Metal complexation of bilirubin and its dimethyl ester in N,N-Dimethylformamide

Abstract

This study deals with the formation and identification of bilirubin-metal complexes in the solvent N, N-Dimethylformamide, DMF. Extensive qualitative experiments were performed in order to discover metal salts that would react with bilirubin to give a shift in the well known visible spectrum of bilirubin. A fair number of such metal salts were found but only after a great deal of trial and error. There were a number of reasons for the difficulty in finding these bilirubin-metal complexes. Among these were: 1) the rapid decomposition of DMF, 2) the change in the oxidation state of some metal ions in DMF, 3) the nature of the anion of the metal salt, 4) the acid based equilibria of bilirubin in DMF, 5) the crucial molar ratios of bilirubin: base: metal to form complexes, 6) the sensitivity of both bilirubin and DMF to light, and 7) the solubility of metal salts in DMF, all of which will be discussed in some detail.

From this qualitative work it was found that two different types of spectral shift in the bilirubin (450 nm) visible spectrum occurred. One termed the "Zn type" is the shift of the bilirubin maximum absorbance peak to longer wavelengths and the other termed the "Al type" is the shift of the bilirubin (450nm) maximum to shorter wavelengths. Based on this spectral phenomenon of two different shifting patterns of bilirubin's absorbance peak and supported by work with bilirubin dimethyl ester, ethylenediamine, 2.4- pentadione and the quantitative formation of metal complexes, it is proposed here that there are at least two distinct structures for bilirubin-metal complexes in DMF. The techniques used to prepare materials and to analyze the complexes formed will also be discussed. These include visible spectroscopy, potentiometric titrations, polarography, esterification of bilirubin, purification and standardization of the DMF solvent system and fluorometry.