CH Mode Mixing Determines the Band Shape of the Carboxylate Symmetric Stretch in Apo-EDTA, Ca2+-EDTA, and Mg2+-EDTA.

The infrared spectra of EDTA complexed with Ca2+ and Mg2+ contain, to date, unidentified vibrational bands. This study assigns the peaks in the linear and two-dimensional infrared spectra of EDTA, with and without either Ca2+ or Mg2+ ions. Two-dimensional infrared spectroscopy and DFT calculations reveal that, in both the presence and absence of ions, the carboxylate symmetric stretch and the terminal CH bending vibrations mix. We introduce a method to calculate participation coefficients that quantify the contribution of the carboxylate symmetric stretch, CH wag, CH twist, and CH scissor in the 1400-1550 cm-1 region. With the help of participation coefficients, we assign the 1400-1430 cm-1 region to the carboxylate symmetric stretch, which can mix with CH modes. We assign the 1000-1380 cm-1 region to CH twist modes, the 1380-1430 cm-1 region to wag modes, and the 1420-1650 cm-1 region to scissor modes. The difference in binding geometry between the carboxylate-Ca2+ and carboxylate-Mg2+ complex manifests as new diagonal and cross-peaks between the mixed modes in the two complexes. The small Mg2+ ion binds EDTA tighter than the Ca2+ ion, which causes a redshift of the COO symmetric stretches of the sagittal carboxylates. Energy decomposition analysis further characterizes the importance of electrostatics and deformation energy in the bound complexes.