Abstract
Photoremovable protective groups (PPGs) and related “caged” compounds have been recognized as a powerful tool in an arsenal of life science methods. The present review is focused on recent advances in design of “caged” compounds which function in red or near-infrared region. The naive comparison of photon energy with that of organic bond leads to the illusion that long-wavelength activation is possible only for weak chemical bonds like N-N. However, there are different means to overcome this threshold and shift the uncaging functionality into red or near-infrared regions for general organic bonds. We overview these strategies, including the novel photochemical and photophysical mechanisms used in newly developed PPGs, singlet-oxygen-mediated photolysis, and two-photon absorption. Recent advances in science places the infrared-sensitive PPGs to the same usability level as traditional ones, facilitating in vivo application of caged compounds.
Highlights
Single-photon Photoremovable protective groups (PPGs)The ‘‘uncaging” reaction implies the dissociation of covalent bond between PPG and leaving group (LG) and requires energy
Photoremovable protective groups (PPGs) and related ‘‘caged” compounds have been recognized as a powerful tool in an arsenal of life science methods
We highlight recent advances in design of ‘‘caged” compounds sensitive to red and infrared light. ‘‘Caged” compounds are defined as relatively small molecules that can release substance of interest under the action of light
Summary
The ‘‘uncaging” reaction implies the dissociation of covalent bond between PPG and leaving group (LG) and requires energy. In was shown that the rhodamine moiety enables effective light absorption with electron transfer from N-NO to dye fragment which facilitates N-N bond dissociation. This is the basis of NO-Rosa (Fig. 1a) [10] and related compounds [11] which release NO under illumination of 530–590 nm yellow-green light. According to the quantum mechanics calculations and fluorescence measurement for the same compound, its energy level is 2.2–2.3 eV, which corresponds to the photon wavelength of 560 nm While it can be lowered by extending conjugation system, it is still unclear where is the limit of breaking the conical intersection nature of photodissociation. Coumarin-based PPG with ability of intramolecular carbocation trapping was introduced [35]
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