Cholesterol-Induced Variations in the Domain Structure Fluctuations and Microdynamics of Lipid Membranes

Abstract

Broadband ultrasonic attenuation spectra (100 kHz to 2 GHz) of aqueous solutions of vesicles from 1,2-dimyristoyl-l-3-phosphatidylcholine, with different amounts of cholesterol admixed, have been measured at temperatures between 20 and 28 °C. The spectra have been evaluated in terms of suitable relaxation functions. They are discussed in view of the effect of cholesterol on the membrane behavior around the gel-fluid phase transition temperature T(m). In addition to a frequency-independent asymptotic high-frequency term, all spectra reveal a critical term and a Debye-type relaxation term with relaxation time around 0.5 ns. The former is evaluated in the light of the Bhattacharjee-Ferrell dynamic scaling theory. It is assigned to the critical domain structure fluctuations of the membranes. Critical slowing of fluctuations is demonstrated. Also shown are relations of the critical amplitudes to thermodynamic parameters. The Debye term reflects the rotational isomerization of the phospholipid alkyl chains. The relaxation time of isomerization reveals a significant steplike change at T(m). At moderate cholesterol content an additional Debye relaxation term exists. It is assigned to the axial diffusion of the membrane molecules. Because it likewise shows effects of slowing near T(m), the diffusion appears to be coupled to the domain structure fluctuations. A further relaxation term at small cholesterol concentration is assumed to be due to small-range shape fluctuations of vesicles near the phase transition temperature.

Critical Fluctuations of Membranes: Spectroscopy Meets Thermodynamics

“…At that alamethicin content the peptide molecules cluster already to form pores, accompanied by membrane thinning [37,[59][60][61]. Similar results have been obtained for DMPC bilayers with cholesterol added [32] and for two-component phospholipid bilayers [31] in which, in addition to the conformationally distinct domains, compositionally distinct domains are formed [62]. Effects of bilayer composition are found in the relaxation rate Γ which will be discussed in the next section.…”

“…The phospholipids 1,2-dimyristoyl-3-sn-phosphocholine (DMPC), 1,2-dipalmitoyl-3-sn-phosphocholine (DPPC), and 1,2-dioleoyl-3-sn-phosphocholine (DOPC), as well as cholesterol and the peptide alamethicin were used to prepare suspensions of large unilamellar liposomes in doubly distilled (Millipore) water [29][30][31][32] by means of the extrusion method [34]. As verified by quasi-elastic light-scattering measurements, uniform liposomes with small size distribution were obtained [31].…”

“…Measurements of the ultrasonic attenuation coefficient α as a function of frequency ν have been performed between 100 kHz and 2 GHz using a resonator method at 100 kHz ≤ ν ≤ 15 MHz and a pulse-modulated wave transmission method at 7 MHz ≤ ν ≤ 2 GHz [29][30][31][32]. The experimental error in these measurements was on the order of α/α = 0.01.…”

“…Knowledge about the relaxation process with a relaxation time τ 0 (10 ns≤ τ 0 ≤ 170 ns [29,32]) is still incomplete. It has been tentatively assigned to short-scale deformations of the liposome shape in the transition region.…”

“…Ultrasonic spectra of lipid bilayer systems have indeed been measured for a long time [18][19][20][21][22][23][24][25][26][27][28] but the available frequency range was often too small to allow for a reliable subdivision in contributions from different relaxations. This paper summarizes results from recent measurements in sufficiently broad frequency ranges [29][30][31][32] to show that the bilayer domain structure fluctuations near T m can be well treated by analogy with concentration fluctuations of demixing binary liquids near their critical temperature [33]. In addition to single-parameter bilayers, those composed of different phospholipids or those with cholesterol or a peptide enclosed in a phospholipid matrix will be considered in view of effects in the domain structure and dynamics.…”