CHEMICAL PROPERTIES 375 



quickly acquires a pH of 3.1, which stabilizes the alloxan which has not 

 been converted. This marked fall in pH must be considered in in vitro studies 

 and it is possible that some of the inhibitions of enzymes reported for 

 alloxan may have been due to a pH effect. It may also be pointed out that 

 the search for more stable alloxan derivatives has failed: the iV-substituted 

 derivatives decompose more rapidly than alloxan (Briickmann and Isaacs, 

 1949), and alteration of the rest of the molecule abolishes the ability to 

 react with SH groups and the diabetogenicity. 



Reactions with Amino Acids 



Strecker (1862) demonstrated a reaction between alloxan and certain 

 amino acids yielding COg, NH4+, and an aldehyde with one less C atom 

 than the amino acid. Alloxan is transformed into purpuric acid, and this 

 may complex with the amino acids to give red or rose-colored products; 

 indeed, the glycine complex has been isolated (Piloty, 1909). Hurtley and 

 ■Wootton (1911) showed that most amino acids except glycine and proline 

 react in this way. A mixture of alloxan and glycine does not form formal- 

 dehyde in good yield; the reaction is quite complex and the alloxan is 

 reduced to dialurate and hence to alloxantin, this reacting with the ammonia 

 produced to give purpuric acid or murexide. Lieben and Edel (1932) gave 

 the following order of decreasing reactivity with alloxan: cysteine, histidine, 

 glycine, glutamate, tryptophan, phenylalanine, arginine, aspartate, tyrosine, 

 leucine, serine, alanine, valine, and lysine. The presence of a free amino 

 group is necessary but not sufficient, since many amines do not react, and 

 no satisfactory rule for reactivity could be formulated. Abderhalden (1938) 

 isolated some of the aldehydes formed in this reaction and showed that 

 certain polypeptides react to give a rose or blue-violet color if a terminal 

 glycine is present. The more recent study of Schonberg et al. (1948) showed 

 the necessity for the — CO— (CH=CH)„— CO— grouping and the following 

 reaction scheme was advanced: 



R 

 R I 



— C=0 I — C=N— CH— COO- 



I + H2N— CH— COO- -> I 



— C=0 — C=0 



_C=N— CH,— R — C— N=CH— R _C— NH., 



-> I ' :f± II _>. II + R— CHO 



— C=0 — C— OH — C— OH 



Such reactions are now known as Strecker degradations and are analogous 

 to the reactions of ninhydrin with amino acids. The red-purple color of 

 the product arises from murexide, which is the ammonium salt of purpuric 

 acid, and the complexes of the amino acids with these substances; thus 

 the color produced by reaction of alloxan with biological materials is often 

 intensified by the addition of NH4+. 



