The broken ring: reduced aromaticity in Lys-Trp cations and high pH tautomer correlates with lower quantum yield and shorter lifetimes.
The broken ring: reduced aromaticity in Lys-Trp cations and high pH tautomer correlates with lower quantum yield and shorter lifetimes.
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DOI：
10.1021/jp503355h
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发表时间：
2014-06-26
期刊：
影响因子：
--
通讯作者：
Juszczak LJ
中科院分区：
文献类型：
--
作者：
Eisenberg AS;Juszczak LJ
几个非辐射过程与色氨酸荧光发射竞争。光谱解释的困难在于将特定的分子环境特征与这些过程相关联，从而利用荧光光谱数据来识别色氨酸的局部环境。在这里，光谱和分子模拟研究Lys-Trp二肽带电物种表明，骨架环相互作用是不可分辨的。相反，量子力学基态等值面揭示了吲哚π电子分布和密度的变化，其平行于主链和残基上的电荷（作为pK 1、pK 2和pKR的函数）。芳香相关的量子产率和荧光寿命的变化出现的模式。当量子产率高时，等值面具有与吲哚的最高占据分子轨道（HOMO）相似的电荷分布，吲哚是1 La跃迁偶极矩的主要荧光基态。在量子产率低的地方，环上的等值面电荷分布是不均匀的，减少的，甚至在环外发现。在pH 13时，吲哚胺被去质子化，并且由于将电荷集中在吲哚胺上的互变异构体结构，Lys-Trp量子产率极低;等值面电荷分布与吲哚HOMO几乎不相似。已经在吲哚氮是氢键的蛋白质中观察到这种大大减弱的荧光，这也证明了蛋白质中芳香性变化与量子产率降低的关联。因此色氨酸基态等值面是吲哚芳香性的指示剂，发出辐射和非辐射过程之间的激发能分配的信号。
Several nonradiative processes compete with tryptophan fluorescence emission. The difficulty in spectral interpretation lies in associating specific molecular environmental features with these processes and thereby utilizing the fluorescence spectral data to identify the local environment of tryptophan. Here, spectroscopic and molecular modeling study of Lys-Trp dipeptide charged species shows that backbone-ring interactions are undistinguished. Instead, quantum mechanical ground state isosurfaces reveal variations in indole π electron distribution and density that parallel charge (as a function of pK1, pK2, and pKR) on the backbone and residues. A pattern of aromaticity-associated quantum yield and fluorescence lifetime changes emerges. Where quantum yield is high, isosurfaces have a charge distribution similar to the highest occupied molecular orbital (HOMO) of indole, which is the dominant fluorescent ground state of the 1La transition dipole moment. Where quantum yield is low, isosurface charge distribution over the ring is uneven, diminished, and even found off ring. At pH 13, the indole amine is deprotonated, and Lys-Trp quantum yield is extremely low due to tautomer structure that concentrates charge on the indole amine; the isosurface charge distribution bears scant resemblance to the indole HOMO. Such greatly diminished fluorescence has been observed for proteins where the indole nitrogen is hydrogen bonded, lending credence to the association of aromaticity changes with diminished quantum yield in proteins as well. Thus tryptophan ground state isosurfaces are an indicator of indole aromaticity, signaling the partition of excitation energy between radiative and nonradiative processes.
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