The reaction of triplet flavin with indole. A study of the cascade of reactive intermediates using density functional theory and time resolved infrared spectroscopy

As a model for riboflavin, lumiflavin was investigated using density functional theory methods (B3LYP/6-31G* and B3LYP/6-31+G**) with regard to the proposed cascade of intermediates formed after excitation to the triplet state, followed by electron-transfer, proton-transfer, and radical[bond]radical coupling reactions. The excited triplet state of the flavin is predicted to be 42 kcal/mol higher in energy than the singlet ground state, and the pi radical anion lies 45.1 kcal/mol lower in energy than the ground-state flavin and a free electron in the gas phase. The former value compares to a solution-phase triplet energy of 49.8 kcal/mol of riboflavin. For the radical anion, the thermodynamically favored position to accept a proton on the flavin ring system is at N(5). A natural population analysis also provided spin density information for the radicals and insight into the origin of the relative stabilities of the six different calculated hydroflavin radicals. The resulting 5H-LF* radical can then undergo radical[bond]radical coupling reactions, with the most thermodynamically stable adduct being formed at C(4'). Vibrational spectra were also calculated for the transient species. Experimental time-resolved infrared spectroscopic data obtained using riboflavin tetraacetate are in excellent agreement with the calculated spectra for the triplet flavin, the radical anion, and the most stable hydroflavin radical.     

Formation of secondary triplet species after excitation of fluoroquinolones in the presence of relatively strong bases

Laser flash photolysis of 7-(piperazin-1-yl) fluoroquinolones leads to the formation of a triplet excited state (3A*) at the end of the pulse (lambdamax 520, 610, and 620 nm for enoxacin, ciprofloxacin, and norfloxacin, respectively). Phosphate and bicarbonate buffers react with 3A* to form a secondary triplet (3B*, reaction rates (0.8-9.9) x 108 M-1 s-1), whose T-T absorption is red-shifted (lambdamax 670 nm for enoxacin, 700 nm for ciprofloxacin and norfloxacin). The formation of a secondary triplet is not a common process and disagrees with previous work suggesting that electron transfer occurs between phosphate buffer and the primary triplet excited state with the formation of the anion radical of the fluoroquinolone (FQ.-). We have shown that the FQ.- transient absorption spectrum is quite distinct from that of 3B*. The photophysical characteristics of 3B* have been determined by energy transfer to naproxen, and it has been found that its energy is lower than that of 3A*.     

Evaluation of Transient Absorption Spectra of N, N, N′, N′- Tetra - ( p -methylphenyl ) - 4, 4′- diamino - 1, 1′- diphenyl Ether ( TPDAE ) for Electron Transfer from TPDAE to Fullerenes ( C60 / C70 ) by Laser Flash Photolysis

Photoinduced electron transfer processes between fullerenes (C60 / C70) and N, N, N′, N′- tetra - ( p-methylphenyl ) - 4, 4′- diamino - 1, 1′- diphenyl ether ( TPDAE ) have been studied by nanosecond laser flash photolysis. Quantum yields and rate constants of electron transfer from TPDAE to excited triplet state of fullerenes (C60 / C70 ) in benzonitrile have been evaluated by observing the transient absorption bands in the near-IR region where the excited triplet state, radical anion of fullerenes ( C60 / C70 ) and radical cations of TPDAE appear.     

Evaluation of Transient Absorption Spectra of N, N, N, N- Tetra - ( p -methylphenyl ) - 4, 4- diamino - 1, 1- diphenyl Ether ( TPDAE ) for Electron Transfer from TPDAE to Fullerenes ( C_(60) / C_(70) ) by Laser Flash Photolysis

Fullerenes C60 and C70 have high electron affinity ( 2.6 - 2.8 ev ) and readily form anions on electronchemical reduction1, which were famous as electron acceptor in photo-excitation because of symmetrical shape, large size, and properties of its p - electron system2. After observation of molecular ferromagnetism3 in the tetrakis (dimethylamino ) ethylene salt of C60 as well as the occurrence of ultra-fast photoinduced electron transfer within the dimethyl aniline - C60 complex4, prompted us…     

