Effect of Double Bond Position on Dehydroergosterol Fluorescence Intensity Dips in Phosphatidylcholine Bilayers with Saturatedsn-1 and Monoenoicsn-2 Acyl Chains

Abstract

We have investigated the spectroscopic evidence for sterol superlattice formation in bilayers composed of dehydroergosterol (DHE) and a homologous series of phosphatidylcholine, namely, C(18:0):C(18:1∆ n )PC with n ) 6, 7, 9, 11, and 13. These phosphatidylcholines (PCs) possess the same number of carbons and the same number of cis double bonds; however, the position of the double bond of these PCs varies. We have examined the fractional concentration dependence of the steady-state fluorescence intensity of DHE in the liquid-crystalline multilamellar vesicles composed of DHE and this series of PCs. The DHE contents in the membranes were varied by ∼0.2 mol % in two concentration regions, 18.7-21.3 and 23.9-26.0 mol % DHE. In these two regions, 20.0 and 25.0 mol % are the critical sterol mole fractions predicted for maximal superlattice formation. A DHE intensity dip appears at each of these two critical sterol mole fractions in the PC bilayers that have the double bond located between C9 and the terminal carbon. However, no distinct DHE intensity dips are observed in these two concentration regions as well as in the region 18.4-27.5 mol % DHE, when the double bond is located at C6-C7 or C7-C8. These data suggest that sterol superlattice can occur in liquid-crystalline state of unsaturated phospholipid bilayers so long as the cis double bonds are not located between C9 and the carboxyl carbon. It appears that appropriate van der Waals contacts between the steroid ring and the C2-C9 segment of the phospholipid acyl chain are required for sterol superlattice formation.

Time-Resolved Fluorescence and Fourier Transform Infrared Spectroscopic Investigations of Lateral Packing Defects and Superlattice Domains in Compositionally Uniform Cholesterol/Phosphatidylcholine Bilayers

“…A comparison of TRF data obtained for liposomes prepared by the dry film method revealed that the data scatter was signifi-cantly higher than when the LTT method was used (results not shown). Nevertheless, the critical X CHOL 's observed in the present study agree favorably with those observed previously using the dry film method Sugar, 2002, Wang et al, 2002), perhaps because an extensive temperature recycling-incubation protocol was employed in the latter case (Chong and Sugar, 2002).…”

“…All of the above critical cholesterol contents, except the one at 0.55, agree favorably (within 60.03) with the SL compositions of 0.20, 0.33, 0.40, and 0.50 predicted by the SL model (Virtanen et al, 1995;Somerharju et al, 1999;Chong and Sugar, 2002). In agreement with previous fluorescence studies on cholesterol SL using different probes (Tang et al, 1995;Chong et al, 1996;Chong and Sugar, 2002;Wang et al, 2002), these TRF results provide strong support for the proposition that cholesterol may adopt regular, superlattice-like distributions in fluid lipid bilayers. Since POPC, rather than a fully saturated PC was used here, the present data might be relevant for natural membranes as well.…”

Exploration of Molecular Interactions in Cholesterol Superlattices: Effect of Multibody Interactions

“…Our interpretation of the source of this interaction (i.e., acyl chain entropy contribution) is consistent with the experimental observation that superlattices do not form in a gel-phase bilayer, in which entropy change should be minimum. In addition, it is also consistent with the finding that the double bond position on acyl chains can affect cholesterol superlattice formation (Wang et al, 2002). Wang and Chong discovered that if a cis double bond is located between C9 carbon and the carboxyl carbon of acyl chains, superlattices become undetectable.…”

Lateral Distribution of Cholesterol in Dioleoylphosphatidylcholine Lipid Bilayers: Cholesterol-Phospholipid Interactions at High Cholesterol Limit

“…Our observation that the double bonds have little effect on x à chol value is not totally surprising. Recently, Wang et al (2002) measured the formation of cholesterol superlattices at low cholesterol concentration using fluorescence measurements. They found that cholesterol superlattice becomes undetectable when the cis double bond of PC is located at the same level of cholesterol steroid ring, i.e., between C8 and the carboxyl carbon.…”

“…Furthermore, in some mixtures of fluid-phase phospholipids and cholesterol, evidences suggest that cholesterol molecules can form regular distribution domains (e.g., hexagonal or centered rectangular pattern superlattices) to maximize the cholesterol-cholesterol sepa-ration Virtanen et. al., 1995;Liu et al, 1997;Wang et al, 2002;Cannon et al, 2003).…”