CHEMIEXCITATION IN THE PEROXIDATIVE METABOLISM OF DIETHYLSTILBESTROL

When the synthetic estrogen and tumourogenic compound diethylstilbestrol is exposed to horseradish peroxidase (HRP) and H2O2 in the presence of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB), a burst of oxygen consumption and concomitant light emission are observed. The quinone form of the product is not seen in the absorption spectrum because CTAB strongly catalyses its conversion to Z,Z-dienestrol. The emission spectrum shows several peaks. Total emission is dramatically enhanced by chlorophyll and by xanthene dyes. A key intermediate in chemiexcitation is 4-hydroxypropiophenone. The ability to promote chemiexcitation is retained through various generations of metabolites, giving origin to a cascade of excited states. Since the biological effects of diethylstilbestrol appear to be connected with its peroxidative metabolism, chemiexcitation may eventually prove to be of importance in, for example, toxicity of the drug.     

CHEMIEXCITATION IN THE PEROXIDATIVE METABOLISM OF DIETHYLSTILBESTROL. METABOLIC PRODUCTS

In the presence of the surfactant hexadecyltrimethyl ammonium bromide (CTAB) a cascade of electronically excited states accompanies the successive steps in the peroxidative metabolization of the strong estrogenic and tumourogenic diethylstilbestrol. Reversing the order by necessity, we report in this first paper results with the metabolites. Exposure of 4-hydroxypropiophenone, Z,Z-dienestrol or E,E-dienestrol to horseradish peroxidase and H2O2 promotes oxygen uptake and spectral alterations. Light emission is observed provided that the surfactant CTAB is present. With the three substrates, 4-hydroxybenzoic acid and a new metabolite, p-benzoquinone, have been identified. With both dienestrol isomers, 1-(4'-hydroxyphenyl)-propan-1-on-2-ol has been identified. In all cases the emission spectrum indicates the presence of several emitters. Possible chemiexcitation routes are pointed out. From the dramatic increase of the emission by enhancers, values as high as 1 x 10(-5) are inferred for the product of the quantum yields of chemiexcitation and energy transfer.     

CHEMIEXCITATION IN THE PEROXIDATIVE METABOLISM OF N-METHYLCARBAZOLE: MECHANISTIC IMPLICATIONS

Abstract: The peroxidative metabolism of N -methylcarbazole emits light independently of the presence of oxygen. It is likely that two chemiexcited transients are formed by electron transfer to the activated peroxidase, the cation radical by one electron transfer and a cation biradical by two electron transfer consistent with the failure to observe horseradish peroxidase-II in the steady state of the reaction. In the spectral range investigated (390–700 nm) the observed emission (570–700 nm) is ascribed to the biradical, as the latter is equivalent to an excited state of the postulated iminium cation.
While lipoxygenase has no effect upon N -methylcarbazole, it markedly enhances the emission if peroxidase is present. This effect requires oxygen and is ascribed to an excited product formed by lipoxygenase acting upon an intermediate hydroperoxide of the aerobic process promoted by peroxidase.
Our results are of importance on two counts. First they extend to N -rnethylcarbazole the formation of excited species in the peroxidative metabolism of important xenobiotics. Second, the mechanistic information they provide supports the scheme of metabolism postulated by Kedderis et al. (1986, J. Biol. Chem. 261, 15910–15914).     

LIGHT EMISSION ACCOMPANIES OXYGEN UPTAKE DURING THE PEROXIDATIVE METABOLISM OF TETRACYCLINES

Abstract— Tetracycline molecules offer several sites for peroxidative metabolism of the type known to lead to oxygen consumption and electronic cxcitation. Accordingly, when tetracycline and chlortetracycline were exposed to horseradish peroxidase in the presence of hydrogen peroxide, oxygen was taken up and light emission was observed. The overall quantum yield ofchemiluminescencc is on the order of 10 ⁶, but that of chemiexcitation may be orders of magnitude higher as suggested by studies of sensitized emission. Given the widespread distribution of peroxidases, the formalion of highly reactive metabolites of tetracycline may have biological importance.     

