Lekka Ch E, Ren Jun, Meng Sheng, Kaxiras Efthimios
Department of Materials Science and Engineering, University of Ioannina, Ioannina 45110, Greece.
J Phys Chem B. 2009 May 7;113(18):6478-83. doi: 10.1021/jp807948z.
We present density functional theory (DFT) results on the structural, electronic, and optical properties of Cu-flavonoid complexes for molar ratios 1:1, 1:2, and 1:3. We find that the preferred chelating site is close to the 4-oxo group and in particular the 3-4 site followed by the 3'-4' dihydroxy group in ring B. For the Cu-quercetin complexes, the large bathochromic shift of the first absorbance band upon complexation, which is in good agreement with experimental UV-vis spectra, results from the reduction of the electronic energy gap. The HOMO states for these complexes are characterized by pi-bonding between the Cu d orbitals and the C, O p orbitals except for the case of 1:1 complex (spin minority), which corresponds to sigma-type bonds. The LUMO states are attributed to the contribution of Cu p(z) orbitals. Consequently, the main features of the first optical absorption maxima are essentially due to pi --> pi transitions, while the 1:1 complex exhibits also sigma --> pi transitions. Our optical absorption calculations based on time-dependent DFT demonstrate that the 1:1 complex is responsible for the spectroscopic features at pH 5.5, whereas the 1:2 complex is mainly the one responsible for the characteristic spectra at pH 7.4. These theoretical predictions explain in detail the behavior of the optical absorption for the Cu-flavonoid complexes observed in experiments and are thus useful in elucidating the complexation mechanism and antioxidant activity of flavonoids.
我们展示了密度泛函理论（DFT）对摩尔比为1:1、1:2和1:3的铜-类黄酮配合物的结构、电子和光学性质的研究结果。我们发现，优先螯合位点靠近4-氧代基团，特别是B环中的3-4位点，其次是3'-4'二羟基。对于铜-槲皮素配合物，络合后第一个吸收带的大的红移与实验紫外可见光谱很好地吻合，这是由于电子能隙的减小。这些配合物的最高占据分子轨道（HOMO）态的特征是铜d轨道与碳、氧p轨道之间的π键合，但1:1配合物（自旋少数）的情况除外，其对应于σ型键。最低未占据分子轨道（LUMO）态归因于铜p(z)轨道的贡献。因此，第一个光学吸收最大值的主要特征基本上是由于π→π跃迁，而1:1配合物也表现出σ→π跃迁。我们基于含时DFT的光学吸收计算表明，1:1配合物是pH 5.5时光谱特征产生的原因，而1:2配合物主要是pH 7.4时特征光谱产生的原因。这些理论预测详细解释了实验中观察到的铜-类黄酮配合物的光学吸收行为，因此有助于阐明类黄酮的络合机制和抗氧化活性。
