Photochemistry and Photobiology

Cover image for Vol. 91 Issue 4

Edited By: Jean Cadet

Impact Factor: 2.266

ISI Journal Citation Reports © Ranking: 2014: 44/73 (Biophysics); 184/289 (Biochemistry & Molecular Biology)

Online ISSN: 1751-1097

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  • SPECIAL ISSUE RESEARCH ARTICLE: Following Oxygen Consumption in Singlet Oxygen Reactions via Changes in Sensitizer Phosphorescence

    SPECIAL ISSUE RESEARCH ARTICLE: Following Oxygen Consumption in Singlet Oxygen Reactions via Changes in Sensitizer Phosphorescence

    This work reports an examination of singlet oxygen reactions with amino acid substrates by a method involving measurement of the change in phosphorescence intensity of the singlet oxygen sensitizer. The sensitizer, a Ru(II) bipyridyl complex covalently linked to pyrene, has long-lived phosphorescence in N2 purged aqueous solutions (τ0 ~ 20 μs) that is nearly completely quenched by oxygen in aerated solutions. Irradiation of the complex in water containing sub mM concentrations of histidine, tryptophan and methionine results in a dramatic, easily visible increase in the phosphorescence intensity over a period of 10–100 s. Rate constants for singlet oxygen oxidation of each of the substrates can be obtained by using changes in the phosphorescence intensity in initial rate kinetic analysis. Rate constants obtained in this way compare favorably with those reported in the literature. The method represents a very simple approach for obtaining rate constants for singlet oxygen reactions with various substrates and the kinetics can be extended to nonaqueous solvents.

  • SPECIAL ISSUE RESEARCH ARTICLE: Comparison of Templating Abilities of Urea and Thioruea During Photodimerization of Bipyridylethyelene and Stilbazole Crystals

    SPECIAL ISSUE RESEARCH ARTICLE: Comparison of Templating Abilities of Urea and Thioruea During Photodimerization of Bipyridylethyelene and Stilbazole Crystals

    Photodimerization of cocrystals of four bispyridylethylenes and two stilbazoles with urea as a template in the solid state has been investigated following our success with thiourea. Four investigated olefins photodimerized quantitatively to a single dimer in the crystalline state only. The reactivity of urea–olefin crystals is understood on the basis of their packing arrangements in the crystalline state. In reactive crystals the adjacent reactive molecules are within 4.2 Å and parallel, whereas the unreactive ones have their adjacent molecules are farther than 4.6Å and nonparallel. Thus, with the knowledge of crystal packing the reactivity of urea–olefin crystals is predictable on the basis of Schmidt's topochemical postulates. The templating property of urea, similar to thiourea, derives from its ability to form hydrogen bonds with itself and the guest olefins. Despite the similarities in molecular structures of urea and thiourea their subtle electronic properties, yet to be fully understood, affect the crystal packing and consequently their reactivity in the crystalline state. Further work is needed to fully exploit the templating properties of urea.

  • SPECIAL ISSUE RESEARCH ARTICLE: Electronic Interactions of Michler's Ketone with DNA Bases in Synthetic Hairpins

    SPECIAL ISSUE RESEARCH ARTICLE: Electronic Interactions of Michler's Ketone with DNA Bases in Synthetic Hairpins

    The mechanism and dynamics of photoinduced electron transfer in two families of DNA hairpins possessing Michler's ketone linkers have been investigated by means of steady state and time-resolved transient absorption and emission spectroscopies. The excited state behavior of the diol linker employed in hairpin synthesis is similar to that of Michler's ketone in methanol solution. Hairpins possessing only a Michler's ketone linker undergo fast singlet state charge separation and charge recombination with an adjacent purine base, attributed to well-stacked ground state conformations, and intersystem crossing to the triplet state, attributed to poorly stacked ground state conformations. The failure of the triplet to undergo electron transfer reactions on the 7 ns time scale of our measurements is attributed to the low triplet energy and reduction potential of the twisted triplet state. Hairpins possessing both a Michler's ketone linker and a perylenediimide base surrogate separated by four base pairs undergo photoinduced hole transport from the diimide to Michler's ketone upon excitation of the diimide. The efficiency of hole transport is dependent upon the sequence of the intervening purine bases.

  • RESEARCH ARTICLE: Effects of Substituents on Synthetic Analogs of Chlorophylls. Part 4: How Formyl Group Location Dictates the Spectral Properties of Chlorophylls b, d and f

    RESEARCH ARTICLE: Effects of Substituents on Synthetic Analogs of Chlorophylls. Part 4: How Formyl Group Location Dictates the Spectral Properties of Chlorophylls b, d and f

    Photosynthetic organisms are adapted to light characteristics in their habitat in part via the spectral characteristics of the associated chlorophyll pigments, which differ in the position of a formyl group around the chlorin macrocycle (chlorophylls b, d, f) or no formyl group (chlorophyll a). To probe the origin of this spectral tuning, the photophysical and electronic structural properties of a new set of synthetic chlorins are reported. The zinc and free base chlorins have a formyl group at either the 2- or 3-position. The four compounds have fluorescence yields in the range 0.19–0.28 and singlet excited-state lifetimes of ca 4 ns for zinc chelates and ca 8 ns for the free base forms. The photophysical properties of the 2- and 3-formyl zinc chlorins are similar to those observed previously for 13-formyl or 3,13-diformyl chlorins, but differ markedly from those for 7-formyl analogs. Molecular-orbital characteristics obtained from density functional theory (DFT) calculations were used as input to spectral simulations employing the four-orbital model. The analysis has uncovered the key changes in electronic structure engendered by the presence/location of a formyl group at various macrocycle positions, which is relevant to understanding the distinct spectral properties of the natural chlorophylls a, b, d and f.

