Dynamic properties of the haptenic site of lipid haptens in phosphatidylcholine membranes. Their relation to the phase transition of the host lattice

Abstract

The relation between the dynamic properties of the haptenic site of lipid haptens and the phase transition of the host lattice was investigated using head group spin-labeled phosphatidylethanolamines, that is, spin-label lipid haptens (Brûlet, P., and H. M. McConnell, 1976, Proc. Natl. Acad. Sci. USA., 73:2977-2981; Brûlet, P., and H. M. McConnell, 1977, Biochemistry, 16:1209-1217). The electron spin resonance (ESR) spectra of the lipid haptens in liposomal membranes showed three narrow resonance lines, whose widths and hyperfine splitting values suggested that the haptenic site, i.e., the spin-label moiety, should be exposed in the water phase. The line width of each peak depended on the host lipid species and on the incubation temperature. A temperature study using dipalmitoylphosphatidylcholine (DPPC) liposomes showed that the dynamic properties of the haptenic site were related to the main phase transition and the subphase transition of the host lattice but not to the prephase transition. The angular amplitudes of the tumbling motion of the haptenic site were estimated using oriented multibilayer systems. The angular amplitude of dipalmitoyl-phosphatidyl-N-[[N-(1-oxyl-2,2,6, 6-tetramethyl-4-piperidinyl)-carbamoyl]-methyl]-ethanolamine in DPPC membranes was 63 degrees at 2 degrees C, and it increased slightly with an increase in temperature regardless of the phase transition of the host lattice. The value for egg phosphatidylcholine (PC) at 25 degrees C was the same as for DPPC above its main phase transition temperature. Rotational correlation time analysis showed that the axial rotation of the haptenic site was preferable to the tumbling motion of the rotational axis, and the predominance depended on the phase transition, Lc----L beta' and P beta'----L alpha. Elongation of the spacer arm between the haptenic site and phosphate increased the angular amplitude of the tumbling motion but reduced the effect of the host lattice. Spin-label lipid haptens with unsaturated fatty acyl chains were distributed heterogeneously in DPPC membranes, whereas those with the same fatty acyl chain as the host lattice were distributed randomly. The ESR spectrum of a lipid hapten under its prephase transition temperature showed two components, broad and narrow. This suggests that at least two different domains, a hapten-rich domain and a hapten-poor one, may coexist in membranes. ESR measurements at various temperatures suggested that the haptenic site fraction in the hapten-rich domain decreased in part during the phase transition from L beta' to P beta', and disappeared completely in the La phase. The spatial mobility and lateral diffusion of lipid haptens will be discussed in greater detail.

Cholesterol Modulated Antibody Binding in Supported Lipid Membranes as Determined by Total Internal Reflectance Microscopy on a Microfabricated High-throughput Glass Chip

“…With this goal in mind, we used the lipid biochip to investigate binding of a labeled antibody to a POPC/cholesterol bilayer membrane whose surface structures can be modified systematically by its cholesterol content. The interplay between the hapten, cholesterol, and lipid is sufficiently complex [35][36][37][38][39][40][41] that we cannot yet describe the binding behavior quantitatively in great detail. Nevertheless, the observed specific and nonspecific binding behavior can be used to determine a number of qualitative features of the cholesterol-containing membrane.…”

“…35 The angular orientation of the hapten group increases in the liquid-crystalline phase, since it is closer to the membrane normal for the more ordered, gel phase. 37 Cholesterol increases the packing of the acyl chain region and may push the hapten out of the membrane. Thus, the effect of low cholesterol concentration may be to increase hapten surface concentration and make its orientation more favorable for binding.…”

“…The hapten, DNP, is a small, lipophilic molecule that may partition to the interior hydrocarbon region of the lipid bilayer resulting in a marked decrease in antibody binding . The angular orientation of the hapten group increases in the liquid-crystalline phase, since it is closer to the membrane normal for the more ordered, gel phase . Cholesterol increases the packing of the acyl chain region and may push the hapten out of the membrane.…”

“…Thus, the effect of low cholesterol concentration may be to increase hapten surface concentration and make its orientation more favorable for binding. Takeshita et al showed that haptenated lipids with acyl chains different from those of the host lipid are distributed heterogeneously within the bilayer, forming aggregated domains. An antibody spans a membrane area of 60 nm 2 during binding to the antigen, and the average area of the lipid molecule is 0.6 nm 2 .…”

Effect of Cholesterol on Distribution of Stable, Hydrophobic Perchlorotriphenylmethyl Triethylester Radical Incorporated in Lecithin Liposomal Membranes

“…21) Liposomal suspension (14 ml) was taken into a 100 ml disposable micropipette (Drummond Scientific Co., Broomall, PA, U.S.A.), placed in an ESR sample tube, and set in an ESR cavity. Planar multilamellar membranes were placed in an ESR sample tube and set in an ESR cavity so that the orientation of the membrane plane was parallel or perpendicular to the magnetic field.…”