Modeling the Histidine鈥揚henylalanine Interaction: The NH路路路蟺 Hydrogen Bond of Imidazole路Benzene

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• 作者：Maria A. Trachsel ; Philipp Ottiger ; Hans-Martin Frey ; Chantal Pfaffen ; Angela Bihlmeier ; Wim Klopper ; Samuel Leutwyler
• 刊名：Journal of Physical Chemistry B
• 出版年：2015
• 出版时间：June 25, 2015
• 年：2015
• 卷：119
• 期：25
• 页码：7778-7790
• 全文大小：587K
• ISSN：1520-5207

文摘

NH路路路蟺 hydrogen bonds occur frequently between the amino acid side groups in proteins and peptides. Data-mining studies of protein crystals find that 鈭?0% of the T-shaped histidine路路路aromatic contacts are CH路路路蟺, and only 鈭?0% are NH路路路蟺 interactions. We investigated the infrared (IR) and ultraviolet (UV) spectra of the supersonic-jet-cooled imidazole路benzene (Im路Bz) complex as a model for the NH路路路蟺 interaction between histidine and phenylalanine. Ground- and excited-state dispersion-corrected density functional calculations and correlated methods (SCS-MP2 and SCS-CC2) predict that Im路Bz has a C_s-symmetric T-shaped minimum-energy structure with an NH路路路蟺 hydrogen bond to the Bz ring; the NH bond is tilted 12掳 away from the Bz C₆ axis. IR depletion spectra support the T-shaped geometry: The NH stretch vibrational fundamental is red shifted by 鈭?3 cm^鈥? relative to that of bare imidazole at 3518 cm^鈥?, indicating a moderately strong NH路路路蟺 interaction. While the S₀(A_1g) 鈫?S₁(B_2u) origin of benzene at 38鈥?86 cm^鈥? is forbidden in the gas phase, Im路Bz exhibits a moderately intense S₀ 鈫?S₁ origin, which appears via the D_6h 鈫?C_s symmetry lowering of Bz by its interaction with imidazole. The NH路路路蟺 ground-state hydrogen bond is strong, D_e=22.7 kJ/mol (1899 cm^鈥?). The combination of gas-phase UV and IR spectra confirms the theoretical predictions that the optimum Im路Bz geometry is T shaped and NH路路路蟺 hydrogen bonded. We find no experimental evidence for a CH路路路蟺 hydrogen-bonded ground-state isomer of Im路Bz. The optimum NH路路路蟺 geometry of the Im路Bz complex is very different from the majority of the histidine路aromatic contact geometries found in protein database analyses, implying that the CH路路路蟺 contacts observed in these searches do not arise from favorable binding interactions but merely from protein side-chain folding and crystal-packing constraints. The UV and IR spectra of the imidazole路(benzene)₂ cluster are observed via fragmentation into the Im路Bz⁺ mass channel. The spectra of Im路Bz and Im路Bz₂ are cleanly separable by IR hole burning. The UV spectrum of Im路Bz₂ exhibits two 0₀⁰ bands corresponding to the S₀ 鈫?S₁ excitations of the two inequivalent benzenes, which are symmetrically shifted by 鈭?6/+88 cm^鈥? relative to the 0₀⁰ band of benzene.