Control of Irradiation Wavelength and Quantum Yield of Photocleavable Cages
An important component of biological research is the precise regulation of biomolecules in their physiological environment. Recent advances in neuroscience, have made it possible to regulate the activity of neurotransmitters using photocleavable protecting groups (PPGs), which are also known as cages. This enables the use of light as a trigger to activate selected neurons via photo release of agonists with precise control of timing as well as synaptic site, popularly referred to as spatiotemporal control. In recent years, a variety of PPGs have been developed and applied to biomolecular cages. PPGs can be classified according to their absorption wavelengths (from UV to NIR) and photochemical and physical properties (e.g. quantum yield, photo cross-section, solubility). PPGs absorb light and undergo intramolecular electron transitions and bond rearrangements, resulting in the release of agonists. Therefore, designing effective cages for specific substrates is unique and presents a series of challenges. Glutamate is an important neurotransmitter and considered key in being the excitatory agonist in neurons involved in memory. As such, it has been the substrate of focus for these caging designs. One of the most effective cage glutamate in use is the CDNI-Glu with superior photophysical properties. In our lab, we improved the synthetic procedure to access CDNI-Glu to reduce the total number of steps and times. In addition, we also caged the other two functional groups of glutamate and tested all of these three variations of CDNI caged glutamates via 1H NMR and UV/Vis spectroscopy by irradiating under UV. However, due to the disadvantages of UV, it is necessary to continue to explore other PPGs to shift the maximum wavelength toward visible light or even infrared light. Recent studies have shown that the red shift in the absorption spectrum caused by introducing the diethylamino electron donating group (EDG) of coumarin is larger than that of other EDG substituents, such as alkoxy group and that the carbonyl thioacylation of lactones could be used to make the maximum absorption wavelength move further toward the infrared. So we applied 7-(N, N-diethylamino)-4-methyl-thiocoumarinPPG in the caging of glutamate. We also explored other types of visible light responsive cage design such as, 4-arylalkoxy–boron dipyrromethene (BODIPY).A modified BODIPY was expected to release leaving groups when photolyzed with green light. The thiocoumarin type of cage and two different BODIPY structures were employed to cage glutamate and they were tested for their photolysis kinetics via 1H NMR and UV/Vis spectroscopy by irradiation under blue and green light, respectively. meso-BODIPY was used because it has excellent benefits such as being biologically benign, thermally stable in the dark, easy to be synthesized, no chiral center, and an adjustable absorption in the visible/NIR region. However, a significant drawback is that the quantum yield of release during photolysis process is quite low. Therefore, in order to achieve the desired purpose in biological applications, a relatively higher concentration is required. However, high concentrations may cause adverse effects on cells in some cases. To solve this problem, two structural motifs were identified, each for leading to a significant improvement in quantum yields of photorelease. By creatively combining these two structural elements into a single BODIPY structure, the original properties can be greatly optimized, and the quantum yield and absorption properties can be substantially improved.