Parameswaran Sreeja, Wang Ruili, Hastings Gary
Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303, USA.
J Phys Chem B. 2008 Nov 6;112(44):14056-62. doi: 10.1021/jp806115q. Epub 2008 Oct 9.
Chlorophyll a (Chl-a) is at the heart of solar energy capture and conversion in plants. Because of this, Chl-a has been the subject of innumerable studies. Recently, we have been able to use quantum mechanical methods to calculate the vibrational properties of neutral and oxidized Chl-a in the gas phase [Wang, R.; Parameswaran, S.; Hastings, G. Vib. Spectrosc. 2007, 44, 357-368]. The calculated vibrational properties do not agree with experiment, however. One factor ignored in our calculations was how solvents could impact the vibrational properties. Here we calculate the vibrational properties of Chl-a and Chl-a+ in several solvents that span a wide range of dielectric constant. The calculated and experimental (Chl-a+-Chl-a) infrared difference spectra now show a remarkable similarity. However, the composition of the calculated vibrational modes are very different from that suggested from experiment. We therefore use our calculated data to make new suggestions as to the origin of the bands in experimental (Chl-a+-Chl-a) FTIR difference spectra. We indicate why bands in experimental spectra may have been misassigned. We also point to other experimental data that support our new band assignments. Assignment of bands in (Chl-a+-Chl-a) FTIR difference spectra were first made nearly 20 years ago. These assignments have formed the basis for evaluating all "cation minus neutral" FTIR difference spectra obtained for all photosynthetic systems since then. All of these experimental FTIR difference spectra should be re-examined in light of our new assignments.
叶绿素a（Chl-a）是植物捕获和转换太阳能的核心物质。正因如此，Chl-a一直是无数研究的主题。最近，我们已经能够使用量子力学方法来计算气相中中性和氧化态Chl-a的振动特性[Wang, R.; Parameswaran, S.; Hastings, G. Vib. Spectrosc. 2007, 44, 357 - 368]。然而，计算得到的振动特性与实验结果并不相符。我们的计算中忽略的一个因素是溶剂如何影响振动特性。在此，我们计算了在几种具有广泛介电常数的溶剂中Chl-a和Chl-a +的振动特性。计算得到的和实验测得的（Chl-a + - Chl-a）红外差谱现在显示出显著的相似性。然而，计算得到的振动模式的组成与实验结果所暗示的非常不同。因此，我们利用计算数据对实验（Chl-a + - Chl-a）傅里叶变换红外差谱中谱带的起源提出新的建议。我们指出了实验光谱中谱带可能被错误归属的原因。我们还指出了支持我们新的谱带归属的其他实验数据。（Chl-a + - Chl-a）傅里叶变换红外差谱中谱带的归属最早是在近20年前确定的。从那时起，这些归属就成为了评估所有光合系统获得的所有“阳离子减去中性”傅里叶变换红外差谱的基础。鉴于我们的新归属，所有这些实验傅里叶变换红外差谱都应该重新审视。
