Hydride stretch infrared spectra in the excited electronic states of indole and its derivatives:: Direct evidence for the 1πσ* state
Hydride stretch infrared spectra in the excited electronic states of indole and its derivatives:: Direct evidence for the 1πσ* state
复制标题
DOI：
10.1063/1.1536616
复制
发表时间：
2003-02-08
影响因子：
4.4
通讯作者：
Zwier, TS
中科院分区：
文献类型：
--
作者：
Dian, BC;Longarte, A;Zwier, TS
用激发态荧光浸渍红外光谱法记录了色胺（TRA）的主要构象体吲哚、吲哚-H_2O、3-甲基吲哚、3-甲基吲哚-H_2O、N-乙酰基色氨酸酰胺（NATA）的两种构象体和N-乙酰基色氨酸甲酰胺（NATMA）的三种构象体的电子激发单重态氢化物伸缩红外光谱。NATA和NATMA是色氨酸的甲基封端二肽，其具有构象灵活性，并且在其电子光谱中对二肽骨架的构象表现出敏感性。在吲哚单体中，S-1原点的吲哚NH伸缩基频从基态值（3525 cm（-1））移动到3478 cm（-1）。吲哚-H2O复合物中相应的谱带出现在3387 cm（-1），从基态位置（3436 cm（-1））移动了类似的量。S-1起源1500 cm（-1）范围内的较高振动能级，之前已确定[B. J. Fender，Chem. Phys. Lett. 239，31（1995）]，在性质上为L-1（B）或L-1（a），都显示出相似的激发态吲哚NH伸缩振动。3-甲基吲哚、3-甲基吲哚-H_2O、TRA以及NATA和NATMA的C_5构象的相应光谱都缺少吲哚NH伸缩吸收。取而代之的是一个宽的背景吸收，分布在整个2800-3800 cm（-1）区域。在这些分子中，其他CH伸缩或酰胺NH伸缩振动保持尖锐，出现在其预期的频率范围内。最后，在二肽骨架中具有分子内氢键的NATA和NATMA的C7构象具有所有红外跃迁，被更强的宽背景吸收所取代。整个数据集可以通过沿着吲哚NH伸缩坐标解离的激发（1）π σ（*）状态的存在来解释，如最近由Sobolewski和Domcke [Chem.Phys.Lett. 315，293（1999）]。在弱耦合情况下（吲哚，吲哚-H2O），（1）pisigma* 态和S-1原点之间的差距太大，红外激发无法达到。在中间耦合情况下吲哚NH伸缩的选择性损失反映了L-1（B）态NH伸缩（v=1）能级与（1）pisigma* 态的强耦合，该pisigma* 态沿NH伸缩坐标沿着是离解的.在NATA和NATMA C7构象中，电子态的顺序发生反转，将L-1（a）态推到L-1（B）原点以下，并加强所有氢化物伸缩振动能级与（1）pisigma* 解离连续谱的耦合。这些结果突出了多肽骨架的构象对多肽中色氨酸的物理性质的重要影响。(C)2003年，美国物理学会。
The hydride stretch infrared spectra of indole, indole-H2O, 3-methyl indole, 3-methyl indole-H2O, the main conformer of tryptamine (TRA), two conformers of N-acetyl tryptophan amide (NATA), and three conformers of N-acetyl tryptophan methyl amide (NATMA), have been recorded in the electronically excited singlet states using excited-state fluorescence-dip infrared spectroscopy. NATA and NATMA are methyl-capped dipeptides of tryptophan that have conformational flexibility and exhibit sensitivity in their electronic spectra to the conformation of the dipeptide backbone. In the indole monomer, the indole NH stretch fundamental at the S-1 origin is shifted from its ground-state value (3525 cm(-1)) to 3478 cm(-1). The corresponding band in the indole-H2O complex appears at 3387 cm(-1), shifted by a similar amount from its ground-state position (3436 cm(-1)). Higher vibronic levels within 1500 cm(-1) of the S-1 origin, which have been identified previously [B. J. Fender , Chem. Phys. Lett. 239, 31 (1995)] as being L-1(b) or L-1(a) in character, all show similar excited state indole NH stretch absorptions. The corresponding spectra in 3-methyl indole, 3-methyl indole-H2O, TRA, and in the C5 conformers of NATA and NATMA all are missing the indole NH stretch absorption. In its place, a broad background absorption appears, spread over the entire 2800-3800 cm(-1) region. In these molecules, other CH stretch or amide NH stretch absorptions remain sharp, appearing in their expected frequency ranges. Finally, the C7 conformations of NATA and NATMA, which possess an intramolecular hydrogen bond in the dipeptide backbone, have all infrared transitions washed out, replaced by a stronger broad background absorption. The entire data set can be explained by the presence of an excited (1)pisigma(*) state which is dissociative along the indole NH stretch coordinate, as recently predicted by Sobolewski and Domcke [Chem. Phys. Lett. 315, 293 (1999)]. In the weak coupling case (indole, indole-H2O), the gap between the (1)pisigma* state and the S-1 origin is too large to be reached by infrared excitation. The selective loss of the indole NH stretch in the intermediate coupling case reflects the strong coupling of the L-1(b) state NH stretch (v=1) level to the (1)pisigma* state, which is dissociative along the NH stretch coordinate. In the NATA and NATMA C7 conformers, an inversion of ordering of the electronic states occurs, pushing the L-1(a) state below the L-1(b) origin, and strengthening the coupling of all hydride stretch vibrational levels to the (1)pisigma* dissociative continuum. These results highlight the important influence of the conformation of the polypeptide backbone on the photophysics of tryptophan in polypeptides. (C) 2003 American Institute of Physics.