Infrared spectroscopic study of the metal-coordination structures of calcium-binding proteins

Carboxylate (COO−) groups can coordinate to metal ions in of the following four modes: 'unidentate', 'bidentate', 'bridging' and 'pseudo-bridging' modes. COO− stretching frequencies provide information about the coordination modes of COO− groups to metal ions. We review the Fourier-transform infrared spectroscopy (FTIR) of side-chain COO− groups of Ca2+-binding proteins: pike parvalbumin pI 4.10, bovine calmodulin and Akazara scallop troponin C. FTIR spectroscopy of Akazara scallop troponin C has demonstrated that the coordination structure of Mg2+ is distinctly different from that of Ca2+ in the Ca2+-binding site. The assignments of the COO− antisymmetric stretch have been ensured on the basis of the spectra of calcium-binding peptide analogues. The downshift of the COO− antisymmetric stretching mode from 1565 cm-1 to 1555-1540 cm−1 upon Ca2+ binding is a commonly observed feature of FTIR spectra for EF-hand proteins.