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Molecular Simulation Study on the Influence of Dimethylsulfoxide on the Structure of Phospholipid Bilayers
Biophysical Journal, Dec 2003 by Sum, Amadeu K, de Pablo, Juan J
ABSTRACT
Molecular dynamics simulations of dipalmitoylphosphatidylcholine (DPPC) lipid bilayer/water systems were performed in the presence of dimethylsulfoxide (DMSO) at 2, 5, 10, and 100 mol % DMSO (lipid-free basis). The equilibrium structure and several dynamic properties were determined for these systems. Results show that DMSO penetrates much deeper into the bilayer than water does. It is also found that DMSO molecules do not interact with the polar groups of the lipid headgroup, but exhibit a preference to remain either directly below the headgroup or in the aqueous phase, which is a consequence of the chemical characteristics of DMSO. As the temperature increases, a higher DMSO concentration is observed in the bilayer side of the interface. The area per headgroup in the presence of DMSO is significantly increased from 66.8 [Angstrom]^sup 2^ for the pure bilayer to as high as 87.0 [Angstrom]^sup 2^ at 10 mol % DMSO at 350 K. DMSO hydrogen-bonds strongly with water and exhibits unfavorable interactions with the polar headgroups of DPPC, thereby inducing a dehydration of the headgroups.
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INTRODUCTION
Dimethylsulfoxide (DMSO) is often used in biology as a cryoprotectant agent. It is used for preservation of enzymes and cells during freezing, and it is also used for perfusion of organs before transplant. Many of the protocols that are followed for cryopreservation of mammalian cells specify that the media used to freeze the cells should contain 10 wt % (2.5 mol %) DMSO (Freshney, 1987). DMSO exhibits significant pharmacological activity, including anti-inflammatory and analgesic effects. It also acts as a muscle relaxant agent, and it enhances cell differentiation and function (Jacob and Herschler, 1986). The widespread use of DMSO has led to numerous studies and hypotheses about its properties and interactions with biological organisms.
Several experimental studies have investigated the structure of model cell membranes (phospholipid bilayers) in the presence of DMSO (Chang and Dea, 2001; Gordeliy et al., 1998; Kiselev et al., 1999; Shashkov et al, 1999; Tristam-Nagle et al., 1998; Yamashita et al., 2000; Yu and Quinn, 1998, 2000). Most of these studies have used x-ray diffraction and differential scanning calorimetry to probe the structure and phase behavior of lipid bilayers over a wide range of DMSO concentrations. Gordeliy et al. (1998) performed x-ray diffraction measurements on lipid bilayers in the presence of DMSO over the entire concentration range (DMSO/water); they characterized the phase behavior and concluded that there is a strong interaction between DMSO and the bilayer surface. Tristam-Nagle et al. (1998) investigated the equilibrium phases and kinetics of a subgel phase in dipalmitoylphosphatidylcholine (DPPC) with DMSO, and attributed the changes in the phase behavior of the lipid bilayers to the dehydrating effect caused by DMSO. Yu and Quin (1998; 2000) performed x-ray diffraction on bilayers containing up to 50 wt % (18.7 mol %) DMSO and found that the thickness of the bilayer decreases and that the area per headgroup increases. Kiselev et al. (1999) used diffraction, scattering, and calorimetry to determine the influence of DMSO on the structure and phase transitions of DPPC bilayers over the entire DMSO concentration spectrum; their results showed that DMSO molecules do not penetrate the polar headgroup region or its vicinity. Shashkov et al. ( 1999) used infrared spectroscopy in addition to x-ray and calorimetry to probe the competition between DMSO and water for interactions with the bilayer surface, and found that the resulting dehydration of the lipid bilayer is caused by the strong interaction between DMSO and water. Chang and Dea (2001) used calorimetry to study the effect of DMSO on the phase transitions of lipid bilayers, and concluded that the presence of DMSO affects the solvation of the lipid bilayer. Yamashita et al. (2000) looked at the stability of bilayers at low DMSO concentrations (
More generally, it is known that DMSO can hydrogen-bond with water (Martin and Hauthal, 1971; Shashkov et al., 1999); it is also believed that DMSO has the ability to replace water and solvate lipid headgroups (Gordeliy et al., 1998; Kiselev et al., 1999; Tristam-Nagle et al., 1998). DMSO is also known to easily permeate membranes (Anchordoguy et al., 1992), a characteristic that may be favored by its hydrophobic groups (two methyl groups). The small size of DMSO and its dual character (hydrophobic and hydrophilic) facilitate interactions with the different environments present in a lipid bilayer system, that is, the aqueous and bilayer regions.
Only two computational studies of DMSO with lipid bilayers have appeared in the literature. The work by Smondyrev and Berkowitz (1999) considered the properties of a lipid bilayer in the presence of pure DMSO; their results will be discussed later in this work. The study of Paci and Marchi (1994) examined the transport of a single DMSO molecule through a lipid bilayer. This work provides a detailed analysis of DPPC lipid bilayers in the presence of DMSO over a wide range of concentrations commonly encountered in the preservation of biological systems.
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