Photoinduced release of nitroxyl and nitric oxide from diazeniumdiolates

Aqueous photochemistry of diazen-1-ium-1,2,2-triolate (Angeli's anion) and (Z)-1[N-(3-aminopropyl)-N-(3-aminopropyl)amino]diazen-1-ium-1,2-diolate (DPTA NONOate) has been investigated by laser kinetic spectroscopy. In neutral aqueous solutions, 266 nm photolysis of these diazeniumdiolates generates a unique spectrum of primary products including the ground-state triplet (3NO-) and singlet (1HNO) nitroxyl species and nitric oxide (NO*). Formation of these spectrophotometrically invisible products is revealed and quantitatively assayed by analyzing a complex set of their cross-reactions leading to the formation of colored intermediates, the N2O2*- radical and N3O3- anion. The experimental design employed takes advantage of the extremely slow spin-forbidden protic equilibration between 3NO- and 1HNO and the vast difference in their reactivity toward NO*. To account for the kinetic data, a novel combination reaction, 3NO-+1HNO, is introduced, and its rate constant of 6.6x10(9) M-1 s-1 is measured by competition with the reduction of methyl viologen by 3NO-. The latter reaction occurring with 2.1x10(9) M-1 s-1 rate constant and leading to the stable, colored methyl viologen radical cation is useful for detection of 3NO-. The distributions of the primary photolysis products (Angeli's anion: 22% 3NO-, 58% 1HNO, and 20% NO*; DPTA NONOate: 3% 3NO-, 12% 1HNO, and 85% NO*) show that neither diazeniumdiolate is a highly selective photochemical generator of nitroxyl species or nitric oxide, although the selectivity of DPTA NONOate for NO* generation is clearly greater.     

Reaction mechanism of the CCN radical with nitric oxide

To investigate the possibility of the carbyne radical CCN in removal of nitric oxide, a detailed computational study is performed at the Gaussian-3//B3LYP/6-31G(d) level on the CCN + NO reaction by constructing the singlet and triplet electronic state [C(2)N(2)O] potential energy surfaces (PESs). The barrierless formation of the chain-like isomers NCCNO (singlet at -106.5, triplet cis at -48.2 and triplet trans at -47.6 kcal/mol) is the most favorable entrance attack on both singlet and triplet PESs. Subsequently, the singlet NCCNO takes an O-transfer to form the branched intermediate singlet NCC(O)N (-85.6), which can lead to the fragments CN + NCO (-51.2) via the intermediate singlet NCOCN (-120.3). The simpler evolution of the triplet NCCNO is the direct N-O rupture to form the weakly bound complex triplet NCCN...O (-56.2) before the final fragmentation to NCCN + (3)O (-53.5). However, the lower lying products (3)NCN + CO (-105.6) and (3)CNN + CO (-74.6) are kinetically much less competitive. All the involved transition states for generation of CN + NCO and NCCN + (3)O lie much lower than the reactants. Thus, the novel reaction CCN + NO can proceed effectively even at low temperatures and is expected to play a role in both combustion and interstellar processes. Significant differences are found on the singlet PES between the CCN + NO and CH + NO reaction mechanisms.     

Mechanism of pH-dependent decomposition of monoalkylamine diazeniumdiolates to form HNO and NO, deduced from the model compound methylamine diazeniumdiolate, density functional theory, and CBS-QB3 calculations

Isopropylamine diazeniumdiolate, IPA/NO, the product of the reaction of isopropylamine and nitric oxide, NO, decomposes in a pH-dependent manner to afford nitroxyl, HNO, in the pH range of 13 to above 5, and NO below pH 7. Theoretical studies using B3LYP/6-311+G(d) density functional theory, the polarizable continuum and conductor-like polarizable continuum solvation models, and the high-accuracy CBS-QB3 method on the simplified model compound methylamine diazeniumdiolate predict a mechanism involving HNO production via decomposition of the unstable tautomer MeNN+(O-)NHO-. The production of NO at lower pH is predicted to result from fragmentation of the amide/NO adduct upon protonation of the amine nitrogen.     

