Blockage and Ligand Gating

Strong binding of specific ions can lead not only to selectivity but also to blockage of synthetic ion channels. In BLMs, blockage by permeant ions is revealed by saturation behavior of the conductance g with increasing activity Aion of the ion of interest (Fig. II.IOD) [33]. Fit of the salt profile to Eq. (11.8) gives the inhibitory concentration fCso (or the apparent dissociation constant Kd) and the maximal conductance gMAX for a permeant ion blocker. 

Y = fractional activity (Fig. 11.5) Cguest = guest concentration (ligand or blocker) n = Hill coefficient ECso (effective ligand concentration or JC50 for blockage, or apparent Kd = dissociation constant of the host-guest complex). 

Effective and inhibitory concentrations correspond to apparent dissociation constants of the host-guest complexes. These values can be quite different from the true Kd values. Particularly cautious interpretation is recommended for stoichiometric binding, molecular recognition may actually be much better than it appears in these cases [43-45]. Quantitative correlation of the values of JC50 and KD [44] is not common in the field of synthetic ion channels and pores. The Hill coefficient n 

Woodhull analysis is possible in BLMs and polarized LUVs with results being at least qualitatively comparable [47]. The key information obtained is the Woodhull distance from channel/pore entrance to active site. Mechanistic and structural insights accessible with Woodhull analysis include evaluation as to whether or not molecular recognition by synthetic ion channels and pores really takes place in the membrane, discrimination between voltage-sensitive inclusion complexes and voltage-insensitive peripheral association, and measurement of the depth of guest inclusion (see Section 11.4.4) [47]. 

---