Eﬀect of Cholesterol on the Phase Behavior of Solid-Supported Lipid Vesicle Layers P. Losada-Pe ́ rez,* ,†,‡ M. Khorshid, †,§ D. Yongabi, † and P. Wagner †,§ † Institute for Materials Research IMO, Hasselt University, Wetenschapspark 1, B-3590, Diepenbeek, Belgium ‡ Division IMOMEC, IMEC vzw, Wetenschapspark 1, B-3590, Diepenbeek, Belgium § Soft Matter and Biophysics Section, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D bus 2416, B-3001, Leuven, Belgium * S Supporting Information ABSTRACT: The interest in solid-supported biomimetic membranes stems from their utility in nanotechnology and biosensing. In particular, supported lipid vesicles (SLVs) have become popular in both fundamental biophysical studies and pharmaceutical screening applica- tions. It is thus essential to gain information on the structural properties and phase behavior of SLVs. Here we report on a study on the inﬂuence of cholesterol on the phase behavior of SLVs of saturated phospholipids by using quartz crystal microbalance with dissipation monitoring, a label- free and nonintrusive surface-sensitive technique. Two complementary approaches have been used, a Voigt-based viscoelastic model yielding shear viscosity temperature proﬁles and the ﬁrst-order derivative of the frequency (mass-sensitive) shifts. Anomalies in the shear viscosity and extrema in the ﬁrst-order derivative frequency curves stand as a token of the main phase transition and provide information on its gradual suppression upon addition of cholesterol. This method proves convenient for its small sample volume needed, its short temperature equilibration time and the non-necessity of external labels. This work can be regarded as a starting point for further studies on more rare lipid systems and diﬀerent geometries, such as tethered SLVs or biologically relevant vesicles produced by living cells. ■ INTRODUCTION Cholesterol is a lipid from the family of sterols whose presence is ubiquitous in eukaryotic cell plasma membranes. Its molecular characteristics give cholesterol unique properties that motivated intensive research in the last decades, especially on its role in structure and function of lipid bilayer membranes, both natural and model ones. 1-7 Cholesterol has the ability to control the lateral organization of membranes, providing mechanical strength and imparting low permeability barriers to lipid membranes by controlling ﬂuidity and thickness. 8-11 The presence of cholesterol is intimately related to the lipid raft hypothesis. 12-14 Lipid rafts correspond to membrane areas stabilized by cholesterol within a more ordered phase and may serve as platforms for cell signaling and membrane traﬃcking. As a matter of fact, their existence has motivated a plethora of studies to investigate the inﬂuence of lipid rafts on interactions of model membranes with biomolecules such as peptides and nucleic acids (see, for instance, refs 15-20). In this regard, the study of the behavior of static and dynamic thermodynamic properties of model membranes is fundamental for a better understanding of the phase behavior of cholesterol- containing lipid systems. The inﬂuence of cholesterol on lipid phases has been studied for free-standing lipid bilayers systems using several experimental techniques such as calorimetry, 21-24 nuclear magnetic resonance (NMR), 25-30 electron spin resonance, 31 X-ray diﬀraction, 32,33 ﬂuorescence microscopy, 34 laser ultrasonics, 35 and partial volume measurements 36 as well as by molecular simulations. 37-39 Although the eﬀect of cholesterol depends on the type of lipid, a standard dual behavior of cholesterol is observed above and below the melting temperature T m of the lipid bilayer: it promotes ordering of the alkyl chains above T m and disorder below T m . As a result, cholesterol induces the appearance of the liquid- ordered phase, which shares features of both the gel and the liquid-disordered phases. 40-42 Despite the vast number of studies on the phase behavior of free-standing lipid systems, the interest in the phase behavior of diﬀerent lipid geometries such as solid-supported lipid layers is emerging and motivated by their utility in nanotechnology and biosensing. 43 The most commonly used solid-supported systems are supported lipid bilayers (SLBs), supported layers of vesicles (SLVs) and supported lipid monolayers. The inﬂuence of cholesterol on solid-supported lipid layers has been examined for supported lipid bilayers (SLBs) and lipid Received: January 23, 2015 Revised: March 25, 2015 Published: March 26, 2015 Article pubs.acs.org/JPCB © 2015 American Chemical Society 4985 DOI: 10.1021/acs.jpcb.5b00712 J. Phys. Chem. B 2015, 119, 4985-4992