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
Tl. Netzel et al., 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, Journal of the American Chemical Society, 117(36), 1995, pp. 9119-9128
This paper reports syntheses, electronic absorbance and emission spect
ra, 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 e
mbedded in DNA oligomers and duplexes with varying base-sequence compo
sition, The two labels are 1-pyrenyl itself and 1-carboxypyrenyl (1-py
renoyl) which are each joined directly to the 5-position of 2'-deoxyur
idine (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. Thermo
dynamic considerations suggest that the CT photoproduct of 1 which emi
ts in MeOK is pyrene(.+)/dU(.-). The emission kinetics of 1 in MeOH ar
e triexponential, but the wavelength variation of the relative amplitu
des of the different decay lifetimes indicates that the CT-state relax
ations 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 ele
ctron 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 tha
t 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 wi
th emission quantum yields of 0.002 and 0.028, respectively. The emiss
ion kinetics for 2 in THF are at least quadruply exponential having th
e longest emission lifetime of ca. 95 ns. However, only about 10% of t
he relative emission amplitude decays with lifetimes greater than 10 n
s. Approximately 90% of the emission amplitude decays on the same time
scale as for 2 in MeOH, The spectral and emission kinetics data for n
ucleosides 1 and 2 support the conclusions that each has multiple conf
ormers in solution and that the relative orientation of the pyrene and
uridine pi-systems plays a crucial role in determining the rates of b
oth ET quenching of pyrene and charge-recombination in the photoprodu
ct.