Photocycloaddition of the T1 excited state of thioinosine to uridine and adenosine

Abstract

Novel photoadducts were obtained by irradiation of thioinosine (6-thiopurine riboside, TI) in deaerated aqueous solution without and in the presence of uridine and adenosine. Excitation (lambda > 300 nm) of TI to its excited S2 state yields a single bimolecular photoproduct. It is a purine-pyrimidine diriboside in which the purine ring is attached to the amide nitrogen of 6-amino-4-thioxo-5-formamidopyrimidine. When TI was irradiated in the presence of an excess of adenosine, two photoproducts were isolated: diribosides of N-(4,6-diaminopirymidin-5-yl)-N-formyl-6-aminopurine and N-(4-amino-6-formylamino-pyrimidin-5-yl)-6-aminopurine, both containing a purine and a formylaminopyrimidine (Fapy) fragment. The photoreaction of TI with uridine gave two regioisomeric photoproducts identified as diribosides containing either 5- or 6-(purin-6-yl)uracil as aglycones. A multistep mechanism leading to the stable photoproducts is proposed. In the first step of the mechanism, the C=S group of the excited TI undergoes a [2 + 2] cycloaddition regioselectively to the N(7)=C(8) bond of the purine ring or adds in a non-regioselective manner to the C(5)=C(6) bond of uracil. The unstable photoproducts thus formed undergo a series of dark reactions at room temperature. The photocycloaddition reactions originate from the excited T1 state of TI. This conclusion is supported by a combination of evidence from reaction quenching studies using both steady-state quantum yield determinations and kinetics results from nanosecond laser flash photolysis. The T1 state of TI is quenched by other TI molecules in their S0 state (self-quenching) and also by uridine and adenosine, all with large rate constants (0.8-5) x 10(9) M(-1) s(-1). The quantum yields of the reactions are in general very low (phi(R) < or = 8 x 10(-3)). The sources of the inefficiency in the photocycloaddition of TI to uridine and adenosine are discussed. The photoproducts containing the Fapy residue undergo deformylation and isomerization of the ribosyl moiety (anomerization, furanose/pyranose transformation) upon heating in aqueous solution. Products of the transformations were identified.

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

“…The self-quenching constants depend on the viscosity of the solvent, suggesting they are close to the diffusion-controlled limit [195]. Similar results were obtained more recently for 6-thiopurine [208] and 9-propyl-6-thiopurine [21] upon nanosecond laser excitation at 355 nm in aqueous buffer solution at pH 5.8. Attempts to determine the intersystem crossing rate constant of 6-thiopurine by using picosecond transient absorption spectroscopy (35 ps pulse duration) were unsuccessful [195] and the authors concluded that it must have a value greater than 1.3 Â 10 10 s À1 .…”

“…Excitation at 355 nm with 8 ns laser pulses results in the prompt population of the triplet state [13]. It was determined that nonradiative processes play a major role in the deactivation of the triplet state (Table 11), with a fraction of the triplet state population decaying by self-quenching (Table 17) [13], as observed previously in 6-thiopurine [195,208]. Based on the El-Sayed propensity rules [214,215] and on vertical excited-state calculations performed in acetonitrile at the CIS/CPCM/aug-cc-pVTZ level of theory, the authors proposed that intersystem crossing should occur from the S 1 (nπ*) state to the T 1 (ππ*) state upon direct excitation of the S 2 (ππ*) state, i.e., S 2 (ππ*) !…”

Photochemical and Photodynamical Properties of Sulfur‐Substituted Nucleic Acid Bases,

“…Similarly, the lowest‐energy absorption band of 6‐thio‐2′‐deoxyguanosine and 2′,3′,5′‐tri‐ O ‐acetyl‐6,8‐dithioguanosine redshifts by 90 (10 515 cm −1 ) and 130 nm (13 594 cm −1 ) relative to 2′‐deoxyguanosine , whereas that of S 6 ‐methylthioinosine and 6‐thioinosine redshifts by 42 (5796) and 71 nm (8911 cm −1 ) relative to inosine . A low‐energy absorption tail is also observed in the absorption spectra of 6‐thiopurine, 6‐thioguanine, 6‐thioinosine and 6‐thio‐2′‐deoxyguanosine that extends into the near‐visible region and that is redshifted in going from aqueous to organic solvents . Semi‐empirical , single‐reference and multiconfigurational calculations have consistently assigned the absorption tail to the S 0 → S 1 (n S π*) transition, while the lowest‐energy absorption band has been assigned to the S 0 → S 2 (π S π*) transition.…”

“…2 μs in 2‐propanol to ca. 5 μs in ethanol , while that of 6‐thioinosine ranges from 1.25 μs in aqueous solution at pH 5.8 to 3.2 μs in acetonitrile . An intrinsic triplet decay lifetime of 1.1 ± 0.1 μs has been reported for 6‐thio‐2′‐deoxyguanosine in aqueous solution , while the intrinsic triplet decay lifetimes of 6‐thioguanine and azathioprine have not been reported.…”

“…The lowest‐energy absorption maximum of 6‐thioguanine redshifts by 92 nm (10 835 cm −1 ) relative to the absorption maximum of guanine, whereas that of azathioprine, 6‐thiopurine and 2,6‐dithiopurine redshifts by 17 (2309), 57 (6773) and 86 nm (9370 cm −1 ) relative to the purine‐free base . Similarly, the lowest‐energy absorption band of 6‐thio‐2′‐deoxyguanosine and 2′,3′,5′‐tri‐ O ‐acetyl‐6,8‐dithioguanosine redshifts by 90 (10 515 cm −1 ) and 130 nm (13 594 cm −1 ) relative to 2′‐deoxyguanosine , whereas that of S 6 ‐methylthioinosine and 6‐thioinosine redshifts by 42 (5796) and 71 nm (8911 cm −1 ) relative to inosine . A low‐energy absorption tail is also observed in the absorption spectra of 6‐thiopurine, 6‐thioguanine, 6‐thioinosine and 6‐thio‐2′‐deoxyguanosine that extends into the near‐visible region and that is redshifted in going from aqueous to organic solvents .…”