Activation Energy for Permeation of Phosphonium Cations through Phospholipid Bilayer Membrane

The conductance caused by translocation of various phosphonium cations across phospholipid (from soybean) bilayer membrane was measured. Phosphonium cations used were tetraphenylphosphonium (TPP⁺) and triphenylalkylphosphonium cations formulated as (Phe)₃₋P⁺-(CH₂)_nCH₃ (n = 0–5). The conductance was dependent on voltage applied externally to the membrane in accordance with a theory developed by previous authors. Using the theory, values of βk_i were determined, where β and k_i are a linear partition coefficient and a rate constant of transmembrane ion transport, respectively. Since βk_i depended on the phosphonium ion concentration, values extrapolated to infinite dilution, (βk_i)₀, were determined. Temperature dependence of (βk_i)₀ allowed us to estimate the activation energy of transport, E_a. For TPP⁺, thermodynamic values obtained were consistent with values calculated by Flewelling and Hubbell [(1986) Biophys. J. 49, 541–552]. When (Phe)₃₋P⁺-(CH₂)_nCH₃ (n = 0–5) were used, E_a depended on the odd or even of n. This “odd and even” pattern was observed in a variety of phenomena such as solubility in water, equivalent ionic conductivity in water, and ³¹P NMR chemical shift.