A kinetic study on the inhibitory action of sympathomimetic drugs towards photogenerated oxygen active species. The case of phenylephrine

Kinetics and mechanism of the aerobic Riboflavin (Rf, vitamin B2) sensitized photodegradation of Phenylephrine (Phen), a phenolamine belonging to the sympathomimetic drugs family, has been studied in water, employing continuous photolysis, polarographic detection of oxygen uptake, steady-state and time-resolved fluorescence spectroscopy, time-resolved IR-phosphorescence and laser flash photolysis. Results indicate the formation of a weak dark complex Rf-Phen, with an apparent association constant of 5.5+/-0.5M(-1), only detectable at Phen concentrations much higher than those employed in the photochemical experiments. Under irradiation, an intricate mechanism of competitive reactions operates. Phen quenches excited singlet and triplet states of Rf, with rate constants of 3.33+/-0.08 and 1.60+/-0.03x10(9)M(-1)s(-1), respectively. With the sympathomimetic drug in a concentration similar to that of dissolved molecular oxygen in water, Phen and oxygen competitively quench triplet excited Rf, generating superoxide radical anion and singlet molecular oxygen (O2((1)Deltag)) by processes initiated by electron- and energy-transfer mechanisms respectively. As a global result, the photodegradation of the vitamin, a known process taking place from its excited triplet state, is retarded, whereas the phenolamine, practically unreactive towards these oxidative species, behaves as a highly efficient physical deactivator of O2((1)Deltag). The phenolamine structure in Phen appears as an excellent scavenger of activated oxygen species, comparatively superior, in kinetic terms, to some commercial phenolic antioxidants.     

乙硼烷离子和自由基结构的量子化学研究

用B₃LYP/6-311G(d,p)密度泛函方法对B₂H₅⁺阳离子和B₂H₅^·自由基的几何异构体的空间构型进行了优化,并在此基础上用QCISD(T)/6-311++G(3df,2p)偶合簇法进行了单点能计算和零点能校正.结果表明,B₂H₅⁺单态有2种稳定的几何构型(D_3h,C₁),其中C₁构型是新发现的.B₂H₅⁺三重态阳离子除已知C_s构型外,又发现两种稳定构型(C₁).对于B₂H₅^·自由基体系,共有4种异构体(包括两种新发现的构型C_s),其中,具有单桥结构的C_2v最稳定.用二级多体微扰理论和密度泛函方法对前人所认为稳定的B₂H₅⁺单态的C_2v构型进行了全优化,结果发现该构型始终具有一个虚频,不是稳定构型.对B₂H₅^-阴离子体系的单态和三重态进行的全优化,理论上得出单态时具有C_2v和C_s两种稳定构型,而三重态只有C_2v一种稳定构型.     

Theoretical study on the reaction mechanism of (CH_3)_3CO~· radical with NO

Alkoxy radicals RO? form an important class of hydrocarbon oxidation intermediates in combustion processes, interstellar and atmospheric chemistry[1—8]. The number of NO to NO2 conversions can take place during the oxidation of the parent hydrocarbon in the atmosphere and hence affecting tropospheric ozone production. The experimental and theoretical investi-gations of the reactions of alkoxy radicals will be ad-vantageous for safeguarding the environment. Despite the importance of alkoxy r…     

Theoretical study on the reaction mechanism of (CH3)3CO(.) radical with NO

The reaction mechanism of (CH3)3CO(.) radical with NO is theoretically investigated at the B3LYP/6-31G* level. The results show that the reaction is multi-channel in the single state and triplet state. The potential energy surfaces of reaction paths in the single state are lower than that in the triple state. The balance reaction: (CH3)3CONO←→ (CH3)3CO(.)+NO, whose potential energy surface is the lowest in all the reaction paths, makes the probability of measuring (CH3)3CO(.) radical increase. So NO may be considered as a stabilizing reagent for the (CH3)3CO(.)radical.     

