An important physical characteristic in pharmacology is the fat-blood partition

Question

An important physical characteristic in pharmacology is the fat-blood partition coefficient. This is the equilibrium concentration of a drug in fatty tissues (a relatively oily phase) divided by the concentration of drug in blood (an aqueous phase). If the fat-blood partition coefficient is high, the majority of a drug will be transferred to fatty tissue during administration. The fat-blood partition coefficient of the anesthetic n-butyl 4-aminobenzoate is reported to be 575.4 at pH = 7.4.

Calculate the free energy per mole to transport n-butyl 4- aminobenzoate from fatty tissue to blood. At pH = 7.4, n-butyl 4-aminobenzoate is not protonated, so you may assume that equations for uncharged species can be used.

Explanation / Answer

k/c = 575.4

pH = 7.4

For one mole , c = 1

free energy per mole = RT ln K

                              = 8.314 * (273+25) *ln (575.4)

(suppose temp = 25 degree centigrade)

              free energy per mole = 15745.13 J

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