How to define the parameter 'Solubility gain per charge'

[wangwei1619]

Hi, all
Since I don't need to define any parameter related the ionization step of a molecule, what is this parameter Solubility gain per charge used for? Is it OK to always keep the default value of it?

[HenrikCordes]

Dare @wangwei1619,

the parameter "Solubility gain per charge" is only of relevance if you have a charged molecule.
It basically scales the given solubility value with respect to the charge, depending on the pH.

Kind regards,
Henrik

[wangwei1619]

Dear @HenrikCordes

Thanks for your answer. May I judge your mean as this parameter is for some molecules whose active form is being charged? Could you please give me some example of this kind of drug, because I am not familiar with this field?

Best regards,
Wang Wei

[HenrikCordes]

Dear @wangwei1619,

it does not matter if a compound is active or not. Its a physico-chemical property of any compound.
But lets consider Pravastatin: https://www.drugbank.ca/drugs/DB00175

The molecular structure of the compound contains an acid group (bottom right of the molecules) with an pKa value of about 4. The acid function of the molecule can be of charge 0 or -1, depending on the environmental pH.
As you can see in the graph, when the environment crosses ~ pH 4 more of the compound becomes negatively charged.

Usually, the solubility of charged molecules is higher in an aqueous environment than for uncharged molecules and the factor "Solubility gain per charge" accounts for that behavior.

Kind regards,
Henrik

[wangwei1619]

Dear @HenrikCordes

Thanks for your patience and specific description. However, as the molecule solubility and envrionment pH comform to this relationship:
Cs=[HA]+[A-]
[HA]=C0
Cs=C0+Ka*C0/[H+]
I guess if we know the solubility in a specific pH, we can figure out the C0, then calculate the solubility in other pH. Why do we need define the parameter Solubility gain per charge, since it is also uesed to extrapolate solubility in other pH?

Best regards,
Wang Wei

[msevestre]

Closing. Please reopen if you need more help