Transcript Nugget

Development of Photoremovable Protecting Groups
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.
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.
A Zwitterionic Intermediate that Releases
Carboxylates and Phenolates
CH3
O
LG
N
O
h
H2O
buffer
OH
LG
OH
N
O
O
-LG-
N
O
-H+
CH3
HO
O
NEt +
O
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.
NEt
O
A Photochemical Electrocyclic Reaction that Generates
a Zwitterionic Intermediate
CH3
N
O
PhCO
LG
h
H2O
PhCO
CH3
N
O
-H+
LG
H
PhCO
CH3
N
O
-LGLG
CH3
N
O
PhCO
1,5-H
CH3
N
O
PhCO
LG
H
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.
Electrocyclic Ring Closure of benzothiophenes for the
Generation of Zwitterionic Intermediates
CH3
N
O
LG
1
S
CH3
N
O
h
H2O
CH3CN
-H+
H
-LG-
S
LG
2
CH3
N
O
CH3
N
O
S + LG-H
S
LG
3
4
LG- = Cl-, PhCH2CO2-, PhCH2S-
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.
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.