The mechanism of NO formation from the decomposition of dialkylamino diazeniumdiolates: density functional theory and CBS-QB3 predictions

The mechanism of decomposition of dialkylamino diazeniumdiolate salts, R(2)N[NONO](-)M(+), to form nitric oxide, NO, was explored using theoretical methods. B3LYP/6-311+G(d) density functional theory calculations gave the optimized geometries, and energetics were further evaluated with the high accuracy CBS-QB3 method, when feasible. Relative pK(a) values were estimated using the PCM model for aqueous solvation. The terminal oxygen is the most basic site, followed closely by the internal oxygen. Protonation of these sites does not lead to decomposition. However, protonation of the weakly basic amino nitrogen leads to very rapid decomposition and NO generation.     

Theoretical study of the phototoxicity of naproxen and the active form of nabumetone

Density functional theory using the hybrid functional B3LYP has been employed in order to study the mechanisms of photoinduced decomposition of the closely related nonsteroidal anti-inflammatory drugs naproxen (NP) and 6-methoxy-2-naphthylacetic acid (MNAA; the active form of nabumetone). The photochemical properties and computed energies of various species obtained in this study show that both drugs dominate in their deprotonated forms at physiological pH. The deprotonated acids are unable to decarboxylate from their excited singlets; instead, they decarboxylate from their first excited triplet states with high efficiency, overcoming energy barriers less than 3 and 1 kcal/mol for MNAA and NP, respectively. The ultraviolet and visible spectra of the neutral, deprotonated, and decarboxylated moieties of MNAA and NP are more-or-less similar but with higher probabilites (oscillator strength) for the latter. This fact, as well as the higher reactivity of NP, is explained in terms of the electron-donating effect of the additional methyl group present in NP. Singlet oxygen, superoxide radical anion, and corresponding peroxyl radical species are expected to be formed in different steps throughout the proposed photodegradation pathways of both drugs, which give rise to their effects on biomolecules, for example, lipid peroxidation.     

Clar goblet and related non-Kekulé benzenoid LPAHs. A theoretical study

The results of an ab initio and semiempirical study of Clar Goblet (1), a C(38)H(18) non-Kekulé diradical LPAH, and its constitutional isomers 4 and 5 are reported. Planar D(2)(h)-1 was only 87.4 (triplet) and 83.8 (singlet) kJ/mol less stable than its planar Kekulé isomer C(2)(v)-6 (at (U)B3LYP/6-31G). Planar C(s)-4 was 63.6 (triplet) and 76.5 (singlet) kJ/mol less stable than 6. Overcrowded C(1)-5 was 80.1 (triplet) and 98.1 (singlet) kJ/mol less stable than 6. In concealed non-Kekulé 1, the singlet was more stable then the triplet by 3.6 kJ/mol, while in nonconcealed non-Kekulé 4 and 5, the triplets were more stable than the corresponding singlets by 12.9 and 18.1 kJ/mol, respectively, in accordance with theory. The spin density in 1, 4, and 5 is delocalized throughout the positions corresponding to active peri-peri coupling positions of the radical anion of naphthanthrone (2). The bond lengths in 1, 4, and 5 are in the range expected for aromatic compounds, except for the central carbon-carbon bonds, which are considerably elongated. A certain stabilization is evident in the homodesmotic reaction singlet-1 + 10 + 10 --> 11 + 3 + 3, indicating a "communication" between the two benzo[cd]pyrenyl radical (3) units of diradical 1. The HOMA indices indicate that in both singlet 1 and triplet 1 all of the rings except the central one have a significant aromatic character. The central ring is essentially antiaromatic, having negative HOMA index (-0.140 at UB3LYP/6-31+G). The stabilities of 1(2)(-) and 1(2+) are decreased relative to 3(-) and 3(+), respectively.     

Primary photophysical properties of ofloxacin

Steady-state fluorescence has been used to study the excited singlet state of ofloxacin (OFLX) in aqueous solutions. Fluorescence emission was found to be pH dependent, with a maximum quantum yield of 0.17 at pH 7. Two pKa*s of around 2 and 8.5 were obtained for the excited singlet state. Laser flash photolysis and pulse radiolysis have been used to study the excited states and free radicals of OFLX in aqueous solutions. OFLX undergoes monophotonic photoionization from the excited singlet state with a quantum yield of 0.2. The cation radical so produced absorbs maximally at 770 nm with an extinction coefficient of 5000 +/- 500 dm3 mol-1 cm-1. This is confirmed by one-electron oxidation in the pulse radiolysis experiments. The hydrated electron produced in the photoionization process reacts with ground state OFLX with a rate constant of 2.0 +/- 0.2 x 10(10) dm3 mol-1 s-1, and the anion thus produced has two absorption bands at 410 nm (extinction coefficient = 3000 +/- 300 dm3 mol-1 cm-1) and at 530 nm. Triplet-triplet absorption has a maximum at 610 nm with an extinction coefficient of 11,000 +/- 1500 dm3 mol-1 cm-1. The quantum yield of triplet formation has been determined to be 0.33 +/- 0.05. In the presence of oxygen, the triplet reacts to form both excited singlet oxygen and superoxide anion with quantum yields of 0.13 and < or = 0.2, respectively. Moreover, superoxide anion is also formed by the reaction of oxygen with the hydrated electron from photoionization. Hence the photosensitivity due to OFLX could be initiated by the oxygen radicals and/or by OFLX radicals acting as haptens.     

Photoreactivity of biologically active compounds. XVI. Formation and reactivity of free radicals in mefloquine

The formation and reactivity of the triplet state and free radicals of mefloquine hydrochloride (MQ) have been investigated by pulse radiolysis and flash photolysis. The excited triplet, cation radical and anion radical have been produced and their absorption characteristics determined. The triplet-triplet absorption spectrum of MQ showed a maximum at 430 nm, with a molar absorption coefficient of 3600 M(-1) cm(-1) and the quantum yield for intersystem crossing was determined to be close to unity. Deactivation of the triplet, in the absence of oxygen, led to the formation of MQ cation and/or anion radicals. The molar absorption coefficient of the cation radical at 330 nm was determined to be 2300 M(-1) cm(-1), whilst that for the anion radical was 2400 M(-1) cm(-1) at 620 nm and 3600 M(-1) cm(-1) at 350 nm. The molar absorption coefficients of the proposed neutral radical at 320 nm and 520 nm were 4000 M(-1) cm(-1) and 1300 M(-1) cm(-1) respectively. The quantum yield for the formation of singlet oxygen, sensitized by MQ triplet, was determined to be close to unity. Aqueous solutions of MQ were found to photoionize to yield hydrated electron and cation radical of MQ in a biphotonic process. The influences of pH, buffer concentration, oxygen concentration and addition of sodium azide on the formation and reactivity of the transients were evaluated. The reactions between MQ and solvated electrons and superoxide anion were also studied.     

Ground- and excited-state proton transfer and rotamerism in 2-(2-hydroxyphenyl)-5-phenyl-1,3,4-oxadiazole and its O/"NH or S"-substituted derivatives

The intramolecular proton-transfer process, rotational process, and optical properties of 2-(2-hydroxyphenyl)-5-phenyl-1,3,4-oxadiazole (HOXD) and its O/"NH"- and O/"S"-substituted derivatives, 2-(2-hydroxyphenyl)-5-phenyl-1,3,4-triazole (HOT) and 2-(2-hydroxyphenyl)-5-phenyl-1,3,4-thiadiazole (HOTD), respectively, have been studied. DFT (B3LYP/6-31+G**) single-point energy calculations were performed using HF- and DFT-optimized geometries in the ground state (S0). TD-B3LYP/6-31+G** calculations using CIS-optimized geometries were carried out to investigate the properties of the first singlet excited state (S1) and first triplet excited state (T1). The computational results revealed that a high-energy barrier inhibits the proton transfer from cis-enol (Ec) to keto (K) form in S0, whereas the proton transfer in S1 can take place through a very-low-energy barrier. The rotation between Ec and trans-enol (Et) can occur in S0 through a low-energy barrier, whereas it is prohibited because of the high-energy barrier in S1 for each of the three molecules. The vertical excitation energies were calculated using the TD-B3LYP/6-31+G** method based on the HF- and CIS-optimized geometries. Absorption and fluorescence wavelengths of HOT show a hypsochromic shift (6-15 nm) relative to HOXD, while those of HOTD show a bathochromic shift (21-29 nm). The phosphorescence wavelength of HOTD shows a significant bathochromic shift relative to that of HOXD.     

(CH3)3C+NO2反应机理的理论研究

用量子化学密度泛函方法,在B3LYP/6-31G*水平下研究了叔丁基自由基(CH3)3C和NO2气体的反应机理.研究表明,该反应是在单、三态势能面上的多通道反应.不同反应通道的产物不同,单态下反应更容易发生.常温下对于一个敞开体系(例如在大气当中),(CH3)3C自由基和NO2作用主要生成比较稳定的化合物(CH3)3CONO和(CH3)3CNO2.这对于消除大气污染起到一定的作用.     

Au+(1S, 3D)与N2O(1Σ+)反应机理的理论研究

采用密度泛函B3LYP方法, O和N用6-311+G*基组, Au+用赝势基组(8s7p6d)/[6s5p3d], 研究了Au+(1S, 3D)离子和N2O(1Σ+)分子的反应机理. 报道了在基态单重态和激发三重态势能面上各反应物、中间体和过渡态的构型特征及能量. 结果表明, 两个主反应通道Au+(1S)+ N2O(1Σ+)→1NA-Complex-1→1NA-TS1→1NA-Complex-2→1NA-Crossing→[3OAuNN]+和Au+(1S)+ N2O(1Σ+)→1NB-Complex→1NB-Crossing→[AuNN(1Σ+)]++O(3P)都需经过反应交叉势能面, 出现“系间窜越”. 用内禀坐标单点计算垂直激发态的方法确定了势能面交叉点, 并用含时密度泛函TD-B3LYP方法进一步探讨了自旋翻转机理.     

Radical reaction C3H+NO: a mechanistic study

Although a number of hydrocarbon radicals including the heavier C(3)-radicals C(3)H(3) and C(3)H(5) have been experimentally shown to deplete NO effectively, no theoretical or experimental attempts have been made on the reactivity of the simplest C(3)-radical towards NO. In this article, we report our detailed mechanistic study on the C(3)H+NO reaction at the Gussian-3//B3LYP/6-31G(d) level by constructing the singlet and triplet electronic state [H,C(3),N,O] potential energy surfaces (PESs). The l-C(3)H+NO reaction is shown to barrierlessly form the entrance isomer HCCCNO followed by the direct O-elimination leading to HCCCN+(3)O on triplet PES, or by successive O-transfer, N-insertion, and CN bond-rupture to generate the product (1)HCCN+CO on singlet PES. The possible singlet-triplet intersystem crossings are also discussed. Thus, the novel reaction l-C(3)H+NO can proceed effectively even at low temperatures and is expected to play an important role in both combustion and interstellar processes. For the c-C(3)H+NO reaction, the initially formed H-cCCC-NO can most favorably isomerize to HCCCNO, and further evolution follows that of the l-C(3)H+NO reaction. Quantitatively, the c-C(3)H+NO reaction can take place barrierlessly on singlet PES, yet it faces a small barrier 2.7 kcal/mol on triplet PES. The results will enrich our understanding of the chemistry of the simplest C(3)-radical in both combustion and interstellar processes, which to date have received little attention despite their importance and available abundant studies on its structural and spectroscopic properties.