44 



NATURE, FORMATION, AND ACTIVITIf:S 



Physical Properties of Antibiotics 



SOLUBILITY 



A survey of the solulMlity of an iintil)iotic 

 ill x'arious organic solvents and in water, at 

 various pH values, is a very useful method 

 of characterization. This method also gives 

 information aV)Out changes in the substance 

 being investigated. 



An acidic substance will be least solulile 

 in water at an acid pH and a basic substance 

 will be least soluble at an alkaline pH. An 

 amphoteric substance will be least soluble 

 at the pH of its isoelectric point. This in- 

 formation can be supplemented with pf)- 

 tentiometric titrations. 



One must remember that the solubility of 

 a crude preparation can be \'ery different 

 from that of the pure antil)iotic. Thus, crude 

 fradicin is soluble in ethanol, and crystalline 

 fradicin is soluble only in the glycols. As a 

 rule, the purer the substance, the less soluble 

 it is. Candicidin is a good example: crude, 

 it is partly soluble even in water; pure, it is 

 one of the least soluble antibiotics. 



STABILITY 



As we have already discussed, knowledge 

 of the stability of an antibiotic at \'arious 

 pH values and at various temperatures is an 

 essential prelude to the study of purification 

 procedures. Stable antibiotics such as chlor- 

 amphenicol and neomycin will not lose 

 activity if autoelaved at a neutral reaction. 

 Other antibiotics, such as mycomycin, are 

 highly unstal)le. The half-life of mycomycin 

 is only o hours at 27°C. In this respect, 

 certain antibiotics are most unusual; for 

 example, upon standing, rifomycin B in- 

 creases in activity as a result of molecular 

 rearrangement and subsequent transforma- 

 tion into a more potent antibiotic. 



COLOR REACTIONS 



Chemists have de^•ised a number of color 

 reactions which are characteristic of certain 



molecular structures, such as the ninhydrin 

 reaction for primary amino groupings, the 

 Sakaguchi reaction for guanido groups, and 

 the I'^ehling test for reducing sugars. The ap- 

 plication of such tests to antibiotics and 

 their degradation products is very useful in 

 characterizing the antibiotic. 



In this area, it should be mentioned that 

 certain antibiotics fluore.sce when excited 

 by ultraviolet light. The bright yellow 

 fluorescence of oxytetracycline is one ex- 

 ample. 



LIGHT ABSORPTION 



The light absorption of a substance can be 

 measured in the ultraviolet, the visible, and 

 the infrared range. To be of any significance, 

 the infrared spectrum of a substance must 

 be measured on the pure compound. In 

 certain cases, such as that of the neomy- 

 cins, the infrared spectrum gives little infor- 

 mation. In other cases, it will give the 

 chemist consitlerable information on the 

 various functional groupings which are 

 present in the molecule of the antibiotic. In 

 contrast, certain antibiotics have such char- 

 acteristic and intense visible or ultraviolet 

 light spectra that they can be detected in 

 the crudest extracts. This is true of the 

 polyenic antifungal antibiotics. If two sub- 

 stances have similar absorption spectra, this 

 indicates the presence in the two molecules 

 of similar groupings but does not mean 

 necessarily that the two substances are 

 identical. Vov example, trichomycin and 

 candicidin are two different antibiotics with 

 the same light-absorption spectrum. 



Chromatography 



Ohromatography is a method which can 

 be used not only to purify antibiotics, l)ut 

 also to characterize them. In this respect, 

 paper chromatography has been of great 

 value. It can be carried out in either the 

 vertical (ascending and descending) or the 

 horizontal plane (circular and centrifugal). 



