A Zinc(II) Photocage Based on a Decarboxylation Metal Ion Release Mechanism for Investigating Homeostasis and Biological Signaling

Abstract

Metal ion signaling in biology has been studied extensively with ortho-nitrobenzyl photocages; however, the low quantum yields and other optical properties are not ideal for these applications. We describe the synthesis and characterization of NTAdeCage, the first member in a new class of Zn(2+) photocages that utilizes a light-driven decarboxylation reaction in the metal ion release mechanism. NTAdeCage binds Zn(2+) with sub-pM affinity using a modified nitrilotriacetate chelator and exhibits an almost 6 order of magnitude decrease in metal binding affinity upon uncaging. In contrast to other metal ion photocages, NTAdeCage and the corresponding Zn(2+) complex undergo efficient photolysis with quantum yields approaching 30 %. The ability of NTAdeCage to mediate the uptake of (65) Zn(2+) by Xenopus laevis oocytes expressing hZIP4 demonstrates the viability of this photocaging strategy to execute biological assays.

Keywords: X-ray diffraction; cage compounds; coordination compounds; photolysis; zinc.

Figure 1

Uncaging action of [Zn(NTAdeCage)]⁻. Photodecarboxylation followed by deamination produces both m-nitrobenzaldehyde and a more weakly Zn²⁺-binding iminodiaceticacid (IDA) ligand. The NTAdeCage nomenclature refers to the nitrilotriacetate (NTA) chelator as well as the decarboxylation mechanism of the metal ion photocage.

Figure 2

ORTEP diagram of [Cu(6)(OH₂)₂]·H₂O showing 50% thermal ellipsoids and selected atom labels. The unbound water molecule and the hydrogen atoms are omitted for clarity, except H11 that designates the protonated carboxylate ligand. The axial carboxylate carbonyl oxygen atom O3 and water oxygen atom O9 exhibit longer Cu bonds than the equatorial ligands owing to Jahn–Teller distortion.

Figure 3

Photolysis of [Zn(NTAdeCage)]⁻ (80 μm) under simulated physiological conditions (40 mm HEPES, 100 mm KCl, pH 7.5). Irradiation at 365 nm (LED, 3 Wcm⁻²) leads to the erosion of the absorption band at 274 nm associated with the photocaged complex and the concurrent formation of a band at 230 nm characteristic of photo-product 2. Inset: change in absorbance at 230 nm with respect to photolysis time in seconds.

Figure 4

⁶⁵Zn²⁺ uptake in oocytes expressing hZIP4. Oocytes were subjected to ⁶⁵Zn²⁺ by incubating them with 250 nm ⁶⁵ZnCl₂ or 250 nm ⁶⁵ZnCl₂ in the presence of 250 μm of NTAdeCage, NTA or IDA. All oocytes except the [Zn(NTAdeCage)]⁻ control set were exposed to 3 min of 365 nm light (LED, 3 Wcm⁻²). The assay was performed as described previously.^[33] Each bar represents the data from the analysis of 9 to 12 oocytes. The uptake values shown are the mean values with standard error.

Scheme 1

Synthesis of NTAdeCage.