Smyk B, Mędza G, Kasparek A, Pyrka M, Gryczynski I, Maciejczyk M
Department of Physics and Biophysics, University of Warmia and Mazury , Oczapowskiego 4, 10-719 Olsztyn, Poland.
Department of Cell Biology and Immunology, Center for Fluorescence Technologies and Nanomedicine, University of North Texas Health Science Center , Fort Worth, Texas 76107, United States.
J Phys Chem B. 2017 Aug 3;121(30):7299-7310. doi: 10.1021/acs.jpcb.7b05508. Epub 2017 Jul 25.
A methyl ester of sinapic acid (MESA) has recently attracted attention due to its antioxidant action. This article presents results of a study on the spectral and physicochemical properties of MESA, using quantum chemistry (QC), steady-state (absorption and fluorescence), and time-resolved fluorescence techniques (TCSPC). The pK of the phenol group in the ground state was determined (8.6). The pK* values in the excited state calculated from the Förster cycle (1.9) and from fluorescence spectra (8.5) differed significantly but the experimental data suggested that the first was the more probable one. Quantum yields (QYs) for both forms have been determined. The QYs were very low (0.0017 and 0.0007) for nondissociated and dissociated forms, respectively and lifetimes were very short ≤10 ps for both forms. The differences in the probability of H-bond formation in the ground and the excited states were estimated by the application of the SdP polarity scale. Dipole moments in the ground state were calculated using QC. The ratio between dipole moments in the ground and the excited state for free molecule was obtained from Bilot-Kawski (B-K) method. Analysis of all collected results suggests that radical route (through hydrogen atom abstraction) of antioxidant activity of MESA is the more probable one in a water environment at pH below 6.
芥子酸甲酯（MESA）因其抗氧化作用最近受到关注。本文介绍了一项关于MESA光谱和物理化学性质的研究结果，采用了量子化学（QC）、稳态（吸收和荧光）以及时间分辨荧光技术（TCSPC）。测定了基态下酚基团的pK值（8.6）。通过Förster循环计算的激发态pK值（1.9）和通过荧光光谱计算的pK值（8.5）有显著差异，但实验数据表明前者更有可能。测定了两种形式的量子产率（QYs）。未解离和解离形式的QYs分别非常低（0.0017和0.0007），两种形式的寿命都非常短≤10 ps。通过应用SdP极性标度估计了基态和激发态中氢键形成概率的差异。使用QC计算了基态下的偶极矩。通过Bilot-Kawski（B-K）方法获得了自由分子基态和激发态偶极矩之间的比率。对所有收集结果的分析表明，在pH低于6的水环境中，MESA抗氧化活性的自由基途径（通过氢原子提取）更有可能。
