title: 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.
abstract: 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-S(N)1 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 S(1) and the corresponding coumarinylmethyl cation. The rate constant for the ester cleavage and liberation of cAMP is about 10(9) s(-1), estimated for the axial isomer of 6 by analysis of the fluorescence increase of the alcohol 12 formed upon laser pulse photolysis.
authors.affiliation: Institute of Molecular Pharmacology, Robert-Rössle-Strasse 10, D-13125 Berlin, Germany.
authors.forename: Torsten
authors.initials: T
authors.lastname: Eckardt
authors.forename: Volker
authors.initials: V
authors.lastname: Hagen
authors.forename: Björn
authors.initials: B
authors.lastname: Schade
authors.forename: Reinhardt
authors.initials: R
authors.lastname: Schmidt
authors.forename: Claude
authors.initials: C
authors.lastname: Schweitzer
authors.forename: Jürgen
authors.initials: J
authors.lastname: Bendig
mesh_terms: D003374:Coumarins / Q000737:chemistry*; D000242:Cyclic AMP / Q000031:analogs & derivatives* / Q000737:chemistry; D005453:Fluorescence; D010777:Photochemistry; D013050:Spectrometry, Fluorescence; D021241:Spectrometry, Mass, Electrospray Ionization; D013056:Spectrophotometry, Ultraviolet
chemical_list: D003374:Coumarins; D000242:Cyclic AMP