J. Org. Chem. 2002, 67, 703-710
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Deactivation Behavior and Excited-State Properties of (Coumarin-4-yl)methyl Derivatives. 2. Photocleavage of Selected (Coumarin-4-yl)methyl-Caged Adenosine Cyclic 3′,5′-Monophosphates with Fluorescence Enhancement Torsten Eckardt,† Volker Hagen,† Bjo¨rn Schade,† Reinhardt Schmidt,‡ Claude Schweitzer,‡ and Ju¨rgen Bendig*,§ Institute of Molecular Pharmacology, Robert-Ro¨ ssle-Strasse 10, D-13125 Berlin, Germany, Institute of Physical and Theoretical Chemistry, Johann-Wolfgang-Goethe University Frankfurt/Main, D-60439 Frankfurt/Main, Germany, and Institute of Chemistry, Humboldt University Berlin, Hessische Strasse 1-2, D-10115 Berlin, Germany juergen)
[email protected] Received July 10, 2001
A series of axial and equatorial diastereomers of (coumarin-4-yl)methyl-caged adenosine cyclic 3′,5′monophosphates (cAMPs), 1-6, having methoxy, dialkylamino, or no substituent in the 6- and/or 7-positions, and their corresponding 4-(hydroxymethyl)coumarin photoproducts 7-12 have been synthesized. The photochemical and UV/vis spectroscopical properties (absorption and fluorescence) of 1-6 and 7-12 have been examined in methanol/aqueous HEPES buffer solution. Donor substitution in the 6-position causes a strong bathochromic shift of the long-wavelength absorption band, whereas substitution in the 7-position leads only to a weak red shift. The photochemical cleavage of the caged cAMPs was investigated, and the photoproducts were analyzed. Photochemical quantum yields, fluorescence quantum yields, and lifetimes of the excited singlet states were determined. The highest values of photochemical quantum yields (photo-SN1 mechanism) were obtained with caged cAMPs having a donor substituent in the 7-position of the coumarin moiety, caused by electronic stabilization of the intermediately formed coumarinylmethyl cation. With donor substitution in the 6-position, the resulting moderate electronic stabilization of the coumarinylmethyl cation is overcompensated by the strong bathochromic shift, reducing the energy gap between the excited-state S1 and the corresponding coumarinylmethyl cation. The rate constant for the ester cleavage and liberation of cAMP is about 109 s-1, estimated for the axial isomer of 6 by analysis of the fluorescence increase of the alcohol 12 formed upon laser pulse photolysis. Introduction The controlled photochemical release of bioactive effector molecules from masked inactive derivatives (caged compounds) represents an exceptional method for investigating the mechanisms and kinetics of biomolecular processes inside living cells.1,2 This technique allows the generation of instantaneous concentration increases of the biomolecule in the vicinity of activity, and the biological response is triggered almost without delay. * To whom correspondence should be addressed. † Institute of Molecular Pharmacology. ‡ Johann-Wolfgang-Goethe University Frankfurt/Main. § Humboldt University Berlin. (1) (a) Lester, H. A.; Nerbonne, J. M. Annu. Rev. Biophys. Bioeng. 1982, 11, 151-175. (b) Nerbonne, J. M. In Optical Methods in Cell Physiology; De Weer, P., Salzberg, B. M., Eds.; Wiley: New York, 1986; Vol. 40, pp 417-445. (c) Gurney, A. M.; Lester, H. A. Physiol. Rev. 1987, 67, 583-617. (d) McCray, J. A.; Trentham, D. R. Annu. Rev. Biophys. Biophys. Chem. 1989, 18, 239-270. (e) Wooton, J. F.; Trentham, D. R. In Photochemical Probes in Biochemistry; Nielsen, P. E., Ed.; NATO ASI Series C; Kluwer Academic: Dordrecht, The Netherlands, 1989; Vol. 272, pp 277-296. (f) Kaplan, J. H.; Somlyo, A. P. Trends Neurosci. 1989, 12, 54-58. (g) Adams, S. R.; Tsien, R. Y. Annu. Rev. Physiol. 1993, 55, 755-784. (h) Corrie, J. E. T.; Trentham, D. R. In Bioorganic Photochemistry; Morrison, H., Ed.; Wiley: New York, 1993; Vol. 2, pp 243-305. (i) Kao, J. P. Y.; Adams, S. R. Optical Microscopy: Emerging Methods and Applications; Academic Press: San Diego, 1993; pp 27-85. (2) (a) Hagen, V.; Dzeja, C.; Bendig, J.; Baeger, I.; Kaupp, U. B. J. Photochem. Photobiol., B 1997, 42, 71-78. (b) Hagen, V.; Dzeja, C.; Frings, S.; Bendig, J.; Krause, E.; Kaupp, U. B. Biochemistry 1996, 35, 7762-7771.
