Abstract

We have investigated the interactions between phospholipid monolayers and volatile anest-hatics. Two monolayers (dihexadecyl phosphate (DHP) and dipalmitoyl phosphatidyl choline (DPPC) and two anesthetics (halothane and enflurane) were used to observe these interac-tions using a highly sensitive quartz crystal microbalance (HS-QCM). The concentration of each anesthetic in aqueous solution was kept at 4 mM. The frequency of QCM showed no change when halothane was added to the DHP monolayer, however, it responded and de-creased when interaction occurred with DPPC monolayer. In case of enflurane addition the frequency decreased in both the monolayers of DHP and DPPC. The frequency change followed the following order of monolayer-anesthetic interactions: DHP-halothane

Keywords

Physisorption; Phospholipid Monolayers; Quartz Crystal Microbalance (QCM); Anesthetic Hydrate; Monolayer-Water Interface

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1. INTRODUCTION

Intermolecular interactions: hydrogen bonding, hydrophobic interaction, van der Waals interaction play an important role in various cellular functions such as the construction of tertiary structures of protein, the hybridization of DNA, the molecular recognition of membranes, and the transportation of nutrients and medicines [1-3]. The phenomenon of the interaction between a biomembrane and an anesthetic is also considered to be in the category of the above mentioned interactions. Anesthetics have structures containing moieties: hydroxyl, ether, chloroform, etc. apart from known halothane and enflurance used in this work. Anesthetic phenomenon occurs at high body concentrations in the order of millimol (mM); the effect of anesthetic is reversibility depending on the medication concentration [4,5]. The anesthetic potency is also temperature dependent [6-8]. Therefore the mechanism of anesthesia has been regarded as “physisorption phenomenon” in which an anesthetic aggregation acts indirectly at the interface of biomembrane-body fluid [9].

A quartz crystal microbalance (QCM) is a powerful method to investigate the above interfacial phenomenon occurring in liquid phases. QCM can detect the mass of substances adsorbed onto a quartz crystal oscillator (QCO) in a very minute amount up to the order of nanograms [10]. Many studies on chemisorption processes such as the oxidation and redox processes on a modified self-assembled monolayers [11,12], metal ion binding to langmuir monolayers [13,14], and molecular recognition of DNA strands and lipids [15-18] have been reported over the last several decades. While there are fewer reports on the investigation of physisorption processes because of the high QCM sensitivity that restricts the use of the method. Ebara et al. [19] have investigated the complementary guest pyridine compounds and acid binding process involving hydrogen bonding onto various cyanurate lipid monolayers formed at air/water interface using a 27 MHz QCM assembly and suggested that the microenvironment near the lipid surface in a living body is similar to hydrophobic organic medium. Sato et al. [20] have investigated the interaction between aminopurinethiol monolayers and oligonucleotides using a high sensitively improved 5 MHz QCM device that had stability range ±0.3 Hz. The authors claimed, based on the data and results obtained from highly sensitive QCM apparatus, that the hydrogen bonding interactions between the monolayers and complementary nucleic acid bases dissolved in a solution could be detected by their devices.

In this communication, we report physisorption interactions between two phospholipid monolayers DHP and DPPC and two anesthetics halothane and enflurane using a highly sensitively improved 6 MHz QCM device with a stability range ±0.5 Hz for > 12 h. A quartz crystal oscillator (QCO) was attached [21,22] horizontally to the monolayer formed at water surface. The artificially prepared phospholipid monolayer proved to be a good model that allowed to study a real living phenomenon including anesthesia. The investigation of a model physisorption in monolayer-anesthetic interactions using a QCM yielded important information for the elucidation of anesthesia mechanism.

2. MATERIALS, METHODS, AND APPARATUS

Dihexadecyl phosphate (DHP > 99%) and dipalmitoyl phosphatidyl choline (DPPC; 97%), which were used as the model membrane compounds were purchased from Sigma-Aldrich Corp. (St. Louis, USA) and Fluka Chemical Corp. Inc. (Seelze, Germany), respectively. Both the chemicals for membrane preparation were used without further purification. Halothane (2-bromo2-chloro-1,1,1-trifluoroethane > 99%) and enflurane ((RS)-2-chloro-1,1,2-trifluoroethyl difluoromethyl ether > 99%) which were used as volatile anesthetic were also purchased from Sigma-Aldrich Corp. (St. Louis, USA) and Abbott Japan Corp. Ltd. (Tokyo, Japan), respectively.

Figure 1 shows the molecular structures of monolayer forming compounds and anesthetics used in this work. Ultra pure water with conductance of <0.07 S/cm was obtained from a Super Water Purifying System (WL-21P; Yamato Scientific Corp. Ltd., Tokyo, Japan); the water was boiled for 10 min and subsequently cooled to room temperature [21,22]. The concentration of each anesthetic in aqueous solutions was 4 mM. A 1 mM solution of each DHP and DPPC in chloroform (99%, Wako Pure Chemical Industries Ltd., Osaka, Japan) were spread on

Conflicts of Interest

The authors declare no conflicts of interest.

References