Fluorescence quenching of photoreaction products in the excited singlet state

The properties of steady-state spontaneous luminescence of a quantum system with a photoproduct with recordable fluorescence under the conditions of dynamic quenching of excited states by extraneous substances were considered. It was shown that the dependence of photoproduct fluorescence intensity and yield on quencher concentration was nontrivial and could not be conveniently used to determine the Stern-Volmer constant. At the same time, the initial form of the luminophore and its photoproduct produced in a kinetically controlled reaction are quenched in such a way that the ratio of their fluorescence intensities increases linearly as the quencher concentration grows. The corresponding equation was used to determine the constant of bimolecular quenching of reaction product excited states. The results were used in an analysis of the experimental fluorescence spectra of flavone (3-hydroxiflavone), whose fluorescence was excited under the conditions of dynamic quenching of the S ₁ state. Our analysis was shown to be applicable to a wide range of compounds with photoreactions accompanied by two-band fluorescence (charge transfer, proton transfer, phosphorescence, complex formation, etc.). It could be used to accurately determine bimolecular contact constants for excited states of photoreaction product molecules. Original Russian Text ? V.I. Tomin, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 3, pp. 580–585.     

Structure and photoreaction of photoactive yellow protein, a structural prototype of the PAS domain superfamily

Photoactive yellow protein (PYP) is a water-soluble photosensor protein found in purple photosynthetic bacteria. Unlike bacterial rhodopsins, photosensor proteins composed of seven transmembrane helices and a retinal chromophore in halophilic archaebacteria, PYP is a highly soluble globular protein. The alpha/beta fold structure of PYP is a structural prototype of the PAS domain superfamily, many members of which function as sensors for various kinds of stimuli. To absorb a photon in the visible region, PYP has a p-coumaric acid chromophore binding to the cysteine residue via a thioester bond. It exists in a deprotonated trans form in the dark. The primary photochemical event is photo-isomerization of the chromophore from trans to cis form. The twisted cis chromophore in early intermediates is relaxed and finally protonated. Consequently, the chromophore becomes electrostatically neutral and rearrangement of the hydrogen-bonding network triggers overall structural change of the protein moiety, in which local conformational change around the chromophore is propagated to the N-terminal region. Thus, it is an ideal model for protein conformational changes that result in functional change, responding to stimuli and expressing physiological activity. In this paper, recent progress in investigation of the photoresponse of PYP is reviewed.     

Reinvestigation in the photoreaction of 1-naphthalenecarbonitrile and furan

Photocycloaddition of furan to 1-naphthalenecarbonitrile (1) was reinvestigated. Irradiation of the mixture through Pyrex filter yielded endo-[4+4] adduct (2) and syn-[2+2] adduct (4). Considering the secondary orbital interaction in the singlet-state [4+4] photocycloaddition, the favorable formation of exo-[4+4] cyclodimer (3) over 2 followed by a facile Cope rearrangement was proposed for the formation of 4, and confirmed by a low temperature irradiation experiment.     

SINDO1 study of the photoreaction of tetramethylene sulfone

The mechanism of the photoreaction of tetramethylene sulfone (TMSO₂) was investigated by the semiempirical molecular orbital (MO) method SINDO1. The relevant low-lying potential energy surfaces, which were calculated with limited configuration interaction (CI), were studied by optimizing intermediates and transition structures and by introducing linear interpolations between these stationary points. The main initial reaction step for all important products is an α cleavage of one C? S bond. This leads to an intermediate that can be classified as an excited singlet diradical. Its electronic structure is described with a two-electron, three-orbital model. Starting from this initially generated intermediate, the reaction branches into several pathways leading to various products. Feasible reaction pathways were established for all important products consistent with experiments. © 1995 by John Wiley & Sons, Inc.     

Specificity of the redox photoreaction of dimers of thiadicarbocyanine dyes

The primary steps of the redox reaction of dimers of the thiadicarbocyanine dye and its 5,5′-dichloro derivative in aqueous solutions were studied in the presence of 4-nitroacetophenone, ascorbic acid, or hydroquinone. In water the dye molecules (anion, M^?) mainly exist as dimers M₂ ^2?. The laser pulse irradiation (10 ns, 532 nm) results in the population of the lowest triplet level M₂ ^2?, whose depletion occurs due to both intersystem crossing to the ground state and photoinduced transition to the highest triplet state of the dimer followed by photoionization. Photoionization at low intensities of a laser pulse proceeds via the one-quantum mechanism going to the two-quantum mechanism with an increase in the laser pulse intensity. The photooxidation of the dimer in the lowest triplet state with 4-nitroacetophenone results in the formation of unstable radical anion M₂ ^?· that spontaneously dissociates to monomer M^? and radical M^· of the dye. In the presence of electron donors (ascorbic acid, hydroquinone), the dimers in the triplet state are not photoreduced, but the electron donors reduce M₂ ^?· and M^· to the dye dimer and monomer, respectively.     

