Hydrogen and Deuterium NMR Studies of Carboxylate Coordination to Iron(III) Complexes: Diverse Chemical Shift Values for Coordinated Carboxyl Residues

Proton and deuterium NMR signals for coordinated carboxyl residues of high-spin iron(III) and manganese(III) porphyrin complexes have been unambiguously assigned through examination of the deuteriated carboxylate derivatives. Signals are shifted downfield in the region between 15 and 32 ppm for these paramagnetic compounds contained in chlorinated solvents. However, this chemical shift value is attenuated with respect to that observed at 140 ppm for the acetate proton signal of the (N,N'~ ethylenebis(salicylideneaminato))(acetato)iron(III) Schiff base complex. Other non-heme iron(III) carboxylate complexes also give coordinated acetate proton signals in the 140 ppm region. The metalloporphyrin acetate signal is highly solvent dependent, and the signal does not follow Curie law behavior in variable-temperature experiments. Specific solvation effects for the carbonyl residue are suggested. These effects and the differing magnitudes for carboxylate chemical shifts in iron(III) porphyrin complexes and non-heme iron(III) complexes suggest monodentate carboxylate coordination to the iron(III) porphyrin and bidentate coordination in the other iron(III) complexes investigated. Given the wide range of carboxyl chemical shift values, identification of amino acid carboxylate binding in metalloproteins by NMR chemical shift values alone is thus highly problematic.