New Phototriggers: Extending the p-Hydroxyphenacyl π-π* Absorption Range

Article abstract of DOI:10.1021/ol005856n

(Matrix Presented) Introducing 3-methoxy or 3,5-dimethoxy substituents on the 4-hydroxyphenacyl (pHP) photoremovable protecting group has been explored with two excitatory γ-amino acids, L-glutamic acid and γ-amino butyric acid (GABA). These substituents significantly extend the absorption range of the pHP chromophore, e.g., the tail of absorption bands of 2a,b extend above 400 nm, well beyond the absorptions of aromatic amino acids and nucleotides. Irradiation releases the amino acids with rate constants of ～10⁷ s^-1 and appearance efficiencies (Φ_app) of 0.03-0.04. The photoproducts are formed through the pHP excited triplet and are primarily products of photoreduction and photohydrolysis. 1a,b also rearranged to the phenylacetic acid 3.

Full text of DOI:10.1021/ol005856n

ORGANIC
LETTERS
New Phototriggers:1 Extending the
p-Hydroxyphenacyl π−π* Absorption
Range
2000
Vol. 2, No. 11
1545-1547
Peter G. Conrad II, Richard S. Givens,* Jo1rg F. W. Weber, and Karl Kandler†
Department of Chemistry, UniVersity of Kansas, Lawrence, Kansas 66045
rgiVens@ukans.edu
Received March 24, 2000
ABSTRACT
Introducing 3-methoxy or 3,5-dimethoxy substituents on the 4-hydroxyphenacyl (pHP) photoremovable protecting group has been explored
with two excitatory γ-amino acids, L-glutamic acid and γ-amino butyric acid (GABA). These substituents significantly extend the absorption
range of the pHP chromophore, e.g., the tail of absorption bands of 2a,b extend above 400 nm, well beyond the absorptions of aromatic amino
acids and nucleotides. Irradiation releases the amino acids with rate constants of ∼107 s-1 and appearance efficiencies (Φapp) of 0.03−0.04.
The photoproducts are formed through the pHP excited triplet and are primarily products of photoreduction and photohydrolysis. 1a,b also
rearranged to the phenylacetic acid 3.
We report herein the use of 3-methoxy- and 3,5-dimethoxy-
4-hydroxyphenacyl (MeO-pHP and (MeO)2-pHP) as new
members of the p-hydroxyphenacyl (pHP) series of carboxy-
late and phosphate photoremovable protecting groups. Earlier
we2 introduced the p-hydroxyphenacyl (pHP) phototrigger
as an efficient, rapid photoremovable protecting group for
applications in neurobiology, physiology, and biochemistry.
Since its introduction, a variety of pHP derivatives have been
employed for the release of ATP,2 glutamic acid,3,4 GABA,3
the dipeptide Ala-Ala,1 and the C-terminus of the nonapeptide
bradykinin.1 Several in vitro biochemical studies have shown
that the attachment of pHP to the γ-carboxyl group of
glutamic acid or GABA or the C-terminal carboxylic acid
group of peptides completely suppresses the normal biologi-
cal activity of the natural substrates.1,3
In several studies, we have demonstrated that the photo-
release occurs from the triplet state of the phototrigger since
it can be quenched with either potassium sorbate or sodium
2-naphthalenesulfonate1-5 with excellent Stern-Volmer-
derived kinetic rates of ∼108 s-1 or faster for release for
these substrates. A concomitant rearrangement of the pho-
totrigger to p-hydroxyphenylacetic acid (Scheme 1) was
Scheme 1. Mechanism for the Photorelease from pHP
† Department of Neurobiology, University of Pennsylvania, Pittsburgh,
PA 15261.
(1) For the previous paper, see: Givens, R. S.; Weber, J. F. W.; Conrad,
P. G.; Orosz, G.; Donahue, S. L.; Thayer, S. A. J. Am. Chem. Soc. 2000,
122, 2687-2697.
(2) (a) Givens, R. S.; Park, C.-H. Tetrahedron Lett. 1996, 37, 6259-
6262. (b) Park, C.-H.; Givens, R. S. J. Am. Chem. Soc. 1997, 119, 2453-
2463.
(3) Givens, R. S.; Jung, A. H.; Park, C.-H.; Weber, J. F. W.; Bartlett,
W. J. Am. Chem. Soc. 1997, 119, 8369-8370.
(4) Kandler, K.; Katz, L. C.; Kauer, J. A. Nature Neuro. 1998, 1, 114-
118.
shown to be the major pathway6 for the pHP chromophore.
The rearrangement is preceded by an excited-state deproto-
10.1021/ol005856n CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/10/2000