  • RESEARCH ARTICLE: Minimum Exposure Limits and Measured Relationships Between the Vitamin D, Erythema and International Commission on Non-Ionizing Radiation Protection Solar Ultraviolet

    RESEARCH ARTICLE: Minimum Exposure Limits and Measured Relationships Between the Vitamin D, Erythema and International Commission on Non-Ionizing Radiation Protection Solar Ultraviolet

    The International Commission on Non-Ionizing Radiation Protection (ICNIRP) has established guidelines for exposure to ultraviolet radiation in outdoor occupational settings. Spectrally weighted ICNIRP ultraviolet exposures received by the skin or eye in an 8 h period are limited to 30 J m−2. In this study, the time required to reach the ICNIRP exposure limit was measured daily in 10 min intervals upon a horizontal plane at a subtropical Australian latitude over a full year and compared with the effective Vitamin D dose received to one-quarter of the available skin surface area for all six Fitzpatrick skin types. The comparison of measured solar ultraviolet exposures for the full range of sky conditions in the 2009 measurement period, including a major September continental dust event, show a clear relationship between the weighted ICNIRP and the effective vitamin D dose. Our results show that the horizontal plane ICNIRP ultraviolet exposure may be used under these conditions to provide minimum guidelines for the healthy moderation of vitamin D, scalable to each of the six Fitzpatrick skin types.

  • SPECIAL ISSUE RESEARCH ARTICLE: Regulation and Disregulation of Mammalian Nucleotide Excision Repair: A Pathway to Nongermline Breast Carcinogenesis

    SPECIAL ISSUE RESEARCH ARTICLE: Regulation and Disregulation of Mammalian Nucleotide Excision Repair: A Pathway to Nongermline Breast Carcinogenesis

    Nucleotide excision repair (NER) is an important modulator of disease, especially in constitutive deficiencies such as the cancer predisposition syndrome Xeroderma pigmentosum. We have found profound variation in NER capacity among normal individuals, between cell-types and during carcinogenesis. NER is a repair system for many types of DNA damage, and therefore many types of genotoxic carcinogenic exposures, including ultraviolet light, products of organic combustion, metals and oxidative stress. Because NER is intimately related to cellular metabolism, requiring components of both the DNA replicative and transcription machinery, it has a narrow range of functional viability. Thus, genes in the NER pathway are expressed at the low levels manifested by, for example, nuclear transcription factors. As NER activity and gene expression vary by cell-type, it is inherently epigenetically regulated. Furthermore, this epigenetic modulation is disregulated during sporadic breast carcinogenesis. Loss of NER is one basis of genomic instability, a required element in cellular transformation, and one that potentially influences response to therapy. In this study, we demonstrate differences in NER capacity in eight adult mouse tissues, and place this result into the context of our previous work on mouse extraembryonic tissues, normal human tissues and sporadic early stage human breast cancer.

  • SPECIAL ISSUE RESEARCH ARTICLE: Following Oxygen Consumption in Singlet Oxygen Reactions via Changes in Sensitizer Phosphorescence
  • SPECIAL ISSUE RESEARCH ARTICLE: Comparison of Templating Abilities of Urea and Thioruea During Photodimerization of Bipyridylethyelene and Stilbazole Crystals
  • SPECIAL ISSUE RESEARCH ARTICLE: Electronic Interactions of Michler's Ketone with DNA Bases in Synthetic Hairpins
  • RESEARCH ARTICLE: Effects of Substituents on Synthetic Analogs of Chlorophylls. Part 4: How Formyl Group Location Dictates the Spectral Properties of Chlorophylls b, d and f
  • RESEARCH ARTICLE: Minimum Exposure Limits and Measured Relationships Between the Vitamin D, Erythema and International Commission on Non-Ionizing Radiation Protection Solar Ultraviolet
  • SPECIAL ISSUE RESEARCH ARTICLE: Regulation and Disregulation of Mammalian Nucleotide Excision Repair: A Pathway to Nongermline Breast Carcinogenesis

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Synthetic DNA hairpins possessing a Michler’s ketone linker at one end separated from a perylenediimide base surrogate by four base pairs were designed to investigate transport of charge between the two chromophores. Pulsed laser excitation of Michler’s ketone with 400 nm light results in formation of a charge transfer excited state which returns to the group state without undergoing electron transport to perylenediimide (upper pathway). Excitation of perylenediimide with 545 nm light results in injection of a positive charge which is transported via the base pairs to Michler’s ketone (lower pathway). For details see the paper “Electronic Interactions of Michler’s Ketone with DNA Bases in Synthetic Hairpins” by Almaz S. Jalilov et al. on pages 739-747 in this issue.
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