PHOTOPHYSICS OF 2'-DEOXYURIDINE (DU) NUCLEOSIDES COVALENTLY SUBSTITUTED WITH EITHER 1-PYRENYL OR 1-PYRENOYL - OBSERVATION OF PYRENE-TO-NUCLEOSIDE CHARGE-TRANSFER EMISSION IN 5-(1-PYRENYL)-DU

This paper reports syntheses, electronic absorbance and emission spectra, and emission kinetics results for two types of pyrene-substituted uridine nucleosides as part of an ongoing study which is examining the photophysical and photochemical behaviors of these same nucleosides embedded in DNA oligomers and duplexes with varying base-sequence composition, The two labels are 1-pyrenyl itself and 1-carboxypyrenyl (1-pyrenoyl) which are each joined directly to the 5-position of 2'-deoxyuridine (dU), These direct attachments significantly restrict the range of conformations available to the pyrene label when it is attached to a DNA oligomer or duplex. pi,pi* emission is absent for 5-(1-pyrenyl)-dU, 1, in methanol (MeOH) but present in tetrahydrofuran (THF). For 1 in MeOH, broad charge-transfer (CT) emission is present with a maximum at 470 nm and a quantum yield of 0.027; for 1 in THF, pi,pi* emission is present with a maximum at 395 nm and quantum yield of 0.42. Thermodynamic considerations suggest that the CT photoproduct of 1 which emits in MeOK is pyrene(.+)/dU(.-). The emission kinetics of 1 in MeOH are triexponential, but the wavelength variation of the relative amplitudes of the different decay lifetimes indicates that the CT-state relaxations are biexponential with lifetimes of less than or equal to 50 ps and 0.9 ns, Similarly, the pi,pi* State of 1 in MeOH also has two electron transfer (ET) quenching lifetimes of less than or equal to 50 ps and 2-3 ns. The steady-state emission spectrum of 1 in MeOH shows that emission from the few long-lived pi,pi*: states is dominated at all wavelengths by the 0.9-ns lived CT emission, In contrast, only pi,pi* emission is observed for 5-(1-pyrenoyl)-dU, 2, in both MeOH and THF with emission quantum yields of 0.002 and 0.028, respectively. The emission kinetics for 2 in THF are at least quadruply exponential having the longest emission lifetime of ca. 95 ns. However, only about 10% of the relative emission amplitude decays with lifetimes greater than 10 ns. Approximately 90% of the emission amplitude decays on the same time scale as for 2 in MeOH, The spectral and emission kinetics data for nucleosides 1 and 2 support the conclusions that each has multiple conformers in solution and that the relative orientation of the pyrene and uridine pi-systems plays a crucial role in determining the rates of both ET quenching of pyrene* and charge-recombination in the photoproduct.