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Abstract: UV–VIS absorption, circular dichroism, room-temperature emission and nanosecond transient absorption measurements as well as steady-state photochemical methods were used to determine the photophysical and photochemical properties of the lowest excited triplet state (T1) of 2′,3′,5′-tri-O-acetyl-thioinosine (6-thiopurine 2′,3′,5′-tri-O-acetyl-riboside, TI) in acetonitrile (ACN) solutions. The experimental data were supplemented by ab initio quantum chemical calculations on 9-methyl-6-thiopurine (Me9TP), a model molecule for the nucleoside. The ground state tautomeric structures were computed at the MP2/aug-cc-pVTZ level of theory, and the CPCM model was used for an evaluation of the solvent effects. The results show that, in contrast to the gas phase, the thione tautomer is about 6kcal/mol more stable than the thiol form in ACN solution. The predominance of the thione tautomer of TI in solution was confirmed by the good agreement between the measured UV absorption spectrum of TI and the calculated singlet electronic transitions and intensities of the thione form of Me9TP. In addition there was a very close resemblance between the experimental UV spectrum of TI and that of its derivative fixed by the methyl group in the thione tautomeric form. The T1 state of TI was characterized by its energy, phosphorescence (), nonradiative process (ϕ nr), photochemical reaction (ϕ pch) quantum yields, intrinsic lifetime (), rate constants of self-quenching (k sq), phosphorescence (k p), and nonradiative processes (k nr). The rate constants for the quenching of the T1 state of TI by standard triplet quenchers (O2, KI) and by common constituents of nucleic acids (pyrimidine and purine nucleosides) were also determined. The results show that the T1 state of TI exhibits the properties typical of the 3(ππ*) states of aromatic thiocarbonyl compounds, i.e. weak room-temperature phosphorescence (), fast self-quenching process (k sq =7.4×109 M−1 s−1) and high reactivity towards O2 (k q ∼6.8×109 M−1 s−1). Mechanistic studies of the steady-state photolysis of TI in air-equilibrated ACN solutions revealed that TI acts as a sensitiser and an acceptor of singlet oxygen. [Copyright &y& Elsevier]

The absorption, phosphorescence and phosphorescence excitation spectra, phosphorescence quantum yields, and T(1) excited state lifetimes of four 4-thiouracil derivatives were measured for the first time in chemically inert and very weakly interacting perfluoro-1,3-dimethylcyclohexane at room temperature. The set of the 4-thiouracil derivatives comprises 1-propyl-4-thiouracil (PTU) and the related compounds having a methyl substituent at the position α to the thiocarbonyl group, namely 1-propyl-4-thiothymine (PTT), 1,3-dimethyl-4-thiouracil (DMTU), and 1-methyl-3-trideuteriomethyl-4-thiouracil ([D(3)]DMTU). Quantitative information on the intramolecular decay of the T(1) excited state of the four 4-thiouracil derivatives is presented, and the mechanism and dynamics of this process are discussed. In the absence of self quenching and solvent induced deactivation, the T(1) decay of the four 4-thiouracil derivatives was dominated by intramolecular nonradiative processes (NR). The values of the rate constant k(NR) in DMTU and [D(3)]DMTU are about 4 times larger than that in PTT and about 3 times larger than that in PTU. The reasons for the enhanced nonradiative rate constant in DMTU are discussed. It is concluded that the faster rate of the nonradiative processes in DMTU is related to a larger contribution from mixing of the T(2) (nπ*) state into the lowest energy T(1) (ππ*) state, as compared to the analogous coupling in PTU and PTT. This conclusion is supported by ab initio calculations performed at the EOM-CC2/aug-cc-pVDZ level of theory. The energy spacing between the T(2) (nπ*) and T(1) (ππ*) states is estimated to be about 500, 1100, and 2000 cm(-1) for DMTU, PTU, and PTT, respectively. Among the three compounds in question, the predicted energy spacing is thus the smallest for DMTU., (This journal is © The Royal Society of Chemistry and Owner Societies 2011)

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.