Scheme 2. Synthesis of Methoxy pHP GABA’s and L-Glutamates
Scheme 3. Photorelease of l-glutamic Acid and GABA from 1a,b and 2a,b
nation7 of the phenolic proton yielding the triplet phenolate
hydroxyphenacyl chromophores 1 and 2 are in fact shifted
ion,5 the apparent driving force for the rearrangement.4
To expand the dynamic range for this phototrigger and to
move the effective absorption away from the range of normal
peptides and nucleotides, we have explored the effects of
added methoxy substituents on the pHP chromophore and
the resulting photochemistry. Our results are presented here.
The synthesis of the methoxy pHP esters of L-glutamate
(1a, 2a) and of γ-aminobutyric acid (GABA, 1b, 2b) were
accomplished by the DBU-catalyzed displacement of bro-
mide from the appropriate R-bromoacetophenone followed
by deprotection of the remaining amino acid protecting
groups with TFA to give the methoxy phenacyl protected
neurotransmitters 1a,b and 2a,b (Scheme 2).
to 350 and 370 nm, respectively.
As shown in Figure 1, the absorption maxima for the new
3-methoxy-4-hydroxyphenacyl and the 3,5-dimethoxy-4-
(5) (a) Conrad, P. G.; Givens, R. S.; Hellrung, B.; Rajesh, C. S.;
Ramkseier, M.; Wirz, J. Submitted for publication. (b) See also refs 1 and
2.
(6) Anderson, J. C.; Reese, C. B. Tetrahedron Lett. 1962, 1-4.
(7) A recent report by Corrie and Wan (Zhang, K.; Corrie, J. E. T.;
Munasinghe, V. R. N.; Wan, P. J. Am. Chem. Soc. 1999, 121, 5625-5632)
suggested a singlet state rearrangement preceded by deprotonation. While
we differ on the multiplicity of the reaction and even on the order of the
events, we are in substantial agreement that deprotonation does precede
the rearrangement (see ref 5).
Figure 1. Ultraviolet spectra of three caged L-glutamates, 3-meth-
oxy-pHP 1a, 3,5-dimethoxy-pHP 2a, and pHP 3, and three caged
GABA’s, 3-methoxy-pHP 1b, 3,5-dimethoxy-pHP 2b, and pHP 4.
Photolyses of 1 and 2 were performed in aqueous solutions
with either 300 or 350 nm lamps (Scheme 3) and were
1546
Org. Lett., Vol. 2, No. 11, 2000

