Correlation between C=O Stretching Vibrational Frequency and pK(a) Shift of Carboxylic Acids

[Image: see text] Identifying the pK(a) values of aspartic acid (Asp) and glutamic acid (Glu) in active sites is essential for understanding enzyme reaction mechanisms. In this study, we investigated the correlation between the C=O stretching vibrational frequency (ν(C=O)) of protonated carboxylic acids and the pK(a) values using density functional theory calculations. In unsaturated carboxylic acids (e.g., benzoic acid analogues), ν(C=O) decreases as the pK(a) increases (the negative correlation), whereas in saturated carboxylic acids (e.g., acetic acid analogues, Asp, and Glu), ν(C=O) increases as the pK(a) increases (the positive correlation) as long as the structure of the H-bond network around the acid is identical. The negative/positive correlation between ν(C=O) and pK(a) can be rationalized by the presence or absence of the C=C double bond. The pK(a) shift was estimated from the ν(C=O) shift of Asp and Glu in proteins on the basis of the negative correlation derived from benzoic acids. The previous estimations should be revisited by using the positive correlation derived in this study, as demonstrated by quantum mechanical/molecular mechanical calculations of ν(C=O) and electrostatic calculations of pK(a) on a key Asp85 in the proton-transfer pathway of bacteriorhodopsin.