Liquid ordered and gel phases of lipid bilayers: fluorescent probes reveal close fluidity but different hydration

¹ UMR 7175 du CNRS, Institut Gilbert Laustriat, Faculte de Pharmacie, Universite Louis Pasteur

^* To whom correspondence should be addressed. E-mail: aklymchenko{at}pharma.u-strasbg.fr.

Submitted on December 10, 2007
Revised on January 21, 2008
Accepted on 28 March 2008

	Abstract

Hydration and fluidity of lipid bilayers in different phase states were studied using fluorescent probes selectively located at the interface. The probe of hydration was a recently developed 3-hydroxyflavone derivative, which is highly sensitive to the environment, while the probe of fluidity was the diphenylhexatriene derivative, TMA-DPH. By variation of the cholesterol content and temperature in large unilamellar vesicles composed of sphingomyelin or DPPC, we generated different phases: gel, liquid ordered (raft), liquid crystalline and liquid disordered (considered as liquid crystalline phase with cholesterol). For these four phases, the hydration increases in the following order: liquid ordered << gel liquid disordered < liquid crystalline. The membrane fluidity shows a somewhat different trend, namely liquid ordered gel < liquid disordered < liquid crystalline. Thus, gel and liquid ordered phases exhibit similar fluidity, while the last phase is significantly less hydrated. We expect that cholesterol due to its specific H-bonding interactions with lipids and its ability to fill the voids in lipid bilayers expels efficiently water molecules from the highly ordered gel phase in order to form the liquid ordered phase. In the present report, the liquid ordered (raft) and gel phases are for the first time clearly distinguished by their strong difference in hydration.

Key Words: fluorescent probes, gel phase, hydration, lipid membranes, liquid-ordered phase, raft domains