Unusual course of a type II photoreaction

The type II photoreaction of 1-acetyl-2-ethylidene-1-methoxycyclohexane (1) affords 2-ethylidene-1-methoxy-6-methylbicyclo[3.1.1]heptan-6-ol (2) as a result from ring closure of the least stable 1,4-biradical.     

Involvement of electron transfer in the photoreaction of zebrafish Cryptochrome-DASH

Photoreaction of a blue-light photoreceptor Cryptochrome-DASH (Cry-DASH), a new member of the Cryptochrome family, from zebrafish was studied by UV-visible absorption spectroscopy in aqueous solutions at 293 K. Zebrafish Cry-DASH binds two chromophores, a flavin adenine dinucleotide (FAD) and a N5,N10-methenyl-5,6,7,8-tetrahydrofolate (MTHF) noncovalently. The bound FAD exists in the oxidized form (FAD(ox)) in the dark. Blue light converts FAD(ox) to the neutral radical form (FADH*). Formed FADH* is transformed to the fully reduced form FADH(2) (or FADH(-)) by successive light irradiation, or reverts to FAD(ox). FADH(2) (or FADH(-)) reverts to FADH* or possibly to FAD(ox) directly. The effect of dithiothreitol suggests a possible electron transfer between FAD in zebrafish Cry-DASH and reductants in the external medium. This is the first report on the photoreaction pathway and kinetics of a vertebrate Cry-DASH family protein.     

Structure of some products of the amidothionophosphorylation of galactitol

Conclusions As a result of the successive reaction of galactitol with P(NEt₂)₃ and sulfur we have obtained diastereomeric galactitol 1,6-bis-O-[di(N-diethylamido)thionophosphate] 2,3:4,5-bis-O-(N-diethylamidothionophosphates) (the structure of one of them was established by x-ray diffraction analysis) and galactitol 1,2:3,4:5,6-tris-O-(N-diethylamidothionophosphates).Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 896–903, April, 1983.     

盐酸特拉唑嗪降解产物的结构鉴定

盐酸特拉唑嗪是一种新型选择性 α₁-肾上腺能受体拮抗剂,用于治疗高血压和良性前列腺增生。采用高效液相色谱-高分辨多级质谱(HPLC-HRMSⁿ)联用法对强氧化条件下盐酸特拉唑嗪原料药的降解产物进行了详细的解析和结构确证。共检测到15种降解产物,其中7种与文献报道的有关物质相同,另外8种为新检测到的降解产物,其主要降解途径为酰胺键水解、羟基化、羰基化及3种途径组合的次级降解途径。     

Effects of substituents on the type II photoreaction of tetrasubstituted cyclopropenes

The triplet sensitized behavior of a number of 3-alkyl subststituted cyclopropenes containing a heteroatom on the side chain has been studied in mechanistitic detail. The triplet state was found to undergo H-atom abstraction by a mechanism analogous to the well-known Norrish type II process of carbonyl compounds. Rate constants for H-abstraction were obtained by plotting $\text{Φ}{}_0\text{Φ}{}_{\mathrm{q}}$trans-stilbene at a constant quencher to cyclopropene ratio. The rate constants for H-abstraction were found to be two orders of magnitude less than that at for related phenyl alkyl ketones and increased as the strength of the C-H bond in the γ-position decreased. The possibility of electron transfer from the heteroatom of the side chain to the π-π^* triplet state is discussed.     

Structure of the products of acid hydrolysis of panaxoside A

Conclusions The substances obtained in the acid hydrolysis of panaxosides A, B, and C have been isolated and characterized. The structures of these substances have been established from a combination of chemical data and the results of NMR spectroscopy.Khimiya Prirodnykh Soedinenii, Vol. 4, No. 6, pp. 352–355, 1968     

Organic peroxide production in the Cl_2-ethane-air photoreaction system

HPLC along with FT-IR technique was used to study the formation of organic peroxides in the CI2-ethane-air photoreaction system. Ethyl hydroperoxide (CH3CH2OOH, EHP) and per-oxyacetic acid ( CH3C(O)OOH, PAA) were conformed to be the peroxide product in the reaction system. In addition, methyl hydroperoxide (CH3OOH, MHP), hydroxymethyl hydroperoxide (HOCH2OOH, HMHP) and two unidentified organic peroxides were detected for the first time. EHP and MHP were the dominant peroxide products. The identification of HMHP showed that Criegee biradical CH2OO may be formed as an intermediate in the oxidation of ethane. Simulation results showed that photooxidation of ethane may make substantial contribution to source of organic peroxides in the atmosphere.     

Triplet versus singlet photoreaction mechanism in the barrelene di-pi-methane rearrangement

A detailed study of the potential energy surfaces involved in the di-pi-methane rearrangement (singlet and triplet states) reveals the factors that modulate the mechanisms (pathways) and reactivity in strained di-pi-methane systems such as bicyclo[2.2.2]octa-2,5,7-triene (barrelene). [structure: see text]