A study of the interaction of bilirubin with sodium deoxycholate in aqueous solutions

The solubilization of bilirubin-IXalpha (BR) in submicellar and micellar aqueous solutions of sodium deoxycholate (NaDC) was investigated by circular dichroism (CD) and nuclear magnetic resonance (NMR) measurements. BR exhibits a bisignate CD Cotton effect in micellar solutions of NaDC at pH about 8, thus indicating that NaDC interacts preferentially with the BR molecule having left-handed chirality. However, the CD spectrum changes as a function of pH and BR concentration. Moreover, the spectrum becomes inverted at NaDC concentration below the critical micellar concentration. Potential energy calculations accomplished for the system formed by one-deoxycholate anion (DC-) and the left- or right-handed BR molecule (LBR or RBR, respectively) show that the DC--RBR complex is more stable than the DC--LBR one and are in agreement with the CD results. Interaction models are proposed and checked by NMR measurements. The influence of pH and BR concentration on the CD spectra, NaDC being in submicellar and micellar state, is discussed.