Hofmeister effect in ion transport: reversible binding of halide anions to the roflamycoin channel.

We have studied the anion-dependent gating of roflamycoin ion channels using spectral analysis of noise in currents through multichannel planar lipid bilayers. We have found that in addition to low frequency current fluctuations that may be attributed to channel switching between open and closed conformations, roflamycoin channels exhibit a pronounced higher frequency noise indicating that the open channel conductance has substates with short lifetimes. This noise is well described by a Lorentzian spectrum component with a characteristic cutoff frequency that depends on the type of halide anions according to their position in the Hofmeister series. It is suggested that transitions between the substates correspond to a reversible ionization of the channel by a penetrating anion that binds to the channel structure, more chaotropic anions being bound for longer times. Within a framework of a two-substate model, the duration of the substate with reduced electrostatic barrier for cation current varies exponentially with anion electron polarizability. This explains two features of the roflamycoin channel reported earlier: the increase in apparent single-channel conductance along the series F- < Cl- < Br- < I- and the reverse of channel selectivity from anionic for KF to cationic for KI.