碱性离子与黄素的相互作用：金属-核黄素配合物的红外光谱和光谱,The Journal of Physical Chemistry A

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碱性离子与黄素的相互作用：金属-核黄素配合物的红外光谱和光谱
The Journal of Physical Chemistry A ( IF 2.7 ) Pub Date : 2021-04-08 , DOI: 10.1021/acs.jpca.1c01846
David Müller ₁ , Otto Dopfer ₁

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黄素类化合物因其在催化和光化学过程中的多种功能而在生物化学中备受关注。黄素的固有光学性质敏感地取决于其环境，例如与金属离子的络合。在这里，我们表征了碱金属离子（M ⁺）与核黄素（RF，维生素B ₂）的相互作用。为此，通过与PBE0 / cc-pVDZ水平的密度泛函理论（DFT）计算相比较，采用了两种不同的实验光谱方法来确定M ⁺ RF配合物的结构，振动，能量和光学性质。首先，室温下记录的红外多光子离解（IRMPD）光谱表明，M ⁺结合到RF的两个可用亲核羰基（CO2，CO4）之一，表示为O2和O4 +异构体，如金属化过程中CO拉伸模式的特征性位移所揭示。其次，通过可见光解离（VISPD）在保持在6 K的低温离子阱中，在428和529 nm之间记录K ⁺ RF的光谱。通过与时间相关的DFT计算以及Franck–Condon模拟对VISPD光谱进行分析，结果表明，实际上只有M ⁺ RF的O2异构体是通过电喷雾电离形成的，而O4 +异构体的光谱特征却不存在。VISPD频谱归因于S ₁ ←S ₀O2异构体的（ππ*）跃迁比O4 +异构体稳定得多，这是因为M ⁺与RF的N10处的核糖基（糖）侧链的OH基团发生了额外的多重相互作用。相反，没有证据表明存在O4 +异构体，其中M ⁺形成螯合物，M ⁺与O4和N5均结合。RF（在N10处为骨架）和Lumiflavin（在N10处为LF和CH ₃）之间的比较揭示了侧链对黄素与金属离子相互作用的结构，能量和光学性质的剧烈影响。对于M ⁺ LF，O2和O4 +异构体的能量接近，并且都是通过实验观察到的，对于M ⁺RF由于其与侧链的额外相互作用而强烈推荐使用O2异构体。尽管M ⁺ RF（O2）和M ⁺ LF（O2）的S ₁能量非常相似，但由于两种黄素的ππ*跃迁均位于同一异四恶嗪生色团上，因此振动结构存在很大差异，因为软弯曲势N 10处的核糖基侧链强烈影响M ⁺ ...黄素的相互作用。与倾向于在N1质子化的H ⁺ RF相反，具有核键侧链的较大M ⁺离子的空间排斥会阻止N1处的金属化，从而导致形成O2全局最小值。

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Interaction of Alkali Ions with Flavins: Infrared and Optical Spectra of Metal–Riboflavin Complexes

Flavin compounds are of great interest in biochemistry because of their diverse functions in catalytic and photochemical processes. The intrinsic optical properties of flavins depend sensitively on their environment such as complexation with metal ions. Herein, we characterize the interaction of alkali metal ions (M⁺) with riboflavin (RF, vitamin B₂). To this end, two different experimental spectroscopic approaches are employed to determine the structural, vibrational, energetic, and optical properties of M⁺RF complexes by comparison with density functional theory (DFT) calculations at the PBE0/cc-pVDZ level. First, infrared multiple photon dissociation (IRMPD) spectra recorded at room temperature demonstrate that M⁺ binds to one of the two available nucleophilic carbonyl groups (CO2, CO4) of RF, denoted O2 and O4+ isomers, as revealed by characteristic shifts of the CO stretch modes upon metalation. Second, the optical spectrum of K⁺RF is recorded between 428 and 529 nm in a cryogenic ion trap held at 6 K by visible photodissociation (VISPD). Analysis of the VISPD spectrum by time-dependent DFT calculations coupled to Franck–Condon simulations demonstrates that in fact only the O2 isomer of M⁺RF is formed by electrospray ionization, while the spectroscopic signatures of the O4+ isomer are absent. The VISPD spectrum is attributed to the S₁ ← S₀ (ππ*) transition of the O2 isomer, which is calculated to be much more stable than the O4+ isomer because of additional multiple interactions of M⁺ with the OH groups of the ribityl (sugar) side chain attached at N10 of RF. In contrast, there is no evidence for the presence of the O4+ isomer, in which M⁺ forms a chelate complex, with M⁺ binding to both O4 and N5. A comparison between RF (ribityl at N10) and lumiflavin (LF and CH₃ at N10) reveals the drastic effects of the side chain on the structural, energetic, and optical properties of the flavin interaction with metal ions. While for M⁺LF the O2 and O4+ isomers are close in energy and both observed experimentally, for M⁺RF the O2 isomer is strongly favored due to the additional interaction with the side chain. Although the S₁ energies of M⁺RF(O2) and M⁺LF(O2) are quite similar, because the ππ* transition is localized on the same isoalloxazine chromophore for both flavins, the vibrational structures are strongly different because the soft bending potential for the M⁺···flavin interaction is strongly affected by the ribityl side chain at N10. In contrast to H⁺RF, which prefers protonation at N1, steric repulsion of the larger M⁺ ions with the ribityl side chain prevents metalation at N1, leading to the formation of the O2 global minimum.

更新日期：2021-04-22

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