Site binding - general considerations

Not all ions are mobile within the ionic atmosphere of the polyion. A proportion are localized and site-bound-a concept apparently first suggested by Harris Rice (1954). Localized ion binding is equivalent to the formation of an ion-pair in simple electrolytes. Experimental evidence comes mainly from studies on monovalent counterions. 

This concept is due to Bjerrum, who in 1926 suggested that in simple electrolytes ions of the opposite charge could associate to form ion-pairs (Szwarc, 1965 Robinson Stokes, 1959). This concept of Bjerrum arose from problems with the Debye-Huckel theory, when the assumption that the electrostatic interaction was small compared with IcTwas not justified. 

Bjerrum considered the case of spherical ions in a solvent of dielectric constant e. The probability of finding two ions of opposite charge at a distance A from each other is calculated from the number of ions surrounding a central ion of opposite charge in a spherical shell of thickness dA and radius A. This probability, fV(A), is given by 

For a cation of charge and anion of charge z, this minimum becomes 

When A A the ions are free and the Debye-Huckel theory applies. When A A the two ions tend to approach each other and form an ion-pair, and there is no contribution to the electrostatic energy from the interaction between an ion and its atmosphere. 

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General Considerations for Use of Genuine Binding Constants at Binding Sites for Stepwise Binding Model... 

The comparison of fish bioassays (6) with the calculated effective doses indicate that the two most potent brevetoxins, PbTx-1 and PbTx-7, also are most effective at displacing tritiated probe from its specific site of action. The considerably lower potency of brevetoxins PbTx-5 and PbTx-6 in the rat system suggest that these two toxins may bind with lesser affinity to site 5. In a general sense, this is indicated in Table II, and is summmarized in Table III. 

Huang et al. (7) suggested that the brevetoxin binding site lies in the hydro-phobic portion of the channel, and since PbTx-1 and PbTx-7 are also the most hydrophobic of the toxins, their potency may also be in part due to solubility considerations. In general, the more hydrophobic the toxin, the higher is its potency and ability to displace tritiated PbTx-3 from its specific binding site. It is our contention that substituent character on each toxin s K (type-1) or J (type-2) ring... 

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