The one-electron reduction of bis[1-(2',3',5'-tri-O-acetylribosyl)uracil-4-yl] disulfide, initiated by hydrated electrons in a radiation chemical study, has been shown to yield 1-(2',3',5'-tri-O-acetylribosyl)-4-thiouracil as a stable molecular product. The reduction reaction leads, in the first instance, to a transient, albeit remarkably stable disulfide radical anion. This is characterized by a 2-center-3-electron bond with two bonding sigma-electrons and an antibonding sigma*-electron in the sulfur-sulfur bridge, (-S therefore S-)(-). It receives its stability from a sandwich-structure with the two uracilyl moieties facing each other (possibly further assisted by the 2',3',5'-tri-O-acetylribosyl substituents). A considerable lengthening of the original disulfide bridge from 2.02 to 2.73 A in the radical anion seems to facilitate the interaction of the heterocycles and leads to a gain in stabilization energy of 24 and 33 kcal/mol (100 and 140 kJ/mol) as evaluated by UMP2/cc-pVTZ and UMP2/cc-pVDZ calculations, respectively. The (-S therefore S-)(-) bonded radical anion shows a broad optical absorption band with lambdamax=450 nm, epsilonmax=6000 M(-1) cm(-1), and a half-width of 1.0 eV. It exists in equilibrium with the conjugated 1-(2',3',5'-tri- O-acetylribosyl)uracil-4-yl thiyl radical -S(*), and the corresponding thiolate, -S(-). The rate determining step for the disappearance of the disulfide radical anion appears to be protonation of both the radical anion and the free thiolate by reaction with H(+)aq. Absolute rate constants have been measured for these protonation processes, for the formation of the stable thiouridine product, and for the electron transfer from the disulfide radical anion to molecular oxygen. With the (-S therefore S-)(-) <--> -S(*) + -S(-) equilibrium lying very much on the left-hand side, the reduced disulfide system exhibits predominantly reducing properties whereas any oxidizing property of the conjugated thiyl radical has only little if any chance to materialize. Besides attaching directly to the disulfide bridge, the hydrated electrons react also, with about equal efficiency, with the uracil moiety of the investigated compound. This leads to a structurally totally different and electronically distinguishable species than that with the reduced disulfide bridge. In particular, there is no face-to-face interaction between the two heterocyclic moieties and no increased electron density in the S-S bond. The C-centered radicals resulting from the reduction of the uracil and possibly also generated from the ribosyl moieties initiate further cleavage of the S-S bond and thus contribute to the formation of thiouridine. The overall yield of the latter, as determined from steady-state gamma-radiolysis, indicates a small chain process (G=1.54 micromol/J). Possible mechanisms are discussed.

The reactions of a disulfide and a benzylsulfide derived from 4-thiouridine were studied in aqueous acetonitrile using stationary and laser flash photolysis methods. Irradiation of the compounds results in specific cleavage of the S-S bond in the disulfide and the S-CH(2) bond in the sulfide. Identical pyrimidine-derived intermediates were observed in the transient absorption spectra (lambda(max) = 420 nm, epsilon(max) approximately 2500 M(-1) cm(-1)) recorded for both compounds in laser flash photolysis experiments. The intermediate was identified as the 4-pyrimidinylthiyl radical. Irradiation of the disulfide in the absence of oxygen gives 4-thiouridine while the sulfide under identical conditions produced, additionally, 3-benzyl-4-thiouridine as a stable photoproduct. The formation of the latter photoproduct provides evidence for the existence of the N-centered 4-thioxopyrimidynyl radical formed from the initially produced S-centered (thiyl) radical. The 4-thiouridine is formed from the radicals generated in the primary photochemical step by an H abstraction reaction from the solvent (acetonitrile) or from additives (alcohols) that were purposely added. Interestingly, in contrast to the benzylsulfide, the photoreaction of the disulfide is quenched by molecular oxygen with the concomitant formation of uridine. However it appears that uridine is not produced as a result of the reaction of the radicals with oxygen. A mechanism is proposed for the photochemical transformations of the disulfide and benzylsulfide derived from 4-thiouridine. The proposed mechanism is based on the structures of the identified stable photoproducts, the values of the photoreaction quantum yields determined under differing irradiation conditions, and the flash photolysis results.

