Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, Gronostajowa 7, 30-387, Kraków, Poland.
Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387, Kraków, Poland.
Angew Chem Int Ed Engl. 2018 May 28;57(22):6501-6506. doi: 10.1002/anie.201802094. Epub 2018 May 2.
Ab initio DFT computations reveal that the essential structural and photophysical features of the conjugated π-electron system of retinal and carotenoids are dictated by "innocent" methyl substituents. These methyl groups shape the conformation and symmetry of the isoprenoid chromophores by causing a sigmoidal distortion of the polyene skeleton and increasing its flexibility, which facilitates fitting to their binding pockets in proteins. Comparison of in vacuo conformations of the chromophores with their native (protein-bound) conformations showed, surprisingly, that the peripheral groups and interactions with the protein environment are much less significant than the methyl side groups in tuning their structural features. The methyl side groups also contribute to a loss of symmetry elements specific to linear polyenes. In effect, the symmetry-imposed restrictions on the chromophore electronic properties are disabled, which is of tremendous relevance to their photophysics. This is evidenced by their non-negligible permanent dipole moments and by the simulated and experimentally measured circular dichroism spectra, which necessarily reflect the chirality of the conjugated π-electron system.
从头算 DFT 计算表明，视黄醛和类胡萝卜素共轭π电子体系的基本结构和光物理性质是由“无辜”的甲基取代基决定的。这些甲基基团通过使聚烯骨架发生弯曲变形并增加其柔韧性，从而塑造类异戊二烯发色团的构象和对称性，这有利于它们在蛋白质中的结合口袋中拟合。将发色团的真空构象与其天然（与蛋白质结合）构象进行比较，令人惊讶的是，外围基团与蛋白质环境的相互作用远不如甲基侧基在调节其结构特征方面重要。甲基侧基也导致了对线状聚烯特有的对称元素的损失。实际上，对发色团电子性质施加的对称性限制被禁用，这对它们的光物理性质具有巨大的意义。这可以通过它们不可忽略的永久偶极矩以及模拟和实验测量的圆二色性光谱来证明，这必然反映了共轭π电子体系的手性。
