The influence of cholesterol (CHOL) on the main phase transition in single shell dipalmytoylphosphatidylcholine (DPPC) vesicles was investigated in equilibrium and kinetic experiments. CHOL increases the optical density and causes a slight hysteresis in turbidity transition curves. Static fluorescence anisotropy measurements showed interesting differences for three probes sensing different parts in the hydrophobic region of the phospholipid bilayer. Differential scanning calorimetry (DSC) peaks can be separated into a narrow and a broad component. The narrow component, which decreases linearly with increasing CHOL content and disappears at 20 mol %, is attributed to the transition of free phospholipid, while the broad component, being associated with the transition of CHOL-lipid units, increases monotoniously from 0 to 20%. Kinetic experiments were performed on our iodine-laser T-jump arrangement with turbidity detection. Three cooperative relaxation signals in the microsecond and millisecond time range were detected for pure DPPC vesicles as well as vesicles containing 7.5 and 16.5 mol % CHOL. All three relaxation processes were changed by CHOL: the superposition of the three relaxation amplitudes can be separated into a narrow and a broad component, as in DSC experiments. A speculative model is presented which assumes an inhomogeneous CHOL distribution fluctuating on a millisecond time scale in the temperature region of the main phase transition.

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The kinetics of the main phase transition in dipalmytoylphosphatidylcholine (DPPC) vesicles have been investigated using our iodine laser-T-jump technique with fluorescence detection. A set of three fluorescent probes has been used to sense different parts of the bilayer hydrocarbon chain region. The well established membrane probes DPH and TMADPH as well as DPHPC, a labelled DPPC molecule. We report three relaxation signals in the microseconds and ms time range, which are detected with all three probes. This result supports our model of the main phase transition in DPPC vesicles.

Abstract:The iodine-laser temperature-jump technique has been used to investigate the main phase transition in single shell vesicles of dimyristoylphosphatidylcholine, The probe molecules DPH and TMA-DPH were incorporated into the lipid bilayer and laser T-jump experiments with turbidity and flourescence polarization detection were performed. We found three well separated relaxation processes between 5 ~s and 10 ms. The relaxation signals showed strong cooperativity in the relaxation times as well as in their corresponding amplituedes. We attributed the relaxation to the formation and dissolution of clusters of different order inside the bilayer.

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