The spectral and photophysical behavior of four fluorescent 9-oxo-imidazo[1,2-a]purine derivatives containing pyridyl, pyridylphenyl, phenyl, and biphenylyl substituents at the C(6) position of the tricyclic skeleton is described. The studies were performed in several aprotic and protic organic solvents using absorption spectroscopy as well as steady-state and time-resolved fluorescence spectroscopy. The results are also presented of TDDFT calculations on singlet-singlet excitation energies and oscillator strengths for two models of 9-oxo-imidazo[1,2-a]purine, with phenyl or pyridyl substituents, both in the gas phase and in methanol solution. While the derivatives with aryl substituents did not show any significant dependence of their static and dynamic fluorescence properties on the nature of the solvent, the compounds containing a pyridine residue exhibited a remarkable reduction of their fluorescence quantum yields and lifetimes in the alcoholic solutions. The solute-solvent hydrogen-bonding interaction in the first excited singlet state is responsible for the fast radiationless decay rates determined for pyridyl- and pyridylphenyl-substituted compounds in protic solvents. The results of experimental and theoretical studies show that the hydrogen of the alcohols' hydroxyl group and the nitrogen atom of the pyridine moiety are involved in the interaction. The fluorescence-quenching experiments performed for the pyridyl-substituted 9-oxo-imidazo[1,2-a]purine derivative using trifluoroethanol, methanol, and butanol as quenchers revealed that the quenching efficiencies, expressed by the Stern-Volmer quenching constants, correlate with the H-bond donating abilities of the alcohols. The quenching is a dynamic process, and the H-bonded complex formed is nonfluorescent. The experimentally determined and the calculated values of the dipole moment change associated with the electronic excitation indicate that the excited S(1) states of all of the molecules studied in this work have an intramolecular charge-transfer character and that electronic charge is transferred to the C(6) substituent upon excitation. Thus, the ability of the pyridyl substituent nitrogen atom to act as an H-bond acceptor in the excited S(1) state is enhanced. The 6-pyridyl-9-oxo-imidazo[1,2-a]purine presents a novel fluorophore, which, besides its medical applications, may be useful as a sensor of hydroxyl groups in microorganized systems.

A novel solvatochromic betaine dye has been synthesized from xanthosine and characterized spectroscopically by UV-vis in a broad range of solvents. The dye 9-(2',3',5'-tri-O-acetyl-beta-d-ribofuranosyl)-2-(pyridinium-1-yl)-9H-purin-6-olate, 1a, exhibits solvent-induced spectral band shifts that are (2)/(3) as large as that of the betaine known as Reichardt's dye, which forms the basis of the E(T)(30) solvent polarity scale. Moreover, the dye 1a is a ribonucleoside and hence has the potential application as a polarity probe for application in RNA oligonucleotides. The isomeric dye 6-(pyridinium-1)-yl-9H-purin-2-olate, 2a, has also been synthesized and exhibits slightly smaller solvatochromic band shifts. The new betaine dyes have also been studied by comparing the experimental and calculated solvatochromic shifts based on the calculation of the UV/vis absorption spectra, using a combination of methods with density functional theory (DFT). The COSMO continuum dielectric method, an applied electric field term in the Hamiltonian, and time-dependent density functional theory (TD-DFT) methods were used to obtain absorption energies, ground-state dipole moments, and the difference dipole moment between the ground and excited states. The calculations predict a lower energy absorption band of charge-transfer character that is highly solvatochromic, and a higher energy absorption band that has pi-pi character which is not solvatochromic, in agreement with the experimental data. For Reichardt's dye the difference dipole moment between the ground and excited state (Deltamu = mu(e) - mu(g)) was also calculated and compared to experiment: Deltamu(calcd) = -6 D and Deltamu(exptl) = -9 +/- 1 D.(1) The ground-state dipole moment was found to be mu(g)(calcd) = 18 D and mu(g)(exptl) = 14.8 +/- 1.2 D.(1).

