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 characterizes the importance of electrostatics and deformation energy in the bound complexes.

Automated summary

This study analyzed the vibrational characteristics of EDTA complexed with Ca2+ and Mg2+ using two-dimensional infrared spectroscopy and DFT calculations, introducing a method to calculate participation coefficients to quantify the contribution of different vibration modes. The differences in binding geometry and energy characteristics between the two complexes were also investigated.