1
monitored by HPLC or H NMR (D2O). The release of the
amino acids occurred with quantum efficiencies8 of 0.03 to
0.04, somewhat below those that we reported for the parent
pHP amino acids.3
Photolysis of 2a in phosphate buffer (pH 7.2) in the
presence of 0.001-0.010 M potassium sorbate quenched the
photorelease of the amino acids and gave good Stern-
Volmer kinetics (KSV ) 343 M-1) resulting in a calculated
triplet lifetime of 46 ns and rate constant for release of 2.2
× 107 s-1 (assuming kdiff ) kq ) 7.4 × 109 M-1 s-1).9
Phosphorescence spectra gave triplet energies of 69 kcal/
mol for 1a and 71 kcal/mol for 2a. The phosphorescence
emission for 1a was also quenched by potassium sorbate.
These results clearly indicate a reaction pathway through the
triplet excited state of these new phototriggers and are in
accord with our recent results for p-hydroxyphenacyl (pHP).4
In contrast to the earlier results, however, rearrangement to
the phenylacetic acid has become a minor pathway for 1
and is not observed at all for 2. These alternative photo-
chemical pathways have been reported in a wide array of
other phenacyl derivatives.10
The biological efficacy of these new phototriggers has
likewise been tested using electrophysiological whole-cell
patch clamp recordings for CA1 neurons in acute hippo-
campal brain slices4 and from cultured neocortical neurons.11
Photolysis12 of a 200 mM solution of 3,5-dimethoxy-4-
hydroxyphenacyl GABA (2b) bathing a CA1 hippocampal
neuron from a 7-day-old rat elicited a response from the cell
as indicated by the detection of outward currents as shown
in Figure 2. In all neurons tested (n ) 12), photolysis of 2b
consistently elicited fast membrane currents. These responses
were due to the specific stimulation of GABA A-receptors
(a ligand gated chloride channel) because application of
Bicuculline abolished all inward currents. Figure 2 depicts
the responses stimulated by the release of GABA from 2b.
Figure 2. Peak amplitude of currents elicited by released GABA
is plotted against stimulus number (interval between stimuli 20 s)
and is the average of 10 consecutive traces. After stimulus #22, 10
M Bicuculline14 was added to the bath solution, which completely
blocked the responses elicited by photorelease of GABA. The
relatively constant amplitude (114.2 pA ( 1.4 SEM) for the 20
flashes prior to receptor shut down by Bicuculline demonstrates
that the cell is not harmed by the transient exposure to the UV
flashes or to the appearance of the phototrigger side products during
photolysis.
These preliminary results for the 3-methoxy-4-hydroxy-
phenacyl and 3,5-dimethoxy-4-hydroxyphenacyl phototrig-
gers demonstrate the potential for effective release of certain
R-amino acids14 and for C-terminal protected peptides.
Furthermore, release of amino acids, oligopeptide, and
proteins in biological environments employing laser flash
photolysis and other fast kinetic techniques using pHP
phototriggers such as 1 or 2 is possible at λ > 300 nm and
even beyond λ ) 400 nm.
(8) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London A 1956, A220,
518.
(9) For the calculated estimate of the bimolecular rate for diffusion in
H2O, see: Simons, J. P. Photochemistry and Spectroscopy; J. Wiley-
Interscience: New York, 1971; pp 212-213.
Acknowledgment. This research was supported by the
University of Kansas and by the NSF (OSR-9255223).
(10) See the following and references therein for examples of alternative
photoinduced R-cleavage reactions of substituted acetophenones, for
example: (a) Scaiano, J. C.; Netto-Ferreira, J. C. J. Photochem. 1986, 32,
253-259. (b) Sheehan, J. C.; Umezawa, K. J. Org. Chem. 1973, 38, 3771-
3774. (c) Banerjee, A.; Falvey, D. E. J. Am. Chem. Soc. 1998, 120, 2965-
2966.
(11) Hartnett, K. A.; Stout, A. K.; Rajdev, S.; Rosenberg, P. A.; Reynolds,
I. J.; Aiszeman, E. J. Neurochem. 1997, 68, 1836-1845.
(12) The experimental apparatus and wavelength range are described
elsewhere. Kandler, K.; Givens, R.; Katz, L. C. Photostimulation with Caged
Glutamate (Chapter 27). In Imaging Neurons: A Laboratory Manual; Yuste,
Lanni, R. F., Konnerth, A., Eds.; Cold Spring Harbor Laboratory Press:
1997. Typically, 2 ms UV flashes were employed.
Supporting Information Available: Details of the syn-
thesis and 1H and 13C NMR, UV, IR, and HRMS data for 1,
2, and 6-14. This material is available free of charge via
the Internet at http://pubs.acs.org. Complete experimental
details and additional examples of substituted pHP deriva-
tives will be provided in our full paper.
OL005856N
(14) Givens, R. S.; Weber, J. F. W.; Jung, A. H.; Park, C.-H. New
Photoprotecting Groups: Desyl and p-Hydroxyphenacyl Phosphate and
Carboxylate Esters. In Methods in Enzymology: Caged Compounds;
Marriott, G., Ed.; 1998, 291, 1-29.
(13) Bicuculline is a known GABA A-receptor antagonist. The fact that
the cell shows no response after the antagonist is added indicates that only
free GABA causes the responses.
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