pH-dependent formation of ethylenediaminetetraacetic acid supramolecular aggregates.

Pure ethylenediaminetetraacetic acid tetrasodium salt (EDTA.Na4) in water showed a pH-dependent change of optical density detected in the ultraviolet region. The most important change was found between pH 4 and 8, corresponding to the deprotonation of the third ionizable group having a pK of 6.2. The absorbance change was reversible and depended on the addition/removal of only 1 proton equivalent. The presence of supramolecular aggregates was suggested by size-exclusion chromatography, dialysis and cryo-electronmicroscopy. The molecular weight of EDTA supramolecular aggregates was 10-15,000 and 8-14,000, as determined by gel chromatography and dialysis, respectively. Cryo-electronmicroscopy revealed structures of 2-4 nm in diameter.