Technische Universität Braunschweig, Institute for Physical and Theoretical Chemistry, Department for Biophysical Chemistry, Gaußstr. 17, 38106 Braunschweig, Germany.
Phys Chem Chem Phys. 2021 Apr 14;23(14):8731-8738. doi: 10.1039/d1cp00656h. Epub 2021 Mar 30.
In addition to (bacterio)chlorophylls, (B)Chls, photosynthetic pigment-protein complexes bind carotenoids (Cars) that fulfil various important functions which are not fully understood, yet. However, certain excited states of Cars are optically one-photon forbidden ("dark") and can potentially undergo excitation energy transfer (EET) to (B)Chls following two-photon absorption (TPA). The amount of EET is reflected by the differences in TPA and two-photon excitation (TPE) spectra of a complex (multi-pigment) system. Since it is technically and analytically demanding to resolve optically forbidden states, different studies reported varying contributions of Cars and Chls to TPE/TPA spectra. In a study using well-defined 1 : 1 Car-tetrapyrrole dyads TPE contributions of tetrapyrrole molecules, including Chls, and Cars were measured. In these experiments, TPE of Cars dominated over Chl a TPE in a broad wavelength range. Another study suggested only minor contributions of Cars to TPE spectra of pigment-protein complexes such as the plant main light-harvesting complex (LHCII), in particular for wavelengths longer than ∼600/1200 nm. By joining forces and a combined analysis of all available data by both teams, we try to resolve this apparent contradiction. Here, we demonstrate that reconstruction of a wide spectral range of TPE for LHCII and photosystem I (PSI) requires both, significant Car and Chl contributions. Direct comparison of TPE spectra obtained in both studies demonstrates a good agreement of the primary data. We conclude that in TPE spectra of LHCII and PSI, the contribution of Chls is dominating above 600/1200 nm, whereas the contributions of forbidden Car states increase particularly at wavelengths shorter than 600/1200 nm. Estimates of Car contributions to TPA as well as TPE spectra are given for various wavelengths.
除了（细菌）叶绿素（BChls）外，光合作用色素-蛋白复合物还结合类胡萝卜素（Cars），这些类胡萝卜素具有各种尚未完全了解的重要功能。然而，某些 Cars 的激发态在光学上是单光子禁戒的（“暗”），并且可以在双光子吸收（TPA）后潜在地将激发能转移（EET）到（B）Chls。复合物（多色素）系统的 TPA 和双光子激发（TPE）光谱的差异反映了 EET 的量。由于从技术和分析上难以分辨光学禁戒态，不同的研究报告称 Cars 和 Chls 对 TPE/TPA 光谱的贡献不同。在一项使用明确定义的 1:1 Car-四吡咯二聚体的研究中，测量了四吡咯分子（包括 Chls 和 Cars）的 TPE 贡献。在这些实验中，在宽波长范围内，Cars 的 TPE 主导了 Chl a 的 TPE。另一项研究表明，Cars 对色素-蛋白复合物（例如植物主要的光捕获复合物（LHCII））的 TPE 光谱的贡献很小，特别是对于波长大于约 600/1200nm。通过联合力量和两个团队对所有可用数据的联合分析，我们试图解决这一明显的矛盾。在这里，我们证明了 LHCII 和光系统 I（PSI）的 TPE 宽光谱范围的重建需要 Car 和 Chl 的重要贡献。在这两项研究中获得的 TPE 光谱的直接比较表明原始数据具有良好的一致性。我们得出结论，在 LHCII 和 PSI 的 TPE 光谱中，Chls 的贡献在 600/1200nm 以上占主导地位，而禁戒 Cars 态的贡献在波长小于 600/1200nm 时特别增加。给出了各种波长下 TPA 以及 TPE 光谱的 Cars 贡献的估计值。
