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Interactions of lipid membranes and macromolecules in general depend on the pH of buffered solutions. However, pH buffers modify molecular interactions in more than one way [1,2]. Here we conduct in vitro experiments with mammalian and bacterial cells to determine how common buffers such as 3-(N-morpholino)propanesulfonic acid (MOPS) affect cell viability. We find that primary rat lung microvascular cells do not survive in concentrations greater than 125mM for MOPS compared to 40mM for KCl, and 50mM for PEG400. In contrast, E. coli cells survived in concentrations up to 0.75 M for KCl and 1M MOPS. This is ascribed to a protective effect from the bacterial cell wall. Results are interpreted by comparison with previous measurements of model membrane interactions in buffer solutions.[1] Koerner et al., Biophys. J. 101, 2011.[2] Peiro-Salvador et al. Biochemistry, 48, 2009.[3] van Haaren et al., Am. J. Physiol. Heart Circ. Physiol., 289, 2005.

Gramicidin A (gA) is a cation selective ion channel that has been used in many biophysical studies of lipid bilayers, in particular for investigations of lipid-protein interactions [1, 2] and membrane electrostatics [3]. In addition, it was found that ionic interactions with neutral lipid membranes also affect the kinetics of gA channels [4]. Here we report measurements of gA ion-channels for a series of sodium and potassium salts that show an anion-dependence of gA conductance. We find that gA conductance varies significantly with the anion type with ClO4 and SCN producing distinctly larger conductance values than Cl, F, and H2PO4. These results can provide new insights into ion-lipid membrane interactions and ion channel functions in general. [1] Andersen et al., Annu. Rev. Biophys. Biomol. Struct., 1996, [2] Lunbaek et al., PNAS 2010, [3] Rostovtseva et al. Biophys. J. 1998, [4] Rostovtseva et al. Biophys. J. 2008.

Lithium is a naturally occurring alkali metal which is very easily ionized to Li+, especially in aqueous solutions. Lithium is found mainly in mineral and seawater and in trace amounts in the human body. Lithium ions are being used in various fields ranging from energy storage to the treatment of mental illnesses including bipolar disorder and schizophrenia. The reason why lithium is such a peculiar ion is not exactly known except that its mode of interactions with charged surfaces depends on its hydration properties (i.e. interaction with nearby water molecules). We report two kinds of experimental measurements of lithium interactions with charged lipid membranes. One set of experiments involves X-ray scattering of multilamellar lipid vesicles in solution for which we measure how lithium salts modify van der Waals and electrostatic forces between neighboring membranes. The other set of measurements compare the effect of added lithium on the activity of gramicidin channels in neutral and negatively charged lipid bilayers. We find that in both x-ray and channel measurements, lithium ions modify lipid interactions in a manner that differs from other monovalent posititive ions.

Specific ionic effects, as captured in the Hofmeister series, have been observed in many biological phenomena including protein folding and aggregation and lipid bilayer interactions. Previously we have shown that the Hofmeister effect is present in the activity of gramicidin A channels. In particular, measurements of channel open lifetime and conductance in potassium salts clearly show the existence of two distinct ionic classes which could be identified as kosmotropic and chaotropic. To further investigate this behavior, we have measured the zeta potential of diphytanoyl phosphatidylcholine (DPhPC) liposomes in salt solutions. We observe that anions alter the surface charge of the liposomes depending on the classification of the anion as kosmotropic or chaotropic. Chaotropic anions (SCN-, ClO4-) decrease the surface charge of the liposomes while kosmotropic anions (Cl-, H2PO4-, SO42-) have the opposite effect. These results correlate with our previous studies of cation conductance through gramicidin A channels adding new insight into ionic interactions at the lipid-water interface.