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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
Fig. 1 also shows the density profile for the binary DPPC/DMSO system (without water) at 350 K. In this case, a large number of DMSO molecules penetrate the bilayer; the DMSO concentration decreases from the bulk DMSO region to the middle of the bilayer region, where a steady concentration of DMSO is observed. Because of the hydrophobic nature of DMSO and its favorable interaction with the lipid aliphatic tails, the bilayer structure is deformed-that is, the bilayer interface is not as well-defined as in the case with lower DMSO concentrations. Furthermore, the integrity of the bilayer structure is modified by the lipid aliphatic tails dissolving into the DMSO bulk region. This is illustrated in Fig. 3, which shows a snapshot of the lipids in the simulation box (for clarity, DMSO molecules are not shown). Several lipid tails can be seen protruding from the bilayer into the bulk DMSO region. Smondyrev and Berkowitz (1999) have also performed simulations of a lipid bilayer in pure DMSO (a system with 64 DPPC molecules at 323 K and for 2 ns of simulation time); these authors pointed out that only a small number of DMSO molecules are able to penetrate the bilayer region. Note, however, that Smondyrev and Berkowitz used a different force field and performed their calculations at a lower temperature.
Fig. 5 shows how the local DMSO concentration in the bilayer system changes as a function of temperature. At the lower temperature of 325 K, DMSO tends to concentrate more in the aqueous region than in the bilayer; at 400 K, there is a shift in the relative concentrations, and DMSO is found predominantly in the bilayer. The hydrogen bonds between DMSO and water are favored at low concentrations and low temperatures. At higher temperatures, this hydrogen-bonding becomes weaker, DMSO adopts a more hydrophobic character, and it prefers to interact with the bilayer tails. This change in relative concentration with temperature is a manifestation of the hydrophobic effect, and has been invoked before to explain the toxicity of DMSO in cellular cryopreservation (Anchordoguy et al., 1992, 1991). It should also be pointed out that even though DMSO and water molecules are strongly hydrogen-bonded, there is no indication that water molecules penetrate the bilayer interface; in other words, DMSO is unable to act as a carrier for water into the bilayer. In the process of crossing the bilayer interface, DMSO must release the water molecules to which it is bound, thereby assuming an instantaneous transformation from hydrophilic to hydrophobic. This is also indicated from the fast crossing of DMSO molecules through the interface (see Fig. 2).
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