Substituent effects on energetics of peptide-carboxylate hydrogen bonds as studied by 1H NMR spectroscopy: implications for enzyme catalysis.

Substituent effects in N-H···O hydrogen bonds were estimated by comparing the acidities of two series of model compounds: N-benzoylanthranilic acids (A) and 4-benzoylamidobenzoic acids (B). Intramolecular N-H···O hydrogen bonds were found to be present in the A series of compounds, while B acids were used as control models. The respective pK(a) values for A and B acids were determined experimentally in DMSO solution using proton NMR spectroscopy. With X = H, the pK(a) for A and B acids were observed to be 7.6 and 11.6, respectively, a difference of 4.0 units (ΔpK(a)). However, with X = p-NO2, the ΔpK(a) value between A and B acids increased to 4.7 units: the pK(a) values for A and B acids were determined as 6.7 and 11.4, respectively. The ΔpK(a) values between A and B acids as a function of the X substituents were studied in 10 other examples. The effects of X substituents in A acids could be predicted on the basis of the observed linear Hammett correlations, and the sensitivity of each substituent effect was found to be comparable to those observed for the ionization of substituted benzoic acids (ρ = 1.04 for A acids, and ρ = 1.00 for benzoic acids).