**1. Introduction**

The biological significance of carbohydrate molecules and developments of their biomedical and industrial applications have stimulated intensive research toward gaining knowledge of the alteration of membrane physicochemical properties by the presence of sugars. The protection of higher plant cells from the consequences of prolonged dehydration with the help of oligo- and polysaccharides represents a prominent example of the stabilizing effect exerted by sugars on biological membranes [1–3]. Simple carbohydrates represent one of the major protecting excipients in preservation of cells during freeze-drying [4]. The cryoprotective efficiency of sugars is related to the molecular mechanisms of their interaction with lipid membranes. It has been established that even under conditions of tight lipid packing, polysaccharides are able to sustain the liquid-crystalline lamellar phase of phospholipids by penetrating between the lipid headgroups [5], which

**Citation:** Vitkova, V.; Yordanova, V.; Staneva, G.; Petkov, O.; Stoyanova-Ivanova, A.; Antonova, K.; Popkirov, G. Dielectric Properties of Phosphatidylcholine Membranes and the Effect of Sugars. *Membranes* **2021**, *11*, 847. https://doi.org/10.3390/ membranes11110847

Academic Editor: Monika Naumowicz

Received: 30 September 2021 Accepted: 27 October 2021 Published: 30 October 2021

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is coherent with their strong drought-protective role in cellular membranes [1]. Simple carbohydrates (mono- and disaccharides) are also found to exert a protective role against abnormal temperatures and dehydration in biomembranes and synthetic lipid bilayers [6]. Some disaccharides such as trehalose and sucrose are identified as efficient cryoprotectants counteracting the water deficiency via replacement of water molecules at the membrane surface [1,7,8]. Large decrease of the lateral phospholipid mobility by sucrose has been evidenced by fluorescence correlation spectroscopy measurements and molecular dynamic simulations [7]. The strong influence of simple carbohydrates and oligoand polysaccharides on the tension of lipid monolayers has also been reported [8]. Data acquired for mechanical properties of bilayer lipid stacks [9,10] and free-standing lipid membranes [11–13] in the presence of mono- and disaccharides outline a more complex picture on sugar–membrane interactions.

The detailed characterization of the electrical properties of biological membranes contributes to the understanding of the governing mechanisms involved in cellular processes such as transmission of electrical impulses or synaptic activity. Research in this direction is relevant to numerous electroporation-based applications in drug delivery, electrochemotherapy, cell–cell hybridization, food processing, etc. [14]. The lipid bimolecular matrix represents the main structural entity of cell membranes. Since it is impermeable to ions, the lipid bilayer behaves as a capacitor, strongly influencing the electric field distribution in the cell. The quantification of the lipid bilayer capacitance and the estimation of its dependence on external factors allows for evaluating the charging time of membranes and membrane–field interactions, including transmembrane potential difference [15]. The alteration of the bilayer electrical capacitance in sucrose-containing solutions reported recently [16] emerged various questions regarding the effect on membrane electrical properties of dissolving simple carbohydrates in the aqueous surroundings.

Considering the close relation of the specific capacitance to membrane composition, its structure and physical state [17], we probe here the susceptibility of this important electrical parameter of lipid bilayers to the presence of sugar molecules in the aqueous surroundings. The effect of glucose, fructose and sucrose on the degree of hydration and rotational order parameter of lipid molecules is measured with regard to expected alterations of membrane dielectric permittivity [16,18–20]. The specific capacitance of free-standing membranes is measured by frequency-dependent electrodeformation of giant unilamellar lipid vesicles (GUVs) and Fast-Fourier transform electrochemical impedance spectrometry (FFT-EIS) of planar bilayer lipid membranes (BLMs) for acquisition of the impedance spectrum (1 Hz–50 kHz) in a couple of seconds. The simplest physical model of biological membranes is the lipid bilayer [21]. Characterized by diameters in the same range as the typical cell sizes (5–100 µm), GUVs are considered as a basic physical model of biomembranes allowing for investigation of membrane-related phenomena at the scale of single vesicles [22,23]. The elaborated experimental protocols for preparation of GUVs offer a good control of membrane composition and the physicochemical parameters of the aqueous environment. Membrane dipole potential, the degree of hydration and rotational order parameter of lipid molecules are studied in sugar-containing aqueous solutions by fluorescence spectroscopy of large unilamellar vesicles (LUVs), which are characterized by orders of magnitude smaller diameters (~100 nm) compared to GUVs. In addition to the application for modeling biomembranes, lipid vesicles are also recognized for their usage as drug and gene carriers [24], which foregrounds the investigation of their membrane physical properties and stability.

Here, we study the alteration of dielectric properties, the degree of hydration, the rotational order parameter and dipole potential of lipid bilayers in the presence of simple carbohydrates. For the first time, the effect of glucose and fructose is probed on the impedance characteristics of biomimetic lipid membranes. The reported results provide knowledge of sugar–membrane interactions in relation to the bilayer molecular organization, electrical capacitance, resistance and dielectric permittivity. The acquired new data about the alteration of the dielectric properties and molecular organization of lipid

bilayers in sugar solutions are expected to facilitate the selection of the best carbohydrate to meet requirements of applications related to the response of lipid bilayers to external electric fields.