The photochemistry of 2',3',5'-tri-O-acetyl-5-iodo-4-thiouridine (3) in deoxygenated 1:1 CH(3)CN-H(2)O pH 5.8 (phosphate buffer) solution has been studied by means of steady-state and nanosecond laser flash photolysis methods. Under steady-state irradiation (lambda > or = 334 nm), the stable photoproducts were iodide ion, 2',3',5'-tri-O-acetyl-4-thiouridine (4), and two disulfides. The disulfides were the symmetrical bis-(2',3',5'-tri-O-acetyl-5-iodo-4-thiouridine) (5) and unsymmetrical 6, which contains both 4-thiouridine and 5-iodo-4-thiouridine residues. The formation of the dehalogenated photoproduct suggests that C(5)-I bond cleavage is a primary photochemical step. Attempts to scavenge the resulting C(5)-centered radical by suitable addends, bis-(N-alpha-acetyl)cystine-bis-N-ethylamide or benzene, were unsuccessful. Analysis of the photoproducts formed under these conditions showed that the S-atom is the reactive center. The photoproduct 4, obtained by irradiation of 3 in CD(3)CN-H(2)O, followed by reversed-phase HPLC isolation using nonlabeled eluents, did not contain deuterium. An analogous experiment performed in CH(3)CN-D(2)O gave deuterated product 4-d with 88% of the deuterium incorporated at C(5). Transient absorption observed upon laser excitation (lambda= 308 nm) of 3 was assigned to the 4-uridinylthiyl radical on the basis of the similarity of this spectrum with that obtained upon laser photolysis of the disulfide: bis-(2',3',5'-tri-O-acetyl-4-thiouridine) 14. On the basis of the results of steady-state and laser photolysis studies, a mechanism of the photochemical reaction of 3 is proposed. The key mechanistic step is a transformation of the C(5)-centered radical formed initially by C(5)-I bond cleavage into a long-lived S-centered radical via a 1,3-hydrogen shift. Theoretical calculations confirmed that the long-lived S-centered radical is the most stable radical derived from the 4-thiouracil residue.

Tricyclic (T, 3,9-dihydro-9-oxo-SH-imidazo[g2-c]purine) analogues of acyclovir (ACV, 1), substituted in the 6 position with pyrid-4-yl, 4-(pyrid-4'-yl)Ph, 4-(pyrimidin-5-yl)Ph and 4-(thiazol-2'-yl)Ph units were synthesized. For the synthesis of the heteroarylphenyl derivatives, a convenient general route was developed, ie., Suzuki cross-coupling between protected 6-(4-dihydroxyborylphenyl) TACV and easily available bromoheterocycles. Fluorescent properties of newly synthesized TACV aoalogues strongly depend on the nature of a solvent. This sensitivity of fluorescence makes the compounds promising probes of H-bonding in the environment.

Tricyclic (T) analogues of acyclovir (ACV, 1) and ganciclovir (GCV, 2) carrying the 3,9-dihydro-9-oxo-5H-imidazo[1,2-a]purine system [i.e., 6-(4-BrPh)TACV, 5 and 6-(4-BrPh)TGCV, 6] were transformed into 6-[(4'-R2)-4-biphenylyl] derivatives of TACV (7-9) and TGCV (10-12) by Suzuki cross coupling with 4-substituted phenylboronic acids. Compound 11 (R2 = CH2OH) showed a high (approximately 1000) selectivity index against herpes simplex virus type 1 (HSV-1) together with advantageous fluorescence properties (emission in visible region, little overlap with absorption and moderate intensity).

Intramolecular processes of deactivation of 1,3-dimethyl-4-thiouracil (DMTU) from the second excited singlet (S2) (pi, pi*) and the lowest excited triplet (T1) (pi, pi*) states have been studied using perfluoro-1,3-dimethylcyclohexane (PFDMCH) as a solvent. The spectral and photophysical (PP) properties of DMTU in CCl4, hexane and water have also been described. For the first time, the fluorescence from S2 state DMTU has been observed. The picosecond lifetime of DMTU in the S2 state (tau(S2)) in PFDMCH has been proposed to be determined by a very fast intramolecular reversible process of hydrogen abstraction from the ortho methyl group by the thiocarbonyl group. The shortening of tau(S2) in CCl4 is interpreted to be caused by the intermolecular interactions between DMTU (S2) and the solvent. Results of the phosphorescence decay as a function of DMTU concentration were analyzed using the Stern-Volmer formalism, which enabled determination of the intrinsic lifetime of the T1 state (tau0(T1)) and rate constants of self-quenching (k(sq)). The lifetimes, tau0(T1), of DMTU in PFDMCH and CCl4 are much longer than the values hitherto obtained in more reactive solvents. The PP properties of DMTU both in the S2 and T1 states have been shown to be determined by the thiocarbonyl group.