Several investigators have successfully employed caged adenosine cyclic 3′,5′-monophoshates (cAMPs) to study cAMP-dependent cellular processes.2-4 Recently Furuta et al. introduced (7-methoxycoumarin-4-yl)methyl-caged cAMP (7-MCM-caged cAMP) and described its favorable properties such as a long half-life in the dark in physiological buffer solutions and its relatively high efficiency of photorelease.5 Using time-resolved fluorescence measurements, we recently found that the analogues 7-MCMcaged 8-Br-cAMP and 7-MCM-caged 8-Br-cGMP released the corresponding cyclic nucleotides (8-Br-cAMP, 8-BrcGMP) rapidly within a few nanoseconds.6 The photocleavage (Scheme 1) was shown to proceed similarly to that of naphthylalkyl7,8 and benzyl9 phosphates and involves heterolysis of the C-O ester bond (solvent(3) Walker, J. W.; Reid, G. P.; Trentham, D. R. Methods Enzymol. 1989, 172, 288-301. (4) (a) Korth, M.; Engels, J. Naunyn-Schmiedeberg’s Arch. Pharmacol. 1979, 310, 103-111. (b) Nargeot, J.; Nerbonne, J. M.; Engels, J.; Lester, H. A. Proc. Natl. Acad. Sci. U.S.A. 1983, 80, 2395-2399. (c) Nerbonne, J. M.; Richard, S.; Nargeot, J.; Lester, H. A. Nature 1984, 310, 74-76. (d) Wiesner, B.; Hagen, V. J. Photochem. Photobiol., B 1999, 49, 112-119. (e) Frings, S.; Hackos, D. H.; Dzeja, C.; Okyama, T.; Hagen, V.; Kaupp, U. B.; Korenbrot, J. Methods Enzymol. 2000, 315, 797-817. (5) (a) Furuta, T.; Torigai, H.; Sugimoto, M.; Iwamura, M. J. Org. Chem. 1995, 60, 3953-3956. (b) Furuta, T.; Iwamura, M. Methods Enzymol. 1998, 291, 50-63. (6) Hagen, V.; Bendig, J.; Frings, S.; Wiesner, B.; Schade, B.; Helm, S.; Lorenz, D.; Kaupp, U. B. J. Photochem. Photobiol., B 1999, 53, 91102.
10.1021/jo010692p CCC: $22.00 © 2002 American Chemical Society Published on Web 01/15/2002
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Scheme 1.
Mechanism of the Photolysis of Caged cAMPs 1-6
assisted photoheterolysis) and subsequent ion-pair separation by the polar solvent (formation of cAMP), followed by trapping of the coumarinylmethyl carbocation by the solvent (hydroxylation).10 This ionic mechanism is inconsistent with the radical pathway postulated by Furuta11 for (7-methoxycoumarin-4-yl)methyl diethyl phosphate. Furthermore, we reported that 7-MCM-caged cyclic nucleotides are only weakly fluorescent, whereas the photoproduct 4-(hydroxymethyl)-7-methoxycoumarin (7MCM-OH) shows strong fluorescence, which should facilitate monitoring of the release process.6,10,12 In this paper we investigate the influence of substituents in the 6- and 7-positions of the coumarin caging group on (i) the spectroscopic properties (absorption and fluorescence), (ii) the photophysical deactivation behavior (fluorescence ability), and (iii) the photochemical reactions of caged cAMPs. Substituents influence the absorption and fluorescence band positions of coumarin derivatives.13,14 Electrondonating substituents in the 6- and/or 7-positions and electron-withdrawing substituents in the 3-position of the 4-methylcoumarin moiety cause a significant bathochromic shift of the S0-S1 transition. Most of the substituted coumarins are characterized by large fluorescence quan(7) (a) Itoh, Y.; Gouki, M.; Goshima, T.; Hachimori, A.; Kojima, M.; Karatsu, T. J. Photochem. Photobiol., A 1998, 117, 91-98. (b) Pincock, J. A. Acc. Chem. Res. 1997, 30, 43-49. (c) Arnold, B.; Donald, L.; Jurgens, A.; Pincock, J. A. Can. J. Chem. 1985, 63, 3140-3146. (8) Givens, R. S.; Matuszewski, B. J. Am. Chem. Soc. 1984, 106, 6860-6861. (9) Hilborn, J. W.; MacKnight, E.; Pincock, J. A.; Wedge, P. J. J. Am. Chem. Soc. 1994, 116, 3337-3346. (10) Schade, B.; Hagen, V.; Schmidt, R.; Herbrich, R.; Krause, E.; Eckardt, T.; Bendig, J. J. Org. Chem. 1999, 64, 9109-9117. (11) Furuta, T.; Wang, S. S.