Correlation between the Hydrogen-Bond Structures and the CO Stretching Frequencies of Carboxylic Acids as Studied by Density Functional Theory Calculations: Theoretical Basis for Interpretatio

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• 作者：Ken-ichi Takei ; Ryouta Takahashi ; Takumi Noguchi
• 刊名：Journal of Physical Chemistry B
• 出版年：2008
• 出版时间：May 29, 2008
• 年：2008
• 卷：112
• 期：21
• 页码：6725 - 6731
• 全文大小：584K
• 年卷期：v.112,no.21(May 29, 2008)
• ISSN：1520-5207

文摘

Carboxylic groups (COOH) of Asp and Glu side chains often function as key components in enzymatic reactions, and identifying their H-bond structures in the active sites is essential for understanding the reaction mechanisms. In this study, the correlation between the H-bond structures and the Crc="http://pubs.acs.org/images/entities/dbd_2.gif">O stretching (νCrc="http://pubs.acs.org/images/entities/dbd_2.gif">O) frequencies of COOH groups was studied using density functional theory calculations. The νCrc="http://pubs.acs.org/images/entities/dbd_2.gif">O frequencies and their shifts upon OH deuteration were calculated for model complexes of acetic acid and propionic acid H bonded at different sites with various compounds. Calculation results together with some experimental data showed that, upon direct H bonding at the Crc="http://pubs.acs.org/images/entities/dbd_2.gif">O group, the νCrc="http://pubs.acs.org/images/entities/dbd_2.gif">O frequencies downshift from the free value (1770−1780 cm⁻¹ in an Ar matrix) to 1745−1760 cm⁻¹, while H bonding at the OH hydrogen induce even larger downshifts to provide the frequencies at 1720−1745 cm⁻¹. In contrast, when the COH oxygen is H-bonded, the νCrc="http://pubs.acs.org/images/entities/dbd_2.gif">O frequencies upshift to 1785−1800 cm⁻¹. In double and multiple H-bond forms, H-bonding effects at individual sites are basically additive, and complexes in which the Crc="http://pubs.acs.org/images/entities/dbd_2.gif">O and the OH hydrogen are simultaneously H bonded exhibit significantly low νCrc="http://pubs.acs.org/images/entities/dbd_2.gif">O frequencies at 1725−1700 cm⁻¹, while complexes H bonded at the oxygen of the COH in addition to either at the Crc="http://pubs.acs.org/images/entities/dbd_2.gif">O or the OH hydrogen exhibit medium frequencies of 1740−1765 cm⁻¹. The νCrc="http://pubs.acs.org/images/entities/dbd_2.gif">O frequencies linearly correlate with the Crc="http://pubs.acs.org/images/entities/dbd_2.gif">O lengths, which are changed by H bonding at different sites. Upon OH deuteration, all the complexes showed νCrc="http://pubs.acs.org/images/entities/dbd_2.gif">O downshifts mostly by ~10 cm⁻¹ and in some cases as large as ~20 cm⁻¹, and hence deuteration-induced downshifts can be a good indicator, irrespective of H-bond forms, for assignments of the νCrc="http://pubs.acs.org/images/entities/dbd_2.gif">O bands of carboxylic groups. The results in this study provide the criteria for determining the H-bond structures of Asp and Glu side chains in proteins using their νCrc="http://pubs.acs.org/images/entities/dbd_2.gif">O bands in Fourier transform infrared spectra.