BILADIENES-(a,b) AND RELATED SUBSTANCES 123 



Complex salts. It is doubtful whether metal complexes of rubins exist; 

 the green "bilirubin Cu complex salts" of Kuster {1607,1008) are evidently 

 compounds of biliverdin and of biliviolinoid oxidation products of bilirubin. 

 Fischer found no complex salt formation of bilirubin with zinc {778). The 

 addition of zinc acetate to ammoniacal solutions of bilirubin appears to 

 produce a color change, but this has not yet been studied in detail. Certainly 

 no fluorescent zinc complexes are formed. According to Siedel and Fischer 

 {255Ii) the dimethyl ethers of a-hydroxypyrromethenes and of mesobilirubin 

 form, however, complex salts. One can again conclude from this that in 



mesobilirubin the rings I and IV are present in the lactam form °%A and 



H 



thus no tertiary nitrogen is available to coordinate with the metal, while 

 in the ethers the lactim form hoL A '^ fixed and thus two tertiary nitrogen 



N 



atoms which are able to coordinate with the metal are present {1090). 



4.5. Surface Properties and Adsorption 



The spreading of bilirubin on water has been mentioned in Section 1.4. 

 Bilirubin is bound to serum albumin, not to egg albumin {215,2132), com- 

 pletely at pH 7.2, incompletely at pH 8. 2 {2022). The penetration of bili- 

 rubin into cholesterol, octadecylamine, and protein monolayers has been 

 studied by Stenhagen and Rideal {2622). In the protein monolayers no 

 specificity was found, rigid tanned layers being formed with all proteins at 

 pH 7.2. The authors conclude that bilirubin, like the porphyrins, is bound 

 to the €-amino groups of lysine. Serum albumin contains much more lysine 

 than egg albumin and the e-amino groups of egg albumin are assumed to 

 be inaccessible in the native protein owing to interaction with its own car- 

 boxylic acid groups, while in the monolayers they become uncovered. The 

 authors observe that the penetration of bilirubin into monolayers of choles- 

 terol is faster at lower pH, while the reverse is true for its penetration into 

 octadecylamine. This, they assume, may explain why a low ;;H favors 

 stone formation in the bile. Increasing acidity would decrease the inter- 

 action of bilirubin with a carrier of amine nature in the bile (Pedersen), 

 but would increase its interaction with cholesterol. It would also decrease 

 the solubility of bilirubin. 



5. BILADIENES-(a,b) AND RELATED SUBSTANCES 



5.1. Biliviolinoid Substances 



By a great variety of methods a class of bile pigments can be 

 obtained which have the following striking properties in common: 

 they are readily dissolved in neutral organic solvents to give red to 

 red-violet solutions; in mineral acids their solutions are violet to 



