. A time-resolved x-ray diffraction study using microwave-induced temperature-jumps

The dynamics and mechanism of the premelting (L beta'-P beta') and main transitions (P beta'-L alpha) in fully hydrated dipalmitoylphosphatidylcholine were examined by low-angle time-resolved x-ray diffraction (TRXRD) using microwave radiation to effect uniform, internal sample heating. Equilibrium and dynamic aspects of the transitions were investigated. The dynamic studies involved applying a temperature jump of sufficient amplitude to effect the two transitions sequentially. Our findings are as follows. (a) Microwave radiation has proven useful as a means for implementing rapid and uniform internal heating in temperature-jump studies of lipid-phase transition kinetics. (b) Heating rate can be controlled by adjusting microwave power setting. (c) The thermal expansion coefficient of the three lyotropic phases follows the sequence L beta' not equal to O greater than P beta' much greater than L alpha. (d) Regardless of temperature-jump amplitude and sample heating rate the P beta' phase was always in evidence as an intermediate between the L beta' and L alpha phases. (e) The degree of development of the P beta' phase was inversely proportional to temperature-jump amplitude and heating rate. (f) The shortest transit time recorded for the combined L beta'-P beta', and P beta'-L alpha transitions was less than 1 s. (g) Upon cooling from the L alpha phase the onset of the chain disorder/order transition was apparent as a dramatic change of slope in the scattering angle vs. time plot which is interpreted as arising from sample heating by the latent heat of the transition. (h) Based on the shape of the low-angle diffraction pattern of the P beta' phase the P beta'-L alpha transition appears to be reversible with no evidence of metastability as was observed in the slow scan TRXRD measurements of Tenchov et al. (1989. Biophys. J. 56:757-768).

High-frequency ultrasonic absorption spectroscopy on aqueous suspensions of phospholipid bilayer vesicles

Between 1 MHz and 3 GHz the ultrasonic absorption coefficient has been precisely measured as a function of frequency for some aqueous suspensions of single-walled phospholipid bilayer vesicles. All solutions of the specially purified phospholipids clearly show excess absorption, reflecting three molecular relaxation processes with discrete relaxation times. Typical values for these times are 50, 3 and 0.5 ns. The attempt is made to relate these relaxation processes to mechanisms of rotational isomerization in the hydrocarbon chains. Some other molecular mechanisms which could also contribute to the ultrasonic excess absorption spectra are also briefly discussed.

Phospholipase C and the physical states of polar head groups of lipids

Activity of phospholipase C from Clostridium perfringens on liposomes made from sn-3-phosphatidylcholine, dimyristoyl (DMPC), dipalmitoyl (DPPC) or distearoyl (DSPC) was measured at various temperatures and was correlated with their gel/liquid-crystalline phase transitions (Tc:23, 41.5, 52 degrees C for DMPC, DPPC, DSPC, respectively). In all cases, the activity of phospholipase C was high in the gel phases of the substrates and was almost zero in their liquid-crystalline phases. Fluorescence depolarization measurements of N-dansyl-sn-3-phosphatidylethanolamine (DPE) and 1,6-diphenyl-1,3,5-hexatriene (DPH) incorporated into the liposomes showed that both the head group and the alkyl chains of the lipids were immobilized in the gel phases but were highly mobile in the liquid-crystalline phase. These results indicate that the rotational mobility of lipids (both of the head groups and the alkyl chains) was not a major factor in the phospholipase C reaction. It is inferred that some electrostatic and/or hydrophobic interactions might play important roles in regulation of the phospholipase C activity.

