Wellman Sydney M J, Jockusch Rebecca A
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, M5S 3H6, Canada.
Chemistry. 2017 Jun 7;23(32):7728-7736. doi: 10.1002/chem.201605167. Epub 2017 Apr 10.
In nature, the finely tuned photophysical properties of chlorophyll a (Chla) are vital to the capture and transfer of sunlight during photosynthesis. In order to better understand how these properties are influenced by the molecular environment, we have examined the intrinsic spectroscopy of Chla in vacuo. Visible photodissociation action spectra (an indirect measure of absorption) of gaseous protonated Chla and Chla complexed with metal cations are reported. These show that spectral features within the Soret band (∼350-445 nm) have markedly different intensities depending on the identity of the cation. In contrast, fluorescence emission spectra of metalated Chla complexes show only small dependences on the identity of the metal ion, with emission maxima shifting from 661 to 654 nm. Remarkably, replacing the metal ion with a proton turns off the fluorescence of this key pigment. Density functional theory geometry-optimized structures indicate that the most favorable site of protonation differs from that of metal cationization, and may help explain the surprising on/off behavior of Chla's intrinsic fluorescence.
在自然界中，叶绿素a（Chla）经过精细调节的光物理性质对于光合作用过程中太阳光的捕获和传递至关重要。为了更好地理解这些性质如何受到分子环境的影响，我们研究了真空中Chla的本征光谱。报告了气态质子化Chla以及与金属阳离子络合的Chla的可见光光解离作用光谱（吸收的间接测量）。这些结果表明，根据阳离子的种类，索雷特带（约350 - 445 nm）内的光谱特征具有明显不同的强度。相比之下，金属化Chla络合物的荧光发射光谱仅对金属离子的种类有较小的依赖性，发射最大值从661 nm移至654 nm。值得注意的是，用质子取代金属离子会使这种关键色素的荧光熄灭。密度泛函理论几何优化结构表明，最有利的质子化位点与金属阳离子化位点不同，这可能有助于解释Chla本征荧光令人惊讶的开启/关闭行为。
