Development of Photoremovable Protecting Groups

1. Development of Photoremovable Protecting Groups

2. Photoremovable Protecting Groups and Cage Compounds The project develops new “cage compounds” for practical applications in biochemistry, cellular biology, and physiology. Cage compounds provide the means for quickly introducing biological agents into cells. Once released, such biological agents or biomolecules trigger a given biological process, which then may be studied independently. Such release can be achieved nearly instantaneously by use of a flash of light, and the site of the release can be controlled selectively by irradiating a specific region of a cell or tissue. In terms of the chemistry, the exposure to light causes a chemical reaction which releases the biomolecule upon removal of the protecting group. The protecting group blocks the biological activity of the biomolecule prior to its release.

3. Photoremovable Protecting Groups Based on Electrocyclic Reactions that Generate Zwitterionic Intermediates Zwitterionic intermediates have proven capable of expelling various leaving group anions of biological significance. Such anions that have been released are carboxylates and phenolates. The released carboxylates can be neurotransmitter amino acids as the biomolecules. Biological examples for released phenolates are peptides and proteins that have side chain phenolic groups of tyrosine residues.

4. A Zwitterionic Intermediate that Releases Carboxylates and Phenolates Examples of leaving groups LG- studied are various carboxylates including the neurotransmitters glutamate and GABA. Phenolate groups are also released. Quantum yields are 0.2-0.3. Release times of carboxylates are microseconds. see: (a) Ma, C.; Steinmetz, M.G.; Kopatz, E.J.; Rathore, R. J. Org. Chem.2005, 70, 4431-4442. (b) Ma, C.; Chen, Y.; Steinmetz, M.G. J. Org. Chem.2006, 71, 4206-4215.

5. A Photochemical Electrocyclic Reaction that Generates a Zwitterionic Intermediate Examples of leaving groups LG- released are various carboxylates and phenolates. Quantum yields are 0.07 – 0.09. A minor byproduct is formed via 1,5-H shift. Reaction occurs in competition with radiationless decay of the singlet excited state. see: Jia, J.; Sarker, M.; Steinmetz, M.G.; Shukla, R.; Rathore, R. J. Org. Chem.2008, 73, 8867-8879.

6. Electrocyclic Ring Closure of benzothiophenes for the Generation of Zwitterionic Intermediates The triplet excited state cyclizes in the case of benzothiophenes. Release of chloride, phenylacetate, and benzylthiolate occurs quantitatively. The quantum yield for chloride release is 0.22; with air it is 0.078, indicative of triplet excited state quenching by oxygen.

7. Work in Progress The ease of incorporation of thiolate groups and the high release yields suggest that benzothiophene derivatives may be useful photoremovable protecting groups for peptides and proteins. Efforts are underway to incorporate glutathione, which regulates the redox state of cells. The photolysis wavelength must be shifted to > 350 nm by introduction of a benzoyl group into the anilide ring, and triplet yields may be increased by incorporating heavy atom(s) into the benzothiophene ring.