Phase Transitions and Structural Changes in DPPC Liposomes Induced by a 1-Carba-Alpha-Tocopherol Analogue.

Grażyna Neunert,Aneta Baj,Jolanta Tomaszewska-Gras,Stanislaw Witkowski,Krzysztof Polewski,Marlena Gauza-Włodarczyk

doi:10.3390/molecules26102851

Abstract

Steady-state emission spectroscopy of 1-anilino-8- naphthalene sulfonate (ANS) and 1,6-diphenyl-1,3,5-hexatriene (DPH), fluorescence anisotropy, and DSC methods were used to characterize the interactions of the newly synthesized 1-carba-alpha-tocopherol (CT) with a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) membrane. The DSC results showed significant perturbations in the DPPC structure for CT concentrations as low as 2 mol%. The main phase transition peak was broadened and shifted to lower temperatures in a concentration-dependent manner, and pretransition was abolished. Increasing CT concentrations induced the formation of new phases in the DPPC structure, leading to melting at lower temperatures and, finally, disruption of the ordered DPPC structure. Hydration and structural changes of the DPPC liposomes using ANS and DPH fluorescent probes, which are selectively located at different places in the bilayer, were studied. With the increased concentration of CT molecules in the DPPC liposomes, structural changes with the simultaneous formation of different phases of such mixture were observed. Temperature studies of such mixtures revealed a decrease in the temperature of the main phase transition and fluidization at decreasing temperatures related to increasing hydration in the bilayer. Contour plots obtained from concentration–temperature data with fluorescent probes allowed for identification of different phases, such as gel, ordered liquid, disordered liquid, and liquid crystalline phases. The CT molecule with a modified chromanol ring embedded in the bilayer led to H-bonding interactions, expelling water molecules from the interphase, thus introducing disorder and structural changes to the highly ordered gel phase.

Highlights

Applied fluorescent probes of CT in a DPPC membrane changed its thermodynamic and structural parameters, which bind at different positions in the membrane structure; they report the modifications in the lipid microenvironment, reflecting changes induced by the presence of CT
Each CT molecule embedded in the DPPC bilayer changed its hydration by expelling water molecules, creating a cluster with neighboring phospholipids
Increasing the CT concentration generated more such clusters, which eventually joined together with phospholipids, forming a new structure. This process of new structure generation was observed as the formation of composed traces of differential scanning calorimetry (DSC), lowering the intensity, enthalpy, cooperativity, and temperature of the main phase transitions

Summary

Introduction

Vitamin E (α-tocopherol) is regarded as one of the most effective natural lipophilic chain-breaking antioxidants, protecting the integrity of lipids and phospholipid bilayers of biomembranes [1]. It exhibits a broad spectrum of nonantioxidant biological functions [2,3]. The most widely accepted biological function of vitamin E is its antioxidant activity [4]. This functionality is fulfilled by stereoelectronic features of the chroman6-ol system which contributes to the stabilization of the tocopheroxyl radicals that are formed. In search of a new synthetic, more efficient tocopherol analogues and for better evaluation of the tocopheroxyl radical (TocO·)

Methods

Results

Discussion

Conclusion