UV radiation can cause harmful effects to human skin, including premature skin ageing and skin cancer. Historically, sunscreens were developed to filter out UVB (290 nm-320 nm), but now the importance of UVA (320 nm-400 nm) sunscreens is realised. The most common UVA sunscreens are based on dibenzoylmethane (1,3-diphenyl propan-l,3-dione, DBM), of which the most common is Parsol 1789 (4'- methoxy 4'-tertiarybutyl DBM). The photochemistry of these materials has, however, been poorly understood. In this work the photophysics and photochemistry of DBM, Parsol 1789, Parsol DAM and ditertiarybutyl DBM have been studied, along with the respective 0-methylated and C-methylated compounds of DBM and Parsol 1789.DBMs exist primarily as an intra-molecularly bonded enol, which absorbs strongly at λ≈340 nm due to a π,π* transition. The absorption spectra of DBMs also exhibit a smaller peak at λ≈250 nm, due to an n,π* transition of the diketone content. At low temperature the main absorption band of DBMs shifts to longer wavelengths and vibrational structure can be observed. The enol form of DBMs fluorescence at low temperature, (v(_0)’→v’’(_0) at λ≈385 nm), and phosphorescence can be observed from both the diketone (λ(_em)≈495 nm,) and enol forms (λ(_em)≈425 nm). Thus the triplet energies of the diketones and enols of the DBMs studied have been measured. 0-methylated DBMs do not possess an intra-molecular H-bond, and the π,π* absorption band falls to lower wavelengths than for chelated DBMs. C-methylated DBMs exist as a diketone structure, and display photophysics typical of an aromatic ketone. It has been suggested that the main process on irradiation of DBM is the formation of a short-lived non-chelated enol, however no direct evidence as to the structure of this species is reported in the literature. Formation of the diketone form of DBM on prolonged irradiation in acetonitrile solution has also been reported, and in this work the quantum yield of this process has been measured; ɸ≈0.01 ± 0.004. In this work, direct (low temperature) IR spectroscopic evidence is presented to prove that the short-lived species produced on irradiation is indeed a non-chelated enol. The infra-red studies also suggest that the non-chelated enol form of DBM form complexes with polar solvents, as has been proposed in the literature. Quantum yields of non-chelated enol formation in cyclohexane at room temperature have been measured to be approximately ɸ=0.5 + 0.07. This work indicates that the rate of transient decay is enhanced by the interaction of the transient molecules with chelated enol molecules or other transient molecules. IR studies of low temperature transient formation confirm the interaction of transient molecules by the observation of inter-molecular hydrogen-bonding. By comparison with the E and Z isomers of 0-methylated DBM it is suggested that at low temperature DBM initially forms a Z-cis non-hydrogen bonded enol, which then converts to an E-trans non-hydrogen bonded enol with further irradiation. The kinetics and the temperature variation of the enol recovery support the theory that there is more than one species formed. The photochemistry of DBM in emulsions has also been studied in this work. It has been shown that the photochemistry occurring on irradiation is similar to that observed in solutions. This indicates that simple solutions are a good model for actual sunscreen formulations. Singlet oxygen is a highly reactive species capable of causing serious biological damage, however this work shows that DBM sunscreens generate singlet oxygen by photosensitisation, with quantum yields ɸ∆≈0.005-0.01. It has also been shown that the lifetime of the excited state of DBM involved in singlet oxygen production is very short, approximately τ <1 µs.

No description available.

No description available.

This project was designed to investigate the impact of natural levels of ultraviolet-B radiation on the genomic mutation rate in Arabidopsis thaliana. UV-B radiation is a known mutagen, but plants may have evolved mechanisms to cope with any genomic damage induced by routine exposure to this radiation. In an attempt to determine whether the genomic mutation rate in a plant species is elevated in the presence of UV-B, two eleven generation mutation accumulation studies were preformed. One study incorporated levels of UV-B similar to that encountered on a clear mid-summer's day, while the other was performed in the absence of this mutagen. Mutation rate estimates, obtained primarily from maximum likelihood analysis of phenotypic data, were not significantly greