Spectral (DFT-IR, FT-IR and UV) similarities and differences between substrate (pyruvate) and inhibitor (oxamate) of lactic dehydrogenase (LDH)

FT-IR (in KBr disk) and UV (in aqueous solution) spectra were recorded for lithium, sodium, potassium and/or guanidinium pyruvates and oxamates at room temperature. In parallel, quantum-chemical calculations were performed and vibrational frequencies computed at the DFT(B3LYP)/6-31++G** level. In the experimental FT-IR spectra of solid salts, positions and intensities of the ν(αCO) and νas(COO) bands depend on the cation and also on the anion. Oxamates are intermolecularly associated, while pyruvates are not H-bonded. The Li+, Na+ and K+ cations interact preferentially with oxygens of the COO group in pyruvates and with oxygens of the αCO and COO groups in oxamates. In the UV spectra, cations {H+, Li+, Na+, K+ and (H2N)3C+} have slight influence on the position of the n−π* band indicating the same type of interactions in hydrated salts.