Local dielectric properties around polar region of lipid bilayer membranes

Local dielectric constant was evaluated from the Stokes shifts of fluorescence spectra of L-alpha-dansylphosphatidylethanolamine (DPE) incorporated into liposomes made of synthetic phosphatidylcholine (dipalmitoyl or distearoyl) or bovine brain phosphatidylserine. The evaluation was established as follows. First, the Stokes shift of DPE was assured to follow Mataga-Lippert's equation and was a function of the dielectric constant and the refractive index in some standard organic solvents. Second, the change of the refractive index did not contribute much to the change in the Stokes shift. Third, the time resolved fluorescence depolarization of DPE in liposomes showed that the cone wobbling diffusion was rapid relative to the fluorescence lifetime and therefore that the dielectric relaxation did not affect the evaluation of the constant in the polar region of membranes. We then investigated the characteristics of the local dielectric constant in the polar region of the lipid bilayer and found that the dielectric constant varies between 4 and 34 depending upon calcium binding and also gel/liquid-crystal phase transition. Such large changes of the local dielectric constant were further correlated with the dynamic structure of lipid bilayer membranes measured by conventional fluorescence depolarization techniques. The large changes of dielectric constant around the polar region suggest that electrostatic interactions at this region can be altered 10-fold by such ionic or thermotropic factors and therefore that local dielectric properties can play crucial roles in membrane functions.

On the phase transition kinetics of phospholipid bilayers. Relaxation experiments with detection of fluorescent anisotropy

Relaxation experiments were performed on vesicles of dimyristoylphosphatidylcholine in the lipid phase transition region by means of a Joule heating temperature jump technique. The time course of fluorescence anisotropy of the dye 1,6-diphenyl-1,3,5-hexatriene (DPH), incorporated in the bilayer, was observed. Since the dye always exhibits stationary anisotropy in the time range of observation, its anisotropy represents the order of the bilayer during the entire course of the experiment. Two relaxation processes were detected within the 1-100 ms range with maximal time constants at the midpoint of transition. At least one process was faster than the temperature jump dead time. The relaxation times, especially the maximal relaxation times, depend strongly on the bilayer curvature: larger vesicles imply larger time constants. This observation can explain differences between kinetic results of different laboratories. A comparison between the kinetic findings by means of three different dyes located at different sites along the lipid molecules suggests that the slower steps of the lipid phase transition mechanism involve the entire lipid molecule rather than individual parts of the molecule. The results of this present contribution reconfirm the recently published phase transition mechanism which postulates a series of steps of which the first, the kink formation, is fast and noncooperative and the following ones, representing the expansion of the aggregates, are slower and cooperative.

Characterization of a third phase transition in multilamellar dipalmitoyllecithin liposomes

The thermotropism of dipalmitoyllecithin in fully hydrated multilamellar dispersions has been reexamined by differential scanning calorimetry, X-ray diffraction, and 31P nuclear magnetic resonance. Apart from the well-known pretransition and main transition, there exists a third transition at about 11 degrees C with a transition enthalpy of approximately 3.7 kcal/mol. Both adjoining phases are lamellar, but they differ in the lateral acyl chain packing of the lecithin molecules and in the dynamics of the polar head groups. The kinetics of this third phase transition are extremely slow in comparison with those of the other two transitions.

The kinetics of the formation of rotational isomers in the hydrophobic tail region of phospholipid bilayers

Very fast structural changes in dipalmitoyl phosphatidylcholine molecules forming a vesicular bilayer were investigated by means of a laser temperature-jump technique. After temperature increases of about 1 K within 1 ns, the solution turbidity increases with a time constant of about 4 ns. This time constant exhibited no appreciable temperature dependence and represents a noncooperative process. It is interpreted as a local increase in density in the bilayer which results from a shortening of the individual lipid molecule due to formation of rotational isomers (e.g., kinks) without an appropriate expansion of the molecular environment. The final membrane expansion is achieved in consecutive steps with a decrease in turbidity and time constants between 100 mus and several seconds which are maximal in the midpoint of the phospholipid phase transition. These steps represent cooperative processes, namely the molecular interaction leading to the membrane expansion. The rate of kink formation implies that kinks migrate through the membrane by energetic transitions forwarded from lipid to lipid, rather than by hopping of individual lipids thereby carrying a kink.