Excited State Characteristics of 6-Azauracil in Acetonitrile: Drastically Different Relaxation Mechanism from Uracil

Abstract

Excited-state dynamics of 6-azauracil (6-AU) and sensitized singlet oxygen formation in acetonitrile solution with UV irradiation were investigated for the first time. In the transient absorption measurement, the 248 nm laser photolysis gave a relatively intense absorption band at 320 nm (= 1100 +/- 100 dm(3) mol(-1) cm(-1)) and a broadband in the 500-700 nm region due to triplet 6-AU. The triplet 6-AU, decaying with the rate constant of (5.3 +/- 0.2) x 10(6) s(-1) in Ar saturated acetonitrile, was quenched by molecular oxygen with the rate constant of (2.5 +/- 0.1) x 10(9) dm(3) mol(-1) s(-1). The formation quantum yield of excited triplet 6-AU was estimated to be unity by acetone triplet sensitization and actinometry with benzophenone. The time-resolved thermal lensing signal of 6-AU was also observed by 248 nm laser excitation. In the presence of molecular oxygen, the sensitization from triplet 6-AU gave rise to formation of singlet oxygen O(2) ((1)Delta(g)) with a quantum yield of 0.63 +/- 0.03. Drastically different excited-state dynamics of aza-substituted uracil from normal uracil were clarified, and the mechanism for the enhancement of intersystem crossing by aza-substitution is discussed.

Photochemistry of Nucleic Acid Bases and Their Thio- and Aza-Analogues in Solution

“…In addition, a small absorption tail is observed to the red of the lowestenergy absorption band in 6-azauracil and 6-azauridine, indicative of a low-lying electronic transition with reduced oscillator strength. This is in agreement with TD-B3LYP/PCM/6-31G(d,p) calculations performed by Kobayashi et al [12,15] in acetonitrile, which show that the two lowest-energy electronic transitions in the 6-azauracil nucleobase and nucleoside have S 2 (ππ*) and S 1 (nπ*) electronic configurations, with a larger energy gap than the corresponding electronic transitions in uracil and uridine in the same solvent. The calculations further show that the oscillator strength of the S 2 (ππ*) state in 6-azauridine is~33% smaller than in uridine, in good agreement with the experimental observations (Fig.…”

“…However, photosensitivity to these drugs has also been reported [63,64]. As observed in the thiobases and other nucleobase analogues, the photochemical properties of the azabases are noticeably different from those of the natural bases [12,[14][15][16]. Therefore, it is both fundamentally informative and important to examine how a single nitrogen substitution affects the photophysical properties and excited-state dynamics observed in the nucleic acid bases.…”

“…The small fluorescence yields, together with the small photodegradation [90][91][92][93][94][95][96] and triplet quantum yields of ca. 10 -2 in aqueous solution [12,[97][98][99][100][101][102], show that most of the excited-state population decays nonradiatively back to the ground state.…”

“…On the other hand, the photophysics of the azabase derivatives can be significantly simplified by using acetonitrile as the solvent. Suzuki and co-workers [12,[14][15][16] have recently performed the first steady-state and time-resolved experiments with azabase analogues in acetonitrile solution. These studies will be reviewed briefly below.…”

“…The steady-state absorption and emission properties of the azabases in acetonitrile, including 6-azauridine and 6-aza-2-thiothymine, were investigated by Suzuki and co-workers [12,[14][15][16]. In general, the absorption spectra of the azabase derivatives shift slightly to the red compared to those of the natural bases, with the largest redshift observed in 6-aza-2-thiothymine, presumably because of the additional sulfur substitution (Table 18).…”