Characterization of a photoswitching chelator with light-modulated geometric, electronic, and metal-binding properties

Isomerization of bioactive acylhydrazones triggered by light or thiols

The acylhydrazone unit is well represented in screening databases used to find ligands for biological targets, and numerous bioactive acylhydrazones have been reported. However, potential E/Z isomerization of the C=N bond in these compounds is rarely examined when bioactivity is assayed. Here we analysed two ortho-hydroxylated acylhydrazones discovered in a virtual drug screen for modulators of N-methyl-D-aspartate receptors and other bioactive hydroxylated acylhydrazones with structurally defined targets reported in the Protein Data Bank. We found that ionized forms of these compounds, which are populated under laboratory conditions, photoisomerize readily and the isomeric forms have markedly different bioactivity. Furthermore, we show that glutathione, a tripeptide involved with cellular redox balance, catalyses dynamic E⇄Z isomerization of acylhydrazones. The ratio of E to Z isomers in cells is determined by the relative stabilities of the isomers regardless of which isomer was applied. We conclude that E/Z isomerization may be a common feature of the bioactivity observed with acylhydrazones and should be routinely analysed.

Metal-binding hydrazone photoswitches for visible light reactivity and variable relaxation kinetics

The range of applications for photoswitching moieties is diverse, and the ability to design switches with variable photochemical and physical properties is consequently important for realizing their potential. Previously we reported on the photochromism of (E)-N'-(1-(2-hydroxyphenyl)ethylidene)isonicotinohydrazide (HAPI), an aroylhydrazone compound first developed as a transition metal chelator. Herein we report the synthesis of structurally related aroylhydrazone chelators and explore the effect of these modifications on their UVA, UVC and blue light photoreactivity, photostationary state composition, photoisomer thermal stability, and relative iron(iii) binding affinity. These findings will inform the next generation of aroylhydrazone photoswitches for metal-gated photoswitching applications.

Redox Control over Acyl Hydrazone Photoswitches

Photoisomerization provides a clean and efficient way of reversibly altering physical properties of chemical systems and injecting energy into them. These effects have been applied in development of systems such as photoresponsive materials, molecular motors, and photoactivated drugs. Typically, switching from more to less stable isomer(s) is performed by irradiation with UV or visible light, while the reverse process proceeds thermally or by irradiation using another wavelength. In this work we developed a method of rapid and tunable Z→E isomerization of C═N bond in acyl hydrazones, using aromatic thiols as nucleophilic catalysts. As thiols can be oxidized into catalytically inactive disulfides, the isomerization rates can be controlled via the oxidation state of the catalyst, which, together with the UV irradiation, provides orthogonal means to control the E/Z state of the system. As a proof of this concept, we have applied this method to control the diversity of acyl hydrazone based dynamic combinatorial libraries.

Stimulus-Responsive Prochelators for Manipulating Cellular Metals

Metal ions are essential for a wide range of physiological processes, but they can also be toxic if not appropriately regulated by a complex network of metal trafficking proteins. Intervention in cellular metal distribution with small-molecule or peptide chelating agents has promising therapeutic potential to harness metals to fight disease. Molecular outcomes associated with forming metal-chelate interactions in situ include altering the concentration and subcellular metal distribution, inhibiting metalloenzymes, enhancing the reactivity of a metal species to elicit a favorable biological response, or passivating the reactivity of a metal species to prevent deleterious reactivity. The systemic administration of metal chelating agents, however, raises safety concerns due to the potential risks of indiscriminate extraction of metals from critical metalloproteins and inhibition of metalloenzymes. One can estimate that chelators capable of complexing metal ions with dissociation constants in the submicromolar range are thermodynamically capable of extracting metal ions from some metalloproteins and disrupting regular function. Such dissociation constants are easily attainable for multidentate chelators interacting with first-row d-block metal cations in relevant +1, + 2, and +3 oxidation states. To overcome this challenge of indiscriminate metal chelation, we have pursued a prodrug strategy for chelating agents in which the resulting "prochelator" has negligible metal binding affinity until a specific stimulus generates a favorable metal binding site. The prochelator strategy enables conditional metal chelation to occur preferentially in locations affected by disease- or therapy-associated stimuli, thereby minimizing off-target metal chelation. Our design of responsive prochelators encompasses three general approaches of activation: the "removal" approach operates by eliminating a masking group that blocks a potential metal chelation site to reveal the complete binding site under the desired conditions; the molecular "switch" approach involves a reversible conformational change between inactive and active forms of a chelator with differential metal binding affinity under specific conditions; and the "addition" approach adds a new ligand donor arm to the prochelator to constitute a complete metal chelation site. Adopting these approaches, we have created four categories of triggerable prochelators that respond to (1) reactive oxygen species, (2) light, (3) specific enzymes, and (4) biological regulatory events. This Account highlights progress from our group on building prochelators that showcase these four categories of responsive metal chelating agents for manipulating cellular metals. The creation and chemical understanding of such stimulus-responsive prochelators enables exciting applications for understanding the cell biology of metals and for developing therapies based on metal-dependent processes in a variety of diseases.

A multifunctional, light-activated prochelator inhibits UVA-induced oxidative stress

UVA radiation can damage cells and tissues by direct photodamage of biomolecules as well as by initiating metal-catalyzed oxidative stress. In order to alleviate both concerns simultaneously, we synthesized a multifunctional prochelator PC-HAPI (2-((E)-1-(2-isonicotinoylhydrazono)ethyl)phenyl (trans)-3-(2,4-dihydroxyphenyl)acrylate) that contains a trans-(o-hydroxy)cinnamate ester photocleavable protecting group that is cleaved upon UVA exposure to release a coumarin, umbelliferone, and an aroylhydrazone metal chelator, HAPI (N'-[1-(2-hydroxyphenyl)ethyliden]isonicotinoylhydrazide). While the prochelator PC-HAPI exhibits negligible affinity for iron, it responds rapidly to UVA irradiation and converts to an iron-binding chelator that inhibits iron-catalyzed formation of reactive oxygen species and protects cells from UVA damage.

Perspectives in chemistry--aspects of adaptive chemistry and materials

Chemistry, pure and applied, is a science and an industry. By its power over the expressions of matter, it also displays the creativity of art. It has expanded from molecular to supramolecular chemistry and then, by way of constitutional dynamic chemistry, towards adaptive chemistry. Constitutional dynamics allow for adaptation, through component exchange and selection in response to physical stimuli (e.g. light, photoselection), to chemical effectors (e.g. metal ions, metalloselection) or to environmental effects (e.g. phase change) in equilibrium or out-of-equilibrium conditions, towards the generation of the best-adapted/fittest constituent(s) in a dynamic set. Such dynamic systems can be represented by two-dimensional or three-dimensional dynamic networks that define the agonistic and antagonistic relationships between the different constituents linked through component exchange. The introduction of constitutional dynamics into materials science opens perspectives towards adaptive materials and technologies, presenting attractive behavioral features (such as self-healing). In particular, dynamic polymers may undergo modification of their properties (mechanical, optical, etc.) through component exchange and recombination in response to physical or chemical agents. Constitutional adaptive materials open towards a systems materials science and offer numerous opportunities for soft-matter technologies.

Reversible photo-, metallo- and thermo-induced morphological dynamics of bis-acylhydrazones

A reversible interconversion-cycle, making use of the multiple dynamic properties of bisacylhydrazones, is presented.