pH dependence of the water solubility of bilirubin photo-derivatives and its relevance to phototherapy.

Deoxygenated chloroform solutions of bilirubin were irradiated with visible light and continuously extracted with aqueous solutions at different pHs in the range of 7.20-8.20. The aqueous solutions became yellow rapidly and progressively: the higher their pH, the more intense their coloration. The water soluble E-isomers of bilirubin may not represent the only photoproducts transferred into water. It clearly appears from visible absorption measurements that the photopigment formed in our experiments can be partitioned from chloroform into water at pH as low as 7.2-7.4. Although the water solubility of the photopigment cannot be exactly calculated from experimental data, a direct relationship between water partitioning and water solubility can be reasonably assumed. The fact that the water solubility of the photopigment sharply increases in inverse proportion to the hydrogen ion concentration can be of great relevance to the treatment of jaundiced infants with phototherapy.