FREE RADICALS AND EXCITED SPECIES IN THE METABOLISM OF INDOLE-3-ACETIC ACID AND ITS ETHYL ESTER BY HORSERADISH PEROXIDASE AND BY NEUTROPHILS

The peroxidative metabolization of indole-3-acetic acid, a biologically important process, has been followed by EPR spectroscopy with the aim of obtaining information on the mechanism of generation of electronically excited species. The skatole-3-methylene radical detected during oxidation by horseradish peroxidase, does not appear to be involved in a major oxygen consuming process or in the generation of singlet oxygen. The chemiluminescence spectrum exhibits several maxima, which are also observed when the ethyl ester of indole-3-acetic acid is metabolized by horseradish peroxidase or by myeloperoxidase in neutrophils. When the ester is metabolically activated in either of these systems, the EPR spectrum indicates a tertiary carbon-centered radical. This radical centered on the carbon in the 3-position participates in a chemiexcitation/emissive route. Within the cell, this emissive process is responsible for a large part of the oxygen consumed. Some of the emitters originate in the cleavage of the 2,3 double bond. The ester, which is capable of penetrating into the cells, also emits with other myeloperoxidase-containing cells. This compound may have useful applications as an intracellular chemiluminescent probe for the presence of myeloperoxidase.     

Vibrationally mediated photodissociation of ammonia: the influence of N-H stretching vibrations on passage through conical intersections

Velocity map ion imaging of the H atoms formed in the photodissociation of vibrationally excited ammonia molecules measures the extent of adiabatic and nonadiabatic dissociation for different vibrations in the electronically excited state. Decomposition of molecules with an excited symmetric N-H stretch produces primarily ground state NH(2) along with a H atom. The kinetic energy release distribution is qualitatively similar to the ones from dissociation of ammonia excited to the electronic origin or to several different levels of the bending vibration and umbrella vibration. The situation is very different for electronically excited molecules containing a quantum of antisymmetric N-H stretch. Decomposition from that state produces almost solely electronically excited NH(2)*, avoiding the conical intersection between the excited state and ground state surfaces. These rotationally resolved measurements agree with our previous inferences from lower resolution Doppler profile measurements. The production of NH(2)* suggests that the antisymmetric stretching excitation in the electronically excited molecule carries it away from the conical intersection that other vibrational states access.     

Rydberg states of small NaAr(n)* clusters

The 4s and 5s Rydberg excited states of NaAr(n)* clusters are investigated using a pseudopotential quantum-classical method. While NaAr(n) clusters in their ground state are known to be weakly bound van der Waals complexes with Na lying at the surface of the argon cluster, isomers in 4s or 5s electronically excited states of small NaAr(n)* clusters (n< or =10) are found to be stable versus dissociation. The relationship between electronic excitation and cluster geometry is analyzed as a function of cluster size. For both 4s and 5s states, the stable exciplex isomers essentially appear as sodium-centered structures with similar topologies, converging towards those of the related NaAr(n)+ positive ions when the excitation level is increased. This is consistent with a Rydberg-type picture for the electronically excited cluster, described by a central sodium ion solvated by an argon shell, and an outer diffuse electron orbiting around this NaAr(n)+ cluster core.     

A comparison of the decomposition of electronically excited nitro-containing molecules with energetic moieties C-NO2, N-NO2, and O-NO2

