Programme Contents

Mobilities of methanesulfonate, needed for liquid junction potential corrections, and of two related anions, and a mystery solved

Liquid junction potentials (LJP) corrections are especially important in patch-clamp measurements of membrane potentials and the absence, or incorrect application of LJP corrections, can radically affect ion selectivity measurements and lead to totally incorrect selectivity conclusions (Barry & Lynch, 1991). The LJP corrections can be readily and reliably calculated, particularly for monovalent ion salts, provided the mobilities of the significant ions in the appropriate solutions are known (Barry et al., 2013). The anion methanesulfonate is often used as a substitute for chloride (Cl⁻) ions as it is not transported through Cl⁻ channels and, for example, cesium methanesulfonate is often used as a component of the intracellular patch pipette. Mobilities of ions, relative to K⁺ ions, can be readily calculated from the measurement of the limiting equivalent conductivity (λ⁰) of the ions relative to the value for K⁺ and for many this is available in published literature tables. Methanesulfonate is not listed in such tables, though two other somewhat related ions, methylsulfonate (Dean, 1999) and methylsulfate (Vanysek, 2002) are, and their values are given as having an identical limiting equivalent conductance of 48.8 S.cm²equiv^-1, which would give each of them a mobility, u, relative to u_K of 0.66. In addition, methanesulfonate is sometimes assumed to be either the same as methylsulfate or methylsulfonate or at least having a similar ionic mobility to them. However, NaMethanesulfonate (MW = 118.09; CH₃NaO₃S), NaMethylsulfate (MW = 134.09; CH₃NaO₄S) and NaMethylsulfonate (MW = 158.156; CH₂C(CH₃)CH₂SO₃Na) are clearly different salts of different structure and size and would be expected to have different mobilities.

The aim has therefore been to directly determine the mobilities of these three anions, by measuring their LJPs (e.g. Barry et al., 2013) for biionic solution junctions for each 100 mM Na salt versus 100 mM NaCl. The results showed that for NaMethanesulfonate, the corrected measured LJP was -7.4 mV, which corresponded to a methanesulfonate mobility value (relative to K⁺) of 0.60. For NaMethylsulfate, the corrected measured LJP was -7.7 mV, which implied a relative methylsulfate mobility value of 0.59 and for NaMethylsulfonate, the corrected measured LJP was -10.6 mV, which implied a relative methylsulfonate mobility value of 0.45. The last two values are very different from the values determined from the literature of 0.66, implying that the values in the literature for methylsulfate and methylsulfonate values are incorrect.

To confirm these results, we have measured the limiting equivalent conductance (Λ⁰) of the NaMethanesulfonate salt in a range of solutions from 10 mM to 1 mM using a Radiometer 210 Conductivity Meter (see e.g. Ng & Barry, 1995) and obtained an initial Λ⁰ value of 96 ± 1 S.cm²equiv^-1 , which when corrected for the λ⁰_Na value of 50.1 S.cm²equiv^-1 for the Na⁺, gave a value for methanesulfonate⁻ of 45.9 ± 1.0 S.cm²equiv^-1, implying a mobility relative to K⁺ of 0.62 ± 0.02, not very different from the value more directly measured using LJPs.

Further experiments will confirm more precise λ⁰ values for methanesulfonate and measure λ⁰ values for the methylsulfate and methylsulfonate anions.

Barry PH, Lewis TM, Moorhouse AJ. (2013) An optimised 3 M KCl salt-bridge technique used to measure and validate theoretical liquid junction potential values in patch-clamping and electrophysiology. Eur Biophys J 42, 631-646.

Barry PH, Lynch JW. (1991) Topical Review. Liquid junction potentials, and small cell effects in patch clamp analysis. J Memb Biol 121, 101-117.

Dean JA. (1999). Lange’s Handbook of Chemistry, 15^th Edition, McGraw-Hill, New York.

Ng B, Barry PH. (1995) The measurement of ionic conductivities and mobilities of certain less common organic ions needed for junction potential corrections in electrophysiology. J Neurosci Meth 56, 37-41.

Vanysek, P. (2002). Ionic conductivity and diffusion at infinite dilution. In: CRC Handbook of Chemistry and Physics (83rd Edn; ed. D.R. Lide), CRC Press, Boca Raton.