CAGED CATIONS

The molecular concept of caged cations is illustrated on the basis of results obtained on coordination complexes between photo-cleavable, ion-selective, macrobicyclic ligands of the cryptand type and alkali as well as alkaline earth cations. Ligand splitting due to a short UV-light pulse leads to cation liberation in the ms time range as a consequence of the resulting affinity decrease. Such cation concentration jumps can be performed in microvolumes and without any mechanical or pressure perturbances. They can be applied to shift the equilibrium position of, for example, biochemical binding or reaction systems involving the released cation. If the readjustment of the state of equilibrium is slower than the cation release and if the cation concentration jump is small, the kinetic parameters of the system can be determined by applying the chemical relaxation methodology, as shown for alkali ion binding to the membrane transport enzyme Na+,K+-ATPase.