The explanation I keep seeing describes that exactly half of the drug exists in unionized form. Is that to say it’s at it’s most soluble? If not, how do you determine at what pH a drug is most soluble?
In: Chemistry
Yes, you are correct that half exists as in protonated form (usually stylized as HA) and half unprotonated (A). However if one of those isn’t soluble, then pH = pKa would not be the highest solubility point.
A lot of times small molecule pharmaceuticals (I assume that’s what you mean by “drug”) are more soluble further away from their isoelectric point (pI) because ionic species are more polar, and therefore dissolve better in water.
These can be predicted if you know the structure of the molecule, but there’s a common need for experimental verification.
Isolectric point is basically the same as pKa, but for molecules that have multiple acid/base sites. Its when the molecule has a net charge of zero, but some sites may have a charge. An amino acid is like this.
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