Claret J, Ibars O, Lang K, Trull F R, Lightner D A
Departament de Química Física, Facultat de Química, Universitat de Barcelona, Catalunya, Spain.
Biochim Biophys Acta. 1995 Feb 23;1243(2):221-9. doi: 10.1016/0304-4165(94)00131-g.
The cathodic reduction at the mercury electrode of a biliverdin IX alpha-serum albumin complex at physiological pH in an aqueous buffer containing percentages of DMSO ranging from 4% to 20% is studied by cyclic voltametry and controlled potential coulometry. The progression of pigment disappearance and the (stereochemical) nature of the product are monitored by chromatography, UV-visible absorption and circular dichroism 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 complex is reduced at -0.64 V (vs. SCE; 8% DMSO), only a little shifted compared to reduction of free biliverdin IX alpha under the same conditions. In contrast, an analogous bilirubin IX alpha-serum albumin complex is essentially inert towards cathodic reduction under conditions where free bilirubin IX alpha is reduced, indicating a better shielding by the protein of the bilirubin IX alpha molecule from the electrode surface. The presence of relative position (as in the biliverdins IX alpha and XIII alpha) or absence (as in mesobiliverdin IX alpha) of vinyl groups in the pigment does not have a significant effect upon its electroreduction behaviour, indicating that the process is not sensitive to the subtle differences imposed by vinyl groups upon the structure of the corresponding biliverdin-albumin complexes.
采用循环伏安法和控制电位库仑法，研究了在生理pH值下，于含4%至20%二甲基亚砜（DMSO）的水性缓冲液中，胆绿素IXα -血清白蛋白复合物在汞电极上的阴极还原。通过色谱法、紫外可见吸收光谱法和圆二色光谱法监测色素消失的进程以及产物的（立体化学）性质。还原后，与白蛋白结合的胆绿素IXα（稍倾向于P -螺旋结构）生成了具有M -手性构象的相应结合胆红素IXα。该复合物在 -0.64 V（相对于饱和甘汞电极；8% DMSO）处被还原，与相同条件下游离胆绿素IXα的还原相比，仅有轻微偏移。相反，在游离胆红素IXα被还原的条件下，类似的胆红素IXα -血清白蛋白复合物对阴极还原基本呈惰性，这表明蛋白质对胆红素IXα分子的屏蔽作用使其更好地远离电极表面。色素中乙烯基的相对位置（如在胆绿素IXα和XIIIα中）或不存在（如在中胆绿素IXα中）对其电还原行为没有显著影响，这表明该过程对乙烯基给相应胆绿素 -白蛋白复合物结构带来的细微差异不敏感。
