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At least 19 recordsLinked to original sources
PubMed2000

Crystal lattice photochemistry often proceeds in discrete stages. Mechanistic and exploratory organic photochemistry

[formula: see text] While the solid-state photochemistry of crystals has been assumed to proceed to some critical point beyond which the crystal is destroyed, it has now been found that crystal lattice photochemistry often occurs in stages or regimes. In each regime different chemistry generally results. The reaction in different stages may be followed kinetically.

Journal Article↗
PubMed1978

Photochemistry of cytosine derivatives. 1. Photochemistry of thymidylyl-(3' leads to 5')-deoxycytidine.

The photochemistry of thymidylyl-(3' leads to 5')- deoxycytidine (dTpdC) was studied as a model system of adjacent thymine and cytosine bases in DNA. Acetophenonesensitized irradiation causes the cytosine moiety in dTpdC to react with the thymine moiety intramolecularly. Three unstable photoproducts are formed initially which are converted into three isomeric dinucleoside phosphates of thymine-uracil cyclobutane photodimer in a ratio of 4.2:2.2:1. Under the same irradiation condition thymidylyl-(3' leads to 5')-thymidine (dTpdT) yields two products in a ratio of 6:1. The structures of these products are established by chemical and spectroscopic methods. The major product in these reactions has been identified as the stereoisomer which has the same anti,anti relationship between the pyrimidine rings and the deoxyribose group as in the parent dinucleoside phosphates. The efficiency of the intramolecular dimerization of dTpdC is about one-third that of dTpdT. The results suggest that the cytosine base in DNA may be converted to a uracil base via photodimerization with an adjacent pyrimidine base, hydrolysis, and photoreactivation.

Deoxycytidine↗
PubMed2002

Development of experimental and theoretical crystal lattice organic photochemistry: the quantitative cavity. Mechanistic and exploratory organic photochemistry.

The photochemistry of organic reactants in the crystalline state has a long history. What has been lacking is comprehensive theory defining what controls the course of these reactions which quite often afford products not obtainable in solution. Also lacking has been solid-state rearrangement chemistry with accompanying theory. In our research over the past two decades we have developed theoretical treatments of solid-state reactivity. This report describes the development of this research from its primitive beginnings to the present.

Journal Article↗
PubMed1999

Photochemistry of Methane on Pd/Al(2)O(3) Model Catalysts: Control of Photochemistry on Transition Metal Surfaces.

The mean size of the Pd clusters deposited on an Al(2)O(3) thin film influences the efficiency of photodissociation of CD(4) into CD(3) and D as well as the photodesorpion upon irradiation with 193-nm laser light. The picture shows the degree of desorption on Pd clusters of various sizes (as well as on a Pd(111) single-crystal surface) as a function of temperature; the values given refer to the average thickness of the evaporated metal layer.

Journal Article↗
PubMed1989

The photochemistry of 2-alkoxycytosines in phosphate buffer and its link to cytosine photochemistry in alcoholic solution.

The reactions occurring when 2-methoxycytosine (4-amino-2-methoxypyrimidine) (I) and 2-ethoxycytosine (II) are irradiated in aqueous phosphate buffer at pH 7 have been studied. We have verified the previous results of Moore [Can. J. Chem. 41, 1937-1950 (1963)], who studied I and indicated that the product isolatable in greatest yield is N-carbomethoxy-3-hydroxyacrylamidine (IIIa). Similarly, we have found that the product of highest yield, isolated after irradiation of II, is N-carboethoxy-3-hydroxyacrylamidine (IIIb). Evidence is presented indicating that these are secondary products, being produced through hydrolysis of the corresponding N-carboalkoxy-3-aminoacrylamidines. Spectroscopic data are given for N-carboisopropoxy-3-hydroxyacrylamidine (IIIc), as well as the N-carbomethoxy, N-carboethoxy and N-carboisopropoxy derivatives of 3-hydroxy-2-methylacrylamidine. These four compounds were obtained through hydrolysis of the corresponding N-carboalkoxy derivatives of 3-aminoacrylamidine or 3-amino-2-methylacrylamidine, isolated from the photoreaction of cytosine or 5-methylcytosine with the appropriate alcohol.

Alcohols↗
PubMed2000

Supramolecular photochemistry and solar cells

Supramolecular photochemistry as well as solar cells are fascinating topics of current interest in Inorganic Photochemistry and very active research fields which have attracted wide attention in last two decades. A brief outline of the investigations in these fields carried out in our Laboratory of Inorganic Photochemistry and Energy Conversion is given here with no attempt of an exhaustive coverage of the literature. The emphasis is placed on recent work and information on the above mentioned subjects. Three types of supramolecular systems have been the focus of this work: (i) cage-type coordination compounds; (ii) second-sphere coordination compounds, exemplified by ion-pair photochemistry of cobalt complexes and (iii) covalently-linked systems. In the latter, modulation of the photoluminescence and photochemistry of some rhenium complexes are discussed. Solar energy conversion and development of thin-layer photoelectrochemical solar cells based on sensitization of nanocrystalline semiconductor films by some ruthenium polypyridyl complexes are presented as an important application that resulted from specifically engineered artificial assemblies.

Journal Article↗
PubMed1981

Psoralen photochemistry and nucleic acid structure.

