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Pharmacology Guide

Physical chemistry: definitions

cLog P
Calculated log P (i.e. a theoretical value).

Log D
Log P when the test is performed at pH 7.4.

Log P
A measure of the lipophilicity of a compound; its partition coefficient between an organic solvent (usually octanol) and an aqueous buffer.

              Conc. of non-ionised drug in octanol
Log P = Log  _____________________________________

              Conc. of non-ionised drug in buffer

Values usually range from -6 to +6, high positive values indicating high lipid solubility and low or negative values indicating hydrophilic compounds. The test is performed at a pH at which the compound is non-ionised (compare Log D).

Molarity (M)
A measure of the concentration of a solution. A 1M solution contains one mole of the substance per litre of solution.

                                   No. of grams      
Molarity =   _________________________

                            Mol. wt. x volume (litres)

Mole (mol)
The amount of a substance in grams which contains Avogadro's number (6.022 x 10(23)) of atoms or molecules of it.

Molecular Weight
The sum of the atomic weights of the atoms comprising a molecule; alternatively, the number of grams of a compound which contains a mole of it.

Normal solution
An outdated term referring to the concentration of an acid or base. It equals molarity times the number of hydrogen ions or hydroxyl ions generated by each molecule of acid or base respectively. Molarity is the preferred term.

pH at which 50% of an acid is dissociated into H+ and base. High pKa values indicate a weak acid.

Structure-activity relationship. The way in which altering the molecular structure of drugs alters their interaction with a receptor, enzyme, etc.


Forward Greek prefixes to SI units
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