Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303, USA.
Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303, USA.
Biochim Biophys Acta Bioenerg. 2017 Sep;1858(9):804-813. doi: 10.1016/j.bbabio.2017.06.006. Epub 2017 Jun 29.
Time-resolved step-scan FTIR difference spectroscopy at low temperature (77 K) has been used to study photosystem I particles with phylloquinone (2-methyl-3-phytyl-1,4-naphthaquinone) and menadione (2-methyl-1,4-naphthaquinone) incorporated into the A binding site. By subtracting spectra for PSI with phylloquinone incorporated from spectra for PSI with menadione incorporated a (menadione - phylloquinone) double difference spectrum was constructed. In the double difference spectrum bands associated with protein vibrational modes effectively cancel, and the bands in the spectrum are primarily associated with the neutral and reduced states of the two quinones in the A binding site. To aid in the assignment of bands in the experimental double difference spectrum, a double difference spectrum was calculated using three-layer ONIOM methods. The calculated and experimental spectra agree well, allowing unambiguous band assignments to be made. The ONIOM calculations show that both quinones in the A binding site are similarly oriented, with only a single hydrogen bond between the C=O quinone carbonyl group and the backbone NH group of a leucine residue. For the semi-quinone species, but not for the neutral species, this hydrogen bond appears to be very strong. Finally, we have for the first time been able to unmask and identify infrared difference bands associated with neutral naphthoquinone species occupying the A binding site in PSI.
低温（77 K）下时间分辨分步扫描 FTIR 差谱法已被用于研究在 A 结合部位掺入叶绿醌（2-甲基-3-植基-1,4-萘醌）和甲萘醌（2-甲基-1,4-萘醌）的光系统 I 颗粒。通过从含有甲萘醌的 PSI 的光谱中减去含有叶绿醌的 PSI 的光谱，构建了（甲萘醌-叶绿醌）双差光谱。在双差光谱中，与蛋白质振动模式相关的谱带有效地相互抵消，而谱中的谱带主要与 A 结合部位中两个醌的中性和还原态有关。为了帮助确定实验双差光谱中的谱带，使用三层 ONIOM 方法计算了双差光谱。计算和实验光谱吻合良好，允许进行明确的谱带归属。ONIOM 计算表明，A 结合部位中的两个醌的取向相似，只有一个单氢键存在于 C=O 醌羰基和一个亮氨酸残基的骨架 NH 基团之间。对于半醌物种，但不是中性物种，这个氢键似乎非常强。最后，我们首次能够揭示和识别与占据 PSI 中 A 结合部位的中性萘醌物种相关的红外差谱带。
