Tu Bin, Ghosh Brahmananda, Lightner David A
Department of Chemistry, University of Nevada, Reno, NV 89557, USA.
J Org Chem. 2003 Nov 14;68(23):8950-63. doi: 10.1021/jo030252t.
Novel bilirubin analogues with dipyrrinones conjoined to an acetylene rather than a methylene group were synthesized and examined spectroscopically. Despite the increased separation of the dipyrrinones forced by replacing a -CH(2)- by a -C(triple bond)C- unit, molecular dynamics calculations show that, like bilirubin, they may still engage in intramolecular hydrogen bonding to carboxylic acid groups when the propionic acid chains are slightly lengthened, e.g., butanoic acids. Unlike bilirubin, however, which is bent in the middle and has a ridge-tile shape, the acetylene orients the attached dipyrrinones along a linear path, and intramolecular hydrogen bonding preserves a twisted linear molecular shape. The extended planes of the dipyrrinones intersect along the -C(triple bond)C- axis at an angle of 136 degrees for the conformation stabilized by intramolecular hydrogen bonding in the bis-butyric acid rubin (1b). With shorter acid chains (propionic), only one CO(2)H can engage an opposing dipyrrinone in intramolecular hydrogen bonding, and in this energy-minimum conformation of the linear pigment 1a, the intersection of the extended planes of the dipyrrinones has an angle of 171 degrees. Spectroscopic evidence for such linearized and twisted structures was found in the pigments' NMR spectral data and their exciton UV-vis and induced circular dichroism spectra.
合成了二吡咯啉酮与乙炔而非亚甲基相连的新型胆红素类似物，并对其进行了光谱研究。尽管用-C≡C-单元取代-CH₂-会使二吡咯啉酮之间的间距增大，但分子动力学计算表明，与胆红素一样，当丙酸链稍有延长（如丁酸）时，它们仍可能与羧酸基团形成分子内氢键。然而，与呈中间弯曲且具有屋脊形状的胆红素不同，乙炔使连接的二吡咯啉酮沿直线排列，分子内氢键保持了扭曲的线性分子形状。对于双丁酸胆红素（1b）中由分子内氢键稳定的构象，二吡咯啉酮的伸展平面沿-C≡C-轴以136度角相交。对于较短的酸链（丙酸），只有一个COOH能在分子内氢键中与相对的二吡咯啉酮结合，在这种线性色素1a的能量最低构象中，二吡咯啉酮伸展平面的交角为171度。在这些色素的核磁共振光谱数据以及激子紫外可见光谱和诱导圆二色光谱中发现了这种线性化和扭曲结构的光谱证据。
