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 p/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. (c) 2007 Elsevier Inc. All rights reserved.