Many new psoralen derivatives have been synthesized in an effort to enhance their water solubility and their binding to nucleic acids. Availability of the very soluble strongly binding compounds has improved our abilities to follow the optical changes associated with the photochemistry of psoralens with DNA. Changes in both absorbance and fluorescence are presented in this review. A kinetic model for the photochemistry concludes that the detailed kinetics is dominated by the equilibrium constant for intercalation of the psoralen in the DNA, the quantum yield for photoaddition to DNA once intercalated and the quantum yield for photodestruction of the drug in water. With these 3 parameters the kinetics of photochemistry is predictable. The values of these parameters for numerous derivatives of 8-methoxypsoralen and 4,5',8 trimethylpsoralen are presented. Application of this photochemistry to a study of nucleic acid secondary structure in chromatin, fd bacteriophage, and in ribosomes is reviewed.

Antiviral Agents↗
PubMed2002

Photochemistry of racemic and resolved 2-iodooctane. Effect of solvent polarity and viscosity on the chemistry.

The photochemistry of racemic and resolved 2-iodooctane was examined in cyclopentane, methanol, and 2-methyl-2-propanol, media with differing polarities and viscosities. The photochemistry of racemic 2-iodooctane was also examined in the gas phase. The photochemistry of 2-deuterio- and 1,1,1-trideuterio-2-iodooctane in cyclopentane and methanol was also studied. The photoreactions in cyclopentane, 2-methyl-2-propanol, and the gas phase occurred exclusively through homolytic reactions, while in methanol, they occurred predominantly (>53%) through heterolytic reactions. By comparing the disappearance of the optically active substrate with its loss of optical activity, F, the fraction of the initially formed radical pair (RP) or ion pair (IP) resulting in product was determined for the three solvents. Because F contains contributions of both escape of the partners in the RP or IP into the bulk of the solvent and reaction within the RP or IP to yield products other than the substrate, there was no correlation between F and solvent viscosity. The F values will be valuable in assessing the photochemistry of 2-iodooctane in the same media with circularly polarized light.

Journal Article↗
PubMed2006

Femtosecond pump-probe studies of nitrosyl chloride photochemistry in solution.

We present a femtosecond pump-probe study of the primary events of nitrosyl chloride (ClNO) photochemistry in solution. Following 266 nm photolysis, the resulting evolution in optical density is measured for ClNO dissolved in acetonitrile, chloroform, and dichloromethane. The results demonstrate that photolysis results in the production of a photoproduct that has an absorption band maximum at 295 nm in acetonitrile and 330 nm in chloroform and dichloromethane. To determine the extent of Cl production, comparative photochemical studies of methyl hypochlorite (MeOCl) and ClNO are performed. Photolysis of MeOCl in solution results in the production of the Cl:solvent charge-transfer complex; therefore, a comparison of the spectral evolution observed following MeOCl and ClNO photolysis under identical photolysis conditions is performed to determine the extent of Cl production following ClNO photolysis. We find that similar to the gas-phase photochemistry, Cl and NO formation is the dominant photochemical channel in acetonitrile. However, the photochemistry in chloroform and dichloromethane is more complex, with a second product formed in addition to Cl and NO. It is proposed that in these solvents photoisomerization also occurs, resulting in the production of ClON. The results presented here represent the first detailed examination of the solution phase photochemistry of ClNO.

Journal Article↗
PubMed2003

Salinity treatment shows no effects on photosystem II photochemistry, but increases the resistance of photosystem II to heat stress in halophyte Suaeda salsa.

Photosynthetic gas exchange, modulated chlorophyll fluorescence, rapid fluorescence induction kinetics, and the polyphasic fluorescence transients were used to evaluate PSII photochemistry in the halophyte Suaeda salsa exposed to a combination of high salinity (100-400 mM NaCl) and heat stress (35-47.5 degrees C, air temperature). CO(2) assimilation rate increased slightly with increasing salt concentration up to 300 mM NaCl and showed no decrease even at 400 mM NaCl. Salinity treatment showed neither effects on the maximal efficiency of PSII photochemistry (F(v)/F(m)), the rapid fluorescence induction kinetics, and the polyphasic fluorescence transients in dark-adapted leaves, nor effects on the efficiency of excitation energy capture by open PSII reaction centres (F(v)'/F(m)') and the actual PSII effciency (Phi(PSII)), photochemical quenching (q(P)), and non-photochemical quenching (q(N)) in light-adapted leaves. The results indicate that high salinity had no effects on PSII photochemistry either in a dark-adapted state or in a light-adapted state. With increasing temperature, CO(2) assimilation rate decreased significantly and no net CO(2) assimilation was observed at 47.5 degrees C. Salinity treatment had no effect on the response of CO(2) assimilation to high temperature when temperature was below 40 degrees C. At 45 degrees C, CO(2) assimilation rate in control plants decreased to zero, but the salt-adapted plants still maintained some CO(2) assimilation capacity. On the other hand, the responses of PSII photochemistry to heat stress was modified by salinity treatment. When temperature was above 35 degrees C, the declines in F(v)/F(m), Phi(PSII), F(v)'/F(m)', and q(P) were smaller in salt-adapted leaves compared to control leaves. This increased thermostability was independent of the degree of salinity, since no significant changes in the above-described fluorescence parameters were observed among the plants treated with different concentrations of NaCl. During heat stress, a very clear K step as a specific indicator of damage to the O(2)-evolving complex in the polyphasic fluorescence transients appeared in control plants, but did not get pronounced in salt-adapted plants. In addition, a greater increase in the ratio (F(i)-F(o))/(F(p)-F(o)) which is an expression of the proportion of the Q(B)-non-reducing PSII centres was observed in control plants rather than in salt-adapted plants. The results suggest that the increased thermostability of PSII seems to be associated with the increased resistance of the O(2)-evolving complex and the reaction centres of PSII to high temperature.

Adaptation, Physiological↗