-H.; Dantzker, J. L.; Dore, T. M.; Bybee, W. J.; Callaway, E. M.; Denk, W.; Tsien, R. Y. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 1193-1200. (12) Bendig, J.; Helm, S.; Hagen, V. J. Fluoresc. 1997, 7, 357-361. (13) (a) Ito, K.; Maruyama, J. Chem. Pharm. Bull. 1983, 31, 30143023. (b) Giri, R.; Rathi, S. S.; Machwe, M. K.; Murti, V. V. S. Spectrochim. Acta 1988, 44A, 805-807. (c) Arbeola, T. L.; Arbeola, F. L.; Tapia, M. J.; Arbeola, I. L. J. Phys. Chem. 1993, 97, 4704-4707. (d) Rechthaler, K.; Ko¨hler, G. Chem. Phys. 1994, 189, 99-116. (e) Kaholek, M.; Hrdlovic, P. J. Photochem. Photobiol., A 1997, 108, 283288. (f) Raju, B. B.; Eliasson, B. J. Photochem. Photobiol., A 1998, 116, 135-142. (g) Fabian, W. M. F.; Niederreiter, K. S.; Uray, G.; Stadlbauer, W. J. Mol. Struct. 1999, 477, 209-220. (h) Joshi, H. S.; Jamshidi, R.; Tor, Y. Angew. Chem. 1999, 111, 2888-2891. (i) Hagen, V.; Bendig, J.; Frings, S.; Eckardt, T.; Helm, S.; Reuter, D.; Kaupp, U. B. Angew. Chem., Int. Ed. 2001, 40, 1046-1048. (14) Seixas de Melo, J. S.; Becker, R. S.; Macanita, A. L. J. Phys. Chem. 1994, 98, 6054-6058.
Eckardt et al. Scheme 2. Photolysis of the Axial and Equatorial Isomers of (Coumarin-4-yl)methyl Esters of cAMP (1-6), Liberating the (Coumarin-4-yl)Methyl Alcohols 7-12 and cAMP
tum yields. In the absence of heavy atoms15 and for the 6- and 8-bromo-substituted coumarins,16 the triplet population is negligible and fluorescence competes with the nonradiative deactivation (internal conversion).10 The weak fluorescence is explained by an accelerated internal conversion from the excited state caused by mixing of the ππ* and the nπ* states.10,14 Additionally, the low fluorescence quantum yields of some donor-acceptor-substituted coumarins are explained by a nonradiative deactivation pathway via a twisted intramolecular chargetransfer (TICT) state.16 Surprisingly, the influence of coumarin structure on the efficiency of the photochemical bond cleavage has not been investigated so far. Photoactivation of the (coumarin-4-yl)methyl esters of cAMP (1-6) leads to the corresponding 4-(hydroxymethyl)coumarins 7-12, which are liberated during photolysis of the phototrigger compounds (Scheme 2). To study the influence of the electronic properties of donor substituents on the spectroscopic characteristics and photochemical efficiency, we synthesized the axial and equatorial isomers of (coumarin-4-yl)methyl esters of cAMP (1-6) (Scheme 3). 6 was already described in connection with another series of caged cAMPs.13i In addition, the corresponding 4-(hydroxymethyl)coumarins 7-12 were prepared to investigate the photophysical deactivation behavior (Table 1). Furthermore, the phototriggers 2-4 and their corresponding alcohols 8-10 were selected to determine the influence of the number and position of the substituents. Our final goal was to design suitable (coumarin-4-yl)methyl-caged cAMPs with strong longwavelength absorption bands, high photochemical quantum yields, and strong fluorescence enhancement during photolysis. (15) Li, L.-D.; Yang, S.-Z. Anal. Chim. Acta 1994, 296, 99-105. (16) Corrie, J. E. T.; Munasinghe, V. R. N.; Rettig, W. J. Heterocycl. Chem. 2000, 37, 1447-1455.
Properties of (Coumarin-4-yl)methyl Derivatives Scheme 3. Synthesis of the Axial and Equatorial Isomers of (Coumarin-4-yl)methyl Esters of cAMP (1-6)
Table 1. Photophysical Data of the (Coumarin-4-yl)methyl Alcohols 7-12 in MeOH/HEPES, 1:4 compound
τf/ns
τfn/ns
kf/(108 s-1)
CM-OH (7) 6-MCM-OH (8) 7-MCM-OH (9) DMCM-OH (10) DMACM-OH (11) DEACM-OH (12)
