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 interpretation of infrared bands of carboxylic groups in proteins.

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 C=O stretching (nuC=O) frequencies of COOH groups was studied using density functional theory calculations. The nuC=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 C=O group, the nuC=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 nuC=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 C=O and the OH hydrogen are simultaneously H bonded exhibit significantly low nuC=O frequencies at 1725-1700 cm(-1), while complexes H bonded at the oxygen of the COH in addition to either at the C=O or the OH hydrogen exhibit medium frequencies of 1740-1765 cm(-1). The nuC=O frequencies linearly correlate with the C=O lengths, which are changed by H bonding at different sites. Upon OH deuteration, all the complexes showed nuC=O downshifts mostly by approximately 10 cm(-1) and in some cases as large as approximately 20 cm(-1), and hence deuteration-induced downshifts can be a good indicator, irrespective of H-bond forms, for assignments of the nuC=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 nuC=O bands in Fourier transform infrared spectra.