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

we know that

Free energy change = Delta G = -RTln P

P = paritition coefficeint

R = gas constant = 8.314

T = Temperature (assuming standard) = 298 K

So Free energy change = - 8.314 X 298 X 2.303 X log 575.4 = 15747.969 Joules

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