174 BACTERIOLOGICAL CHEMISTRY 



They arc strong monobasic acids with ^^K — 2-8. The 

 probable presence of a masked basic group is shown by 

 a slow titration with perchloric acid in acetic acid. 

 Hydrolysis of penicillin with dilute acid at 30°C. gives 

 the penillic acids, III, which are dicarboxylic acids 

 containing a basic group but no SH group. Treatment 

 of the penillic acids with cold aqueous mercuric chloride 

 causes loss of carbon dioxide and gives rise to penillamines, 

 IV, which contain an 8H group, whilst treatment with 

 baryta converts penillic acid to i^o-penillic acid, X. 

 Hydrolysis of j)enicillin with hot dilute mineral acids 

 gives carbon dioxide, penicillamine, V (<i- j8 jS-dimethyl 

 cysteine) and penillo aldehydes, VII ; that from penicillin 

 I is A 2-hexenoylamino acetaldehyde, whilst penillo- 

 aldehyde II is plien3dacetylamino acetaldehyde. The 

 carbon dioxide and the aldehydes are derived from the 

 corresponding penaldic acids, VI. Treatment of penicillin 

 with alkali or with penicillinase causes opening of the 

 lactone ring with formation of dibasic penicilloic acids, 

 VIII. Treatment of penicillin with methanol gives rise 

 to the inactive methyl ester, IX. 



The potency of penicillin preparations is defined in 

 terms of an arbitrary international unit, the "Oxford unit," 

 which is the amount of activity contained in 0-6 /xg. of a 

 particular standard crystalline preparation. The measure- 

 ment is carried out by comparing the effect of the unknown 

 sample with that of a sample of known potency on cultures 

 of a suitable organism in vitro. Staphylococci are in- 

 hibited by a concentration of penicillin equivalent to 

 0-01 to 0-02 units per ml. The purest solid preparations 

 of penicillin so far described contain about 1650 units 

 per milligram, so that dilutions of 1 in 3 X 10' are 

 sufficient to inhibit staphylococci ; culture filtrates 

 from surface growth contain about 20 to 80 units per 

 millilitre, whilst those from submerged growth ma}^ 

 contain up to 250 units per millilitre, depending on the 

 strain and conditions. 



