Reaction of sulphate radical anion (SO<SUB>4</SUB><SUP>&#8226;-</SUP>) with hydroxy- and methyl-substituted pyrimidines: a pulse radiolysis study

Reactions of sulphate radical anion (SO4&#8226;-) with 4,6-dihydroxy-2-methyl pyrimidine (DHMP), 2,4-dimethyl-6-hydroxy pyrimidine (DMHP), 6-methyl uracil (MU) and 5,6-dimethyl uracil (DMU) have been studied by pulse radiolysis at pH 3 and at pH 10. The transient intermediate spectra were compared with those from the reaction of hydroxyl radical (&#8226;OH). It is proposed that SOSO4&#8226;- produces radical cations of these pyrimidines in the initial stage. These radical cations are short-lived except in the case of DMHP where a relatively longer lived radical cation is proposed to be formed. When there is a hydrogen atom attached to the N(1) or N(3) position, a deprotonation from these sites is highly favored. When there is no hydrogen attached to these sites, deprotonation from a substituted methyl group is favored. At acidic pH, deprotonation from nitrogen is observed for DHMP, MU and DMU. At basic pH, the radical cation reacts with OH- leading to the formation of OH adducts

Reaction of oxide radical ion (O<SUP>&#8226;-</SUP>) with substituted pyrimidines

Pulse radiolysis technique has been used to investigate the reaction of oxide radical ion (O&#8226;-) with 4,6-dihydroxy-2-methyl pyrimidine (DHMP), 2,4-dimethyl-6-hydroxy pyrimidine (DMHP), 5,6-dimethyl uracil (DMU) and 6-methyl uracil (MU) in strongly alkaline medium. The second-order rate constants for the reaction of O&#8226;- with these compounds are in the range 2-5 &#215; 108 dm3 mol-1 s-1. The transient absorption spectra obtained with DHMP have two maxima at 290 and 370 nm and with DMHP have maxima at 310 and 470 nm. The transient spectrum from DMU is characterized by its absorption maxima at 310 and 520 nm and that of MU by its single maximum at 425 nm. The intermediate species were found to react with N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) with high G(TMPD&#8226;+) values ranged between 3.9 &#215; 10-7 molJ-1 and 4.8 &#215; 10-7 molJ-1. These radicals undergo decay by second-order kinetics (2k/&#949; = 1.0-1.7 &#215; 106 s-1). The reaction of O&#8226;- with the selected pyrimidines is proposed to proceed through a hydrogen abstraction from the methyl group forming allyl type radicals. These are mainly oxidizing radicals and hence readily undergo electron transfer reactions with TMPD

Properties of the OH Adducts of Hydroxy-, Methyl-, Methoxy-, and Amino-Substituted Pyrimidines: Their Dehydration Reactions and End-Product Analysis

Reactions of hydroxyl radicals (•OH) with 2-amino-4-methyl pyrimidine (AMP), 2-amino-4,6-dimethyl pyrimidine (ADMP), 2-amino-4-methoxy-6-methyl pyrimidine (AMMP), 2-amino-4-hydroxy-6-methyl pyrimidine (AHMP), 4,6-dihydroxy-2-methyl pyrimidine (DHMP), 2,4-dimethyl-6-hydroxy pyrimidine (DMHP), 6-methyl uracil (MU), and 5,6-dimethyl uracil (DMU) have been studied by pulse radiolysis and steady-state radiolysis techniques at different pH values. The second-order rate constants of the reaction of •OH with these systems are of the order of (2−9) × 10^9 dm^3 mol^(-1) s^(-1) at near neutral pH. The difference in the spectral features of the intermediates at near neutral pH and at higher pH (10.4) obtained with these pyrimidines are attributed to the deprotonation of the OH adducts. The G(TMPD•+) obtained at pH ∼ 6, from the electron-transfer reactions of the oxidizing intermediates with the reductant, N,N,N‘,N‘-tetramethyl-p-phenylenediamine (TMPD), are in the range (0.2−0.9) × 10^(-7) mol J^(-1) which constituted about 3−16% oxidizing radicals. These yields were highly enhanced at pH 10.5 in the case of AHMP, DHMP, DMU, and MU (G(TMPD^(•+)) = 3.8−5.5 ≅ 66−95% oxidizing radical). On the basis of these results, it is proposed that a nonoxidizing C(6)-ylC(5)OH radical adduct is initially formed at pH 6 which is responsible for the observed transient spectra. The high yield of TMPD•+ at higher pH is explained in terms of a base-catalyzed conversion (via a dehydration reaction) of the initially formed C(6)-ylC(5)OH adduct (nonoxidizing) to C(5)-ylC(6)OH adduct which is oxidizing in nature. Among the selected pyrimidines, such a dehydration reaction was observed only with those having a keto (or hydroxy) group at the C(4) position of the pyrimidine ring. Qualitative analyses of the products resulting from the OH adducts of DHMP (at pH 4.5) and DMHP (at pH 6) were carried out using HPLC-ES-MS and a variety of products have been identified. Glycolic and dimeric products were observed as the major end-products. The product profiles of both DHMP and DMHP have shown that the precursors of the products are mainly the C(6)-ylC(5)OH and the H adduct radicals. The identified products are formed mainly by disproportionation and dimerization reactions of these radicals. The mechanistic aspects are discussed

