ELECTROCHEMICAL REDUCTION OF THE BILIVERDIN-SERUM ALBUMIN COMPLEX AS MONITORED BY ABSORPTION AND CIRCULAR-DICHROISM SPECTROSCOPY

The cathodic reduction at the mercury electrode of a biliverdin IX alp ha-serum albumin complex at physiological pH in an aqueous buffer cont aining percentages of DMSO ranging from 4% to 20% is studied by cyclic voltametry and controlled potential coulometry. The progression of pi gment disappearance and the (stereochemical) nature of the product are monitored by chromatography, UV-visible absorption and circular dichr oism spectroscopy. Upon reduction, albumin-bound biliverdin IX alpha, with a slight preference for the P-helicity, affords the corresponding bound bilirubin IX alpha -with an M-chirality conformation. The compl ex is reduced at -0.64 V (vs. SCE; 8% DMSO), only a little shifted com pared to reduction of free biliverdin IX alpha under the same conditio ns. In contrast, an analogous bilirubin IX alpha-serum albumin complex is essentially inert towards cathodic reduction under conditions wher e free bilirubin IX alpha is reduced, indicating a better shielding by the protein of the bilirubin IX alpha molecule from the electrode sur face. The presence and relative position (as in the biliverdins IX alp ha and XIII alpha) or absence (as in mesobiliverdin IX alpha) of vinyl groups in the pigment does not have a significant effect upon its ele ctroreduction behaviour, indicating that the process is not sensitive to the subtle differences imposed by vinyl groups upon the structure o f the corresponding biliverdin-albumin complexes.