The effect of metal binding on the characteristic infrared band intensities of ligands of biological interest

Since ∼40% of all proteins contain metal ions that perform a wide variety of functions indispensable for life processes, a plethora of experimental and theoretical methods have probed the interactions between metal ions and protein residues. Notably, previous studies have focused on how metal binding affects the protein ligands’ vibrational frequencies, but not the corresponding intensities (to the best of our knowledge), even though the latter are generally much more sensitive to structural and environmental alterations than the former. Hence, we have systematically evaluated how metal binding affects not only the protein ligands’ vibrational frequencies, but also the corresponding intensities and how these IR parameters depend on properties of the metal and its host ligand using density functional calculations. The results reveal that for a given protein ligand, the IR intensities, particularly those corresponding to bands without vibrational mode mixing, are quite sensitive to metal binding, and their changes are dictated by the metal’s charge-accepting ability and the ligand’s denticity. The results suggest that IR band intensities, along with vibrational frequencies, may be used to help interpret the IR spectra of protein binding sites in the metal-free and metal-bound states.

► We study the effect of metal binding on IR intensities of amino acid models.
► The role of the metal charge-accepting ability and degree of hydration is assessed.
► The role of the ligand charge-donating ability and denticity is evaluated as well.
► The protein ligands’ IR intensities are quite sensitive to metal binding.
► The IR intensities can be used as sensitive markers in metalloprotein spectroscopy.