Seminaphthofluorescein-Based Fluorescent Probes for Imaging Nitric Oxide in Live Cells

Fluorescent turn-on probes for nitric oxide based on seminaphthofluorescein scaffolds were prepared and spectroscopically characterized. The Cu(II) complexes of these fluorescent probes react with NO under anaerobic conditions to yield a 20–45-fold increase in integrated emission. The seminaphthofluorescein-based probes emit at longer wavelengths than the parent FL1 and FL2 fluorescein-based generations of NO probes, maintaining emission maxima between 550 and 625 nm. The emission profiles depend on the excitation wavelength; maximum fluorescence turn-on is achieved at excitations between 535 and 575 nm. The probes are highly selective for NO over other biologically relevant reactive nitrogen and oxygen species including NO3–, NO2–, HNO, ONOO–, NO2, OCl–, and H2O2. The seminaphthofluorescein-based probes can be used to visualize endogenously produced NO in live cells, as demonstrated using Raw 264.7 macrophages.National Science Foundation (U.S.) (CHE-0611944)National Institutes of Health (U.S.) (K99GM092970

Decomposition of S-Nitrosothiols Induced by UV and Sunlight

Photochemical release of nitric oxide (NO) from the S-nitroso derivatives of glutathione, L-cysteine, N-acetyl-L-cysteine, L-cysteinemethylester, D,L-penicillamine, N-acetyl-D,L-penicillamine, and N-acetylcysteamine has been investigated at neutral and acidic pH. The release of NO from RSNO is one of the key reactions that could be utilized in photodynamic therapy. The UV-VIS and HPLC analyses have shown that under argon saturated conditions, disulfide (RSSR) is the major product of UV as well as sunlight induced decomposition. While in aerated conditions, nitirite—the end product of the oxidation of NO—was also observed along with disulfide. The formation of thiyl radical as the intermediate was reconfirmed by laser flash photolysis. The initial rate of formation of NO was on the order of 10−10dm3mol−1s−1. The quantum yields of these reactions were in the range of 0.2–0.8. The high quantum yields observed in the photo induced release of NO from RSNO using both UV and sunlight demonstrate the potential application of these reactions in photodynamic therapy

Oxidation Reactions of 2‑Thiouracil: A Theoretical and Pulse Radiolysis Study

The reaction of hydroxyl radical (^•OH) with the nucleic acid base analogue 2-thiouracil (<b>1</b>) has been studied by pulse radiolysis experiments and DFT. The generic intermediate radicals feasible for the ^•OH reactions with <b>1</b>, namely, one electron oxidation product (<b>1</b>^•+), ^•OH-adducts (<b>3</b>^•, <b>4</b>^•, and <b>5</b>^•), and H-abstracted radicals (<b>6</b>^• and <b>7</b>^•), were characterized by interpreting their electronic and structural properties along with calculated energetics and UV–vis spectra. Pulse radiolysis experiments showed that the transient formed in the reaction of ^•OH with <b>1</b> in water at pH 6.5 has λ_max at 430 nm. A bimolecular rate constant, <i>k</i>₂ of 9.6 × 10⁹ M^–1 s^–1, is determined for this reaction via competition kinetics with 2-propanol. The experiments suggested that the transient species could be a dimer radical cation <b>2</b>^•+, formed by the reaction of <b>1</b> with the radical cation <b>1</b>^•+. For this reaction, an equilibrium constant of 4.7 × 10³ M^–1 was determined. The transient formed in the reaction of <b>1</b> with pulse radiolytically produced Br₂^•– at pH 6.5 as well as Cl₂^•– at pH 1 has also produced λ_max at 430 nm and suggested the formation of <b>2</b>^•+. The calculated UV–vis spectra of the transient species (<b>1</b>^•+, <b>3</b>^•, <b>4</b>^•, <b>5</b>^•, <b>6</b>^•, and <b>7</b>^•) showed no resemblance to the experimental spectra, while that of <b>2</b>^•+ (λ_max = 420 nm) agreed well with the experimental value and thus confirmed the formation of <b>2</b>^•+. The 420 nm peak was due to σ → σ* electronic excitation centered on a 2-center–3-electron (2c–3e) sulfur–sulfur bond [−S∴S−]. <b>2</b>^•+ is the first reported example of a dimer radical cation in a pyrimidine heterocyclic system. Further, 5-C and 6-C substituted (substituents are −F, −Cl, −NH₂, −N­(CH₃)₂, −OCH₃, −CF₃, −CH₃, −CH₂CH₃, <i>n</i>-propyl, phenyl, and benzyl) and 5,6-disubstituted 2-thiouracil systems have been characterized by DFT and found that the reaction (<b>1</b> + <b>1</b>^•+ → <b>2</b>^•+) is exergonic (1.12–13.63 kcal/mol) for many of them

