Real-time detection of Cu2+ sequestration and release by immobilized apo-metallothioneins using SECM combined with SPR

Scanning electrochemical microscopy (SECM) combined with surface plasmon resonance (SPR), SECM–SPR, was applied for real-time detection of the incorporation of Cu2+ by apo-metallothionein (apo-MT) immobilized on the SPR substrate and release of Cu2+ from surface-confined metallothionein (MT). Cu2+ anodically stripped from a Cu-coated SECM Au tip was sequestered by apo-MT upon its diffusion to the SPR substrate, and release of Cu2+ by MT was accomplished by generating protons via oxidation of hydroquinone at the tip. The high sensitivity of the SPR instrument is capable of following the structural and compositional changes of MT molecules during the metal sequestration and release processes. Due to the enhanced mass transfer rate at the SECM tip, the complication of mass transfer limitation on kinetic measurements, commonly encountered in flow injection SPR, is circumvented. The time-resolved SPR response reveals stepwise changes among three stable MT structures and allows the number of copper ions coordinated in each structure to be determined. The numbers of copper ions incorporated by each MT molecule in the three structures were determined to be 5, 9, and 12. This work expands the SECM–SPR approach to assessments of the dynamics and affinity of binding of small ions to surface-confined proteins and to studies of proteins that do not undergo facile electron transfer reactions.