Kinetic Analysis of a Photosensitive Chelator and its Complex with Zn(II)¶

Abstract

The reversible sequestration and release of metal ions is an important objective in biological and environmental research. Unfortunately, although there have been dramatic examples of metal ion activity control, there are very few quantitative investigations of stoichiometry, equilibria and kinetics. A significant contributor to this lack of quantitative work is the complexity of many photochromic systems. Therefore, we have attempted to create a simple, reversible photochromic metal-ion chelator that can be analyzed quantitatively. The chelator should have certain other attributes as well, namely, that it binds to divalent metal ions (because of their extreme biological importance) and that it binds metal ions in the dark so that light is used to release metal ions rather than sequester them. The photochromic chelator (1) binds to divalent metal ions [Zn(II), Cu(II), Pb(II), Hg(II), Fe(II), Co(II) and Cd(II); other metal ions have not yet been tested] in the dark with a significant binding strength. In both methanol (by spectrophotometry) and methanol-water (by voltammetry), the stoichiometry of the 1-Zn(II) complex is 2:1. The binding constant (K1K2) is on the order of 10(12)-10(14) M(-2) in methanol and 5.0 x 10(8) M(-2) in 50% aqueous methanol. The chelator 1 is photolabile, yielding 2 with a quantum efficiency of 0.91. In a solution containing excess Zn(II), so that over 99% of the ligand exists as the monodentate complex, photolysis produces 2 with a quantum efficiency of 0.15. A kinetic analysis leads to the conclusion that the complex itself is photolabile.

Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy

“…14 The present review covers recent developments in the field, focusing on the scope, limitations, and applications of PPGs, which are used to release organic molecules. Photoactivation of small inorganic species and ions, such as NO, 15 CO, 16 Ca 2+ , 9h,17 Zn 2+ , 18 Cd 2+ , 19 or Cu + , 20 is not covered. Simplified basic structures of the photoremovable protecting groups discussed in this review are listed in Table 1 (the leaving groups are shown in red).…”

Synthetic Biological Approaches for Optogenetics and Tools for Transcriptional Light‐Control in Bacteria

“…Chelators for other divalent metal ions have also been developed. [ 31 ]…”

Measurement of Association and Dissociation Rate Constants for Lead(II)/18-Crown-6 Using Square Wave Voltammetry at a Glassy Carbon Mercury Film Electrode

“…18-Crown-6 and other crowns have been used in high-resolution separations of metal ions. − Chemical rates are needed for understanding band broadening. Crowns and other metal ion chelators are key elements in photoswitching schemes. − Very little kinetic information exists for these systems. , Stephens et al have used kinetic, equilibria and stoichiometric data to model metal ion binding to a photochromic molecule containing an iminodiacetic acid chelator. Knowledge of the metal−ligand complex association- and dissociation-required rate constants were needed to prove that light could force the dissociation of the metal ion complex.…”

“…SWV is a modern electroanalytical technique noted for its sensitivity and high discrimination against charging currents This theory has been used successfully to study metal ion−ligand complexation at a hanging mercury drop electrode (HMDE). , Data are in excellent agreement with theory, but this electrode is not without limitations; for example, the HMDE is unstable in organic solvents and violates regulations against use of bulk mercury. An alternative mercury-based electrode is the glassy carbon mercury film electrode (GCMFE).…”

“…[8][9][10] Very little kinetic information exists for these systems. 11,12 Stephens et al 12 have used kinetic, equilibria and stoichiometric data to model metal ion binding to a photochromic molecule containing an iminodiacetic acid chelator. Knowledge of the metal-ligand complex association-and dissociation-required rate constants were needed to prove that light could force the dissociation of the metal ion complex.…”