Charge transfer from 2-aminopurine radical cation and radical anion to nucleobases: a pulse radiolysis study

Pulse radiolysis study has been carried out to investigate the properties of the radical cation of 2-aminopurine (2AP) and the probable charge transfer from the radical cation and radical anion of 2AP to natural nucleobases in aqueous medium. The radical cation of 2AP was produced by the reaction of sulfate radical anion (SO4&#8226;-). The time resolved absorption spectra obtained by the reaction of SO4&#8226;- with 2AP at neutral pH have two distinct maxima at 380 and 470 nm and is assigned to the formation of a neutral radical of the form 2AP-N2(-H)&#8226; (k2 = 4.7 &#215; 109 dm3 mol-1 s-1 at pH 7). This neutral radical is formed from the deprotonation reaction of a very short-lived radical cation of 2AP. The transient absorption spectra recorded at pH 10.2 have two distinct maxima at 400 and 480 nm and is assigned to the formation of a nitrogen centered radical (2AP-N(9)&#8226;). As the hole transport from 2AP to guanine is a highly probable process, the reaction of SO4&#8226;- is carried out in the presence of guanosine, adenosine and inosine. The spectrum obtained in the presence of guanosine was significantly different from that in the absence and it showed prominent absorption maxima at 380 and 470 nm, and a weak broad maximum centered around 625 nm which match well with the reported spectrum of a neutral guanine radical (G(-H)&#8226;). The electron transfer reaction from the radical anion of 2AP to thymine (T), cytidine (Cyd) and uridine (Urd) was also investigated at neutral pH. Among the three pyrimidines, only the transient spectrum in the presence of T gave a significant difference from the spectral features of the electron adduct of 2AP, which showed a prominent absorption maximum at 340 nm and this spectrum is similar to the electron adduct spectrum of T. The preferential reduction of thymine by 2AP&#8226;- and the oxidation of guanosine by 2AP&#8226;+ clearly follow the oxidation/reduction potentials of the purines and pyrimidines

Reaction of sulphate radical anion (SO .¡ 4 / with hydroxy- and methyl-substituted pyrimidines: a pulse radiolysis study

Abstract-Reactions of sulphate radical anion (SO .¡ 4 / with 4,6-dihydroxy-2-methyl pyrimidine (DHMP), 2,4-dimethyl-6-hydroxy pyrimidine (DMHP), 6-methyl uracil (MU) and 5,6-dimethyl uracil (DMU) have been studied by pulse radiolysis at pH 3 and at pH 10. The transient intermediate spectra were compared with those from the reaction of hydroxyl radical ( . OH). It is proposed that SO .¡ 4 produces radical cations of these pyrimidines in the initial stage. These radical cations are short-lived except in the case of DMHP where a relatively longer lived radical cation is proposed to be formed. When there is a hydrogen atom attached to the N(1) or N(3) position, a deprotonation from these sites is highly favored. When there is no hydrogen attached to these sites, deprotonation from a substituted methyl group is favored. At acidic pH, deprotonation from nitrogen is observed for DHMP, MU and DMU. At basic pH, the radical cation reacts with OH ¡ leading to the formation of OH adducts

Anomalous high reactivity of formyl and acetone ketyl radicals with uracil and its derivatives

CO2&#8226;- and (CH3)2&#183;COH radicals apparently react with uracil and its derivatives, containing two carbonyl groups, with high rate constants (k=1010 dm3 mol-1 s-1). Various mechanistic aspects of such reactions have been probed. The effect of pH and buffer concentration on the initial formation of absorbance points to some keto-enol tautomerism-type fast-reaction between OH- and the substrate, followed by a slower relaxation to the original equilibrium. The radical anions of these compounds react with methyl viologen mainly via an electron transfer reaction with a high rate constant value (4-9)&#215; 109 dm3 mol-1 s-1