α-Helix recognition by rigid-rod β-barrel ion channels with internal arginine-histidine dyads in polarized bilayer membranes

Abstract

The objective of this study was to exploit multifunctional rigid-rod β-barrel ion channels for α-helix recognition by dipole-potential interactions in polarized membranes. Synthesis and evaluation of artificial β-barrel 1 characterized by p-octiphenyl ‘staves,’ leucine residues at the outer and histidine as well as, for the first time, arginine residues at the inner barrel surface are described. Internal arginines were introduced to recognize organic ions such as α-helical poly- l-glutamic acid (α-PLGA) passing through pores formed by barrel 1 at nanomolar concentrations. In unpolarized spherical bilayers (EYPC-LUVs), P-helical α-PLGA blocked pore 1 with a K D=150 nM at pH=4.5. As expected for a highly symmetric supramolecular host, a K D=100 nM determined for M-helical α-PDGA at pH=4.5 did not support substantial chiral guest recognition. Decreasing K D's with increasing pH indicated that, in unpolarized bilayers, pore 1 recognizes anions (rather than α-helices). In polarized spherical membranes, the K D for α-PLGA at pH=4.5 dropped substantially to 13 nM at V ≈−150 mV. This experimental support for operational dipole-potential interactions indicates that α-PLGAs bind within active pores in transmembrane orientation with intravesicular N- and extravesicular C-termini. Independence on bilayer polarization for binding of random-coil PLGA at pH=5.5 corroborated that dipole–potential interactions account for α-helix recognition in polarized membranes.