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. Pho sphonium cations used were tetraphenylphosphonium (TPP+) and triphenyl alkylphosphonium cations formulated as (Phe)3-P+-(CH2)nCH3 (n = 0-5). The conductance was dependent on voltage applied externally to the mem brane in accordance with a theory developed by previous authors. Using the theory, values of betak(i) were determined, where beta and k(i) a re a linear partition coefficient and a rate constant of transmembrane ion transport, respectively. Since betak(i) depended on the phosphoni um ion concentration, values extrapolated to infinite dilution, (betak (i))0, were determined. Temperature dependence of (betak(i))0 allowed us to estimate the activation energy of transport, E(a). For TPP+, the rmodynamic values obtained were consistent with values calculated by F lewelling and Hubbell [(1986) Biophys. J. 49, 541-5521. 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, a nd P-31 NMR chemical shift.