A Chromatographic Technique To Investigate the Lability of Copper Complexes under Steady-State Conditions Using High Specific Activity 64Cu

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Abstract
A novel chromatographic technique is described which has prospects for studying the lability of a metal complex in an aqueous system. It is based on interactions of metal species with an ion-exchange column under steady-state conditions. For this purpose, the column is equilibrated with the sample itself by using it as the mobile phase. With the aid of a high specific activity radiotracer, the characteristics of the metal species interaction with the ion-exchange column can be visualized in a radiochromatogram. Under particular experimental conditions, information regarding the dissociation kinetics of the metal complex can be extracted from this radiochromatogram. Erroneous results due to undesirable interactions of metal species with the chromatographic system are prevented, since the system remains in constant chemical equilibrium with the sample to be analyzed. The potential of this technique was investigated with copper complexes, using high specific activity 64Cu for labeling. Preliminary results obtained with four complexes (Cu−EDTA, Cu−NTA, Cu−citrate, and Cu−glycine) are discussed. For three complexes, dissociation rate constants could be determined: (6.2 ± 0.3) × 10-3 s-1 for Cu−EDTA, (1.0 ± 0.04) × 10-2 s-1 for Cu−NTA, and (3.1 ± 0.2) × 10-2 s-1 for Cu−citrate. No dissociation rate constant could be determined for Cu−glycine owing to incompatible experimental conditions.