Design and Synthesis of a Calcium‐Sensitive Photocage

Abstract

Photolabile protecting groups (or "photocages") enable precise spatiotemporal control of chemical functionality and facilitate advanced biological experiments. Extant photocages exhibit a simple input-output relationship, however, where application of light elicits a photochemical reaction irrespective of the environment. Herein, we refine and extend the concept of photolabile groups, synthesizing the first Ca 2+ -sensitive photocage. This system functions as a chemical coincidence detector, releasing small molecules only in the presence of both light and elevated [Ca 2+ ]. Caging a fluorophore with this ion-sensitive moiety yields an "ion integrator" that permanently marks cells undergoing high Ca 2+ flux during an illumination-defined time period. Our general design concept demonstrates a new class of light-sensitive material for cellular imaging, sensing, and targeted molecular delivery.Small molecules that absorb light have broad utility as tools to probe and perturb biological systems. Chemical fluorophores constitute one important type of light-absorbing molecule.[1] The ability to modify dyes using chemistry allows the construction of numerous probes for specific applications. For example, changing the chemical structure of fluorophores can allow fine-tuning of spectral properties. Likewise, chemical dyes that respond to changes in ion concentration have been prepared. The design and synthesis of such ion indicators involves incorporation of molecular recognition motifs into a fluorophore where the reversible binding of a specific ion alters the absorption and/or fluorescence quantum yield of the dye. This strategy has produced probes for many biologically relevant ions, including Na + , K + , Mg 2+ , Ca 2+ , and Zn 2+ , allowing noninvasive monitoring of ion concentration inside living cells. [2] Photolabile protecting groups or "photocages" comprise another important class of organic chromophore where photon absorption elicits cleavage of a chemical bond. [3] Like fluorophores, the spectral properties of photocages have been manipulated using chemistry, resulting in photolabile groups with longer wavelengths and larger two-photon cross-sections.[4] However, unlike fluorescent dyes, the incorporation of ion-sensitive motifs into photocages is essentially unexplored. Probes built from ion-sensitive photolabile groups could complement reversible ion indicators, functioning as chemical coincidence detectors that would selectively and irreversibly release a small molecule only in the presence of both light and increased ion concentration. In particular, caging a fluorophore with such an ion-sensitive photocage would yield a fluorescent ion "snapshot indicator" or "integrator" that would permanently record increased ion concentration during an illumination-defined time period. Integrator systems based on fluorescent proteins [5] and rhodopsins [6] have been described recently, but small-molecule ion integrators remain unknown. This currently limits the use of such probes to biological systems th...