Photolabile Protecting Groups Based on the Excited State Meta Effect: Development and Application

This review focuses on utilization of the excited state meta effect (ESME) in the development of photolabile protecting groups (PPGs). Structurally simple ESME-based PPGs for release of various functional groups (such as carbonyl, hydroxyl, carboxyl, amino, and thiol groups) are discussed. Examples that demonstrate the appealing advantages of these new PPGs are provided, including their efficient release of "poor" leaving groups such as hydroxyl or amino group directly instead of in their respective carbonate or carbamate form. Applications of these PPGs in synthesis, release of biologically important molecules, materials science, and biomedical engineering are also described.

Using the 3-Diethylaminobenzyl Group as a Photocage in Aqueous Solution

We have demonstrated that the 3-diethylaminobenzyl (DEABn) photolabile protecting group (PPG) is an effective and structurally simple PPG for releasing molecules in aqueous environment. In general, the photoreaction is clean, and the released substrate and the PPG product, i.e., 3-diethylaminobenzyl alcohol, are obtained in high yield. The clean photoreaction can also be achieved under mild ambient conditions with sunlight, while the reactant is stable under indoor lighting. Release of two substrates from one PPG chromophore in aqueous solution has been demonstrated to be feasible. We have also compared the uncaging properties of the DEABn and the widely used o-nitrobenzyl ( o-NB) group, given their comparable structural simplicity. With its clean and efficient photochemical reaction, DEABn should find wide applications, including in the basic and applied research areas where o-NB and its various derivatives are widely used.

High-yielding and photolabile approaches to the covalent attachment of biomolecules to surfaces via hydrazone chemistry

The development of strategies to couple biomolecules covalently to surfaces is necessary for constructing sensing arrays for biological and biomedical applications. One attractive conjugation reaction is hydrazone formation--the reaction of a hydrazine with an aldehyde or ketone--as both hydrazines and aldehydes/ketones are largely bioorthogonal, which makes this particular reaction suitable for conjugating biomolecules to a variety of substrates. We show that the mild reaction conditions afforded by hydrazone conjugation enable the conjugation of DNA and proteins to the substrate surface in significantly higher yields than can be achieved with traditional bioconjugation techniques, such as maleimide chemistry. Next, we designed and synthesized a photocaged aryl ketone that can be conjugated to a surface and photochemically activated to provide a suitable partner for subsequent hydrazone formation between the surface-anchored ketone and DNA- or protein-hydrazines. Finally, we exploit the latent functionality of the photocaged ketone and pattern multiple biomolecules on the same substrate, effectively demonstrating a strategy for designing substrates with well-defined domains of different biomolecules. We expect that this approach can be extended to the production of multiplexed assays by using an appropriate mask with sequential photoexposure and biomolecule conjugation steps.

A hydrazide-anchored dendron scaffold for chemoselective ligation strategies

We report the design and synthesis of a dendron scaffold, enabling the chemoselective decoration of target molecules with multiple copies of functional species, such as peptides, via a hydrazone bond.