Decomposition of electronically excited nitro-containing molecules with different X-NO(2) (X = C, N, O) moieties has been intensively investigated over the past decades; however, their decomposition behavior has not previously been compared and contrasted. Comparison of their unimolecular decomposition behavior is important for the understanding of the reactivity differences among electronically excited nitro-containing molecules with different X-NO(2) (X = C, N, O) bond connections. Nitromethane (NM), dimethylnitramine (DMNA), and isopropylnitrate (IPN) are used as model molecules for C-NO(2), N-NO(2), and O-NO(2) active moieties, respectively. Ultraviolet lasers at different wavelengths, such as 226, 236, and 193 nm, have been employed to prepare the excited states of these molecules. The decomposition products are then detected by resonance enhanced two photon ionization (R2PI), laser induced fluorescence (LIF) techniques, or single photon ionization at 10.5 eV. NO molecules are observed to be the major decomposition product from electronically excited NM, DMNA, IPN using R2PI techniques. The NO products from decomposition of electronically excited (226 and 236 nm) NM and IPN display similar rotational (600 K) and vibrational distributions [both (0-0) and (0-1) bands of the NO molecule are observed]. The NO product from DMNA shows rotational (120 K) and vibrational distributions (only (0-0) transition is observed) colder than those of NM and IPN. At the 193 nm excitation, electronically excited NO(2) products are observed from NM and IPN via fluorescence detection, while no electronically excited NO(2) products are observed from DMNA. Additionally, the OH radical is observed as a minor dissociation product from all three compounds. The major decomposition pathway of electronically excited NM and IPN involves fission of the X-NO(2) bond to form electronically excited NO(2) product, which further dissociates to generate NO. The production of NO molecules from electronically excited DMNA is proposed to go through a nitro-nitrite isomerization pathway. Theoretical calculations show that a nitro-nitrite isomerization for DMNA occurs on the S(1) surface following a (S(2)/S(1))(CI) conical intersection (CI), whereas NO(2) elimination occurs on the S(1) surface following the (S(2)/S(1))(CI) conical intersection for NM and IPN. The present work provides insights for the understanding of the initiation of the decomposition of electronically excited X-NO(2) energetic systems. The presence of conical intersections along the reaction coordinate plays an important role in the detailed mechanism for the decomposition of these energetic systems.     

En Route to Multisurface Chemistry

This article considers what happens when the energy required for a compound to react is supplied by an irradiation lamp instead of by a Bunsen burner. For this purpose real examples are selected from three typical groups of cases. The respective answers obtained should indicate significant moves in organic photochemistry which may be expected to affect the further development of chemistry as a whole in the near future. During this tour d'horizon particular attention is paid to photochemical processes in solids or solvent matrices, light-induced reactions are especially emphasized as key reactions in (natural product) syntheses, and a strong case is made for interpreting the reactions of electronically excited molecules in terms of Salem correlation diagrams.     

Charge exchange ionization in collision cells as a method to detect the presence of long-lived excited electronic states of polyatomic ions

Charge exchange ionization in collision cells installed in a double focusing mass spectrometer with reversed geometry has been used to detect the presence of a long-lived excited electronic state of benzene ion. In particular, the first collision cell located between the ion source and the magnetic sector was modified to serve as an ion source for the reagent ion generated by charge exchange with the primary ion. Strong reagent ion signals were observed when the ionization energies of the reagents (1,3-C4H6, CS2, CH3Cl) were lower than the recombination energy (approximately 11.5 eV) of the excited state benzene ion, while the signals were negligible for reagents (CH3F,CH4) with higher ionization energy. The fact that a strong signal is observable only for electronically exoergic charge exchange is useful for detecting the presence of a long-lived electronically excited state.     

CHEMIEXCITATION IN THE PEROXIDATIVE METABOLISM OF N- METHYLCARBAZOLE: MECHANISTIC IMPLICATIONS

Abstract –The peroxidative metabolism of TV-methylcarbazole emits light independently of the presence of oxygen. It is likely that two chemiexcited transients are formed by electron transfer to the activated peroxidase, the cation radical by one electron transfer and a cation biradical by two electron transfer consistent with the failure to observe horseradish peroxidase-II in the steady state of the reaction. In the spectral range investigated (390–700 nm) the observed emission (570–700 nm) is ascribed to the biradical, as the latter is equivalent to an excited state of the postulated iminium cation.
While lipoxygenase has no effect upon JV-methylcarbazole, it markedly enhances the emission if peroxidase is present. This effect requires oxygen and is ascribed to an excited product formed by lipoxygenase acting upon an intermediate hydroperoxide of the aerobic process promoted by peroxidase.     

Relativistic study on emission mechanism in palladium and platinum complexes

The present study investigates the spin-orbit coupling (SOC) effects in the radiative processes from the electronically excited states of bis[-2-(2-thienyl)-pyridine] platinum (Pt(thpy)2) and palladium (Pd(thpy)2). The transition probabilities among the low-lying spin-mixed states in these complexes are estimated using the discrete variable representation (DVR) method based on the assumption that the system obeys Fermi's golden rule. It is revealed that the low-lying excited singlets and triplets are strongly mixed with each other by SOC in Pt(thpy)2 and, as a result, a fast nonradiative transition occurs to the low-lying excited spin-mixed states. This is followed by the radiative transition from these low-lying spin-mixed states to the lowest spin-mixed state (the ground state); that is to say, a phosphorescence should be observed from these low-lying excited spin-mixed states in Pt(thpy)2. On the contrary, weak SOCs are obtained in Pd(thpy)2 and no phosphorescence at room temperature is expected to be observed in Pd(thpy)2. These results are in good agreement with the experimental reports.     

The role of electronically excited states in recombination reactions

Methods are described for including the participation of bound electronically excited states in calculations on radical recombination reactions. These methods are illustrated by applying them to the reactions For O₂, accurate ab initio potentials are used in calculations which show that the electronic degeneracy and long-range part of the potential are likely to be crucial in determining the contribution of a given electronic state to the overall reaction, as long as the state is not so weakly bound that it dissociates thermally before being electronically quenched. Weak collision effects are allowed for using a Monte Carlo technique and an assumed exponential form for the distribution of energies transferred in collisions with a third body. For larger systems it is evident that the role of bound excited states in the low-pressure regime falls rapidly as the size of the system increases. As the high-pressure limit is approached, however, the contribution of excited states is likely to come close to that expected simply on the basis of electronic degeneracy.     

Photoradical ageing of polymers

The general laws of photoradical ageing of various carbon-chain and heterogeneous-chain polymers caused by reactions of electronically excited radicals have been considered. Information is given on mechanisms, efficiency, and kinetic features of these processes; also the occurrence of photoradical chain processes. It is shown that reactions of electronically excited macroradicals can be made use of in directed changes in the structure and properties of polymers.     

Reconsideration on hydrogen bond strengthening or cleavage of photoexcited coumarin 102 in aqueous solvent: a DFT/TDDFT study

In this work, the excited-state hydrogen bonding dynamics of photoexcited coumarin 102 in aqueous solvent is reconsidered. The electronically excited states of the hydrogen bonded complexes formed by coumarin 102 (C102) chromophore and the hydrogen donating water solvent have been investigated using the time-dependent density functional theory method. Two intermolecular hydrogen bonds between C102 and water molecules are considered. The previous works (Wells et al., J Phys Chem A 2008, 112, 2511) have proposed that one intermolecular hydrogen bond would be strengthened and the other one would be cleaved upon photoexcitation to the electronically excited states. However, our theoretical calculations have demonstrated that both the two intermolecular hydrogen bonds between C102 solute and H(2)O solvent molecules are significantly strengthened in electronically excited states by comparison with those in ground state. Hence, we have confirmed again that intermolecular hydrogen bonds between C102 chromophore and aqueous solvents are strengthened not cleaved upon electronic excitation, which is in accordance with Zhao's works.     

Theory and algorithms for the excited states of large molecules and molecular aggregates

This project aims to attack the frontiers of electronic structure calculations on the excited states of large molecules and molecular aggregates by developing novel theoretical and computational methods. The methodology development is especially based on the time-dependent density functional theory (TDDFT) and valence bond (VB) theory, and is expected to be computationally effective and accurate as well. Research works on the following related subjects will be performed: (1) The analytical energy-derivative approaches for electronically excited state within TDDFT will be developed to reduce bypass the computational costs in the calculation of molecular excited-state properties. (2) The ab initio methods for electronically excited state based on VB theory and hybrid TDDFT-VB method will be developed to overcome the limitations of current TDDFT in simulating photophysics and photochemistry. (3) For larger aggregates, neither ab initio methods nor TDDFT is applicable. We intend to build the effective model Hamiltonian by developing novel theoretical and computational methods to calculate the involved microscopic physical parameters from the first-principles methods. The constructed effective Hamiltonian is then used to describe the excitonic states and excitonic dynamics of the natural or artificial photosynthesized systems, organic or inorganic photovoltaic cell. (4) The condensed phase environment is taken into account by combining the developed theories and algorithms based on TDDFT and VB with the polarizable continuum solvent models (PCM), molecular mechanism (MM), classical electrodynamics (ED) or molecular dynamics (MD) theory. (5) Highly efficient software packages will be designed and developed.     

Electronic excitation as a mechanism of the ion selectivity filter

A correlation between the character of pharmacological activity and the energies of electronic transitions in some biologically active molecules, affecting the nervous system, has been found. In order to explain the correlation, a new principle of the membrane ion selectivity filter has been suggested. The principle is based on the recombination process of a metal cation, passing through the filter, with an electron, when the energy quantum (equal to the metal ionization energy) is emitted. The amino acid residue group, performing the function of the channel filter, absorbs this quantum, transits itself into an electronically excited state, changes its conformation and lets, as a result, the cation pass. The process is possible only in that case when the amino acid residue group has a transition of the same energy, therefore not all of metals can pass through the filter. From the viewpoint of this conception, an active molecule acts because of its transition into an electronically excited state of the same energy and interfering, thereby, with the natural processes.     

ELECTRONICALLY EXCITED PRODUCTS IN THE PHOTODISSOCIATION OF Di-9-METHYLANTHRACENE

Abstract— Although the vast majority of photochemical reactions in condensed phase lead directly to product molecules in the ground states it might be expected that in favorable cases a certain fraction of the reacting species would escape deactivation long enough to attain the product configuration adiabatically in an electronically excited state. In this communication, we report that di-9-methylanthracene, in addition to its normal diabatic course, also leads to small but finite amounts of both excited singlet monomer and excimer species. Oxygen quenching experiments seem to indicate that the monomer and excimer are derived from excited di-9-methylanthracene with little mutual interconversion. The fluorescence yields were determined for these processes in aerated solution at room temperature to be φ₁= 0·00040 ± 0·00005 and φ₂= 0·00037 ± 0·00005 for monomer and excimer species, respectively.     

Neutralization of ions at an electronically excited semiconductor surface

Neutralization of positive ions colliding with a semiconductor surface is studied. It is shown that the neutralization probability can be significantly enhanced if the surface exposed to the impinging ions is electronically excited. The basic reason behind this is that the impinging ions have easier access to the excited surface electrons than to bulk electrons.     

Revisiting the electronic excited-state hydrogen bonding dynamics of coumarin chromophore in alcohols: Undoubtedly strengthened not cleaved

In the present work, the electronic excited-state hydrogen bonding dynamics of coumarin chromophore in alcohols is revisited. The time-dependent density functional theory (TDDFT) method has been performed to investigate the intermolecular hydrogen bonding between Coumarin 151 (C151) and methanol (MeOH) solvent in the electronic excited state. Three types of intermolecular hydrogen bonds can be formed in the hydrogen-bonded C151–(MeOH)₃ complex. We have demonstrated again that intermolecular hydrogen bonds between C151 and methanol molecules can be significantly strengthened upon photoexcitation to the electronically excited state of C151 chromophore. Our results are consistent with the intermolecular hydrogen bond strengthening in the electronically excited state of Coumarin 102 in alcoholic solvents, which has been demonstrated for the first time by Zhao et al. At the same time, the electronic excited-state hydrogen bond cleavage mechanism of photoexcited coumarin chromophores in alcohols proposed in some other studies about the hydrogen bonding dynamics is undoubtedly excluded. Hence, we believe that the two contrary dynamic mechanisms for intermolecular hydrogen bonding in electronically excited states of coumarin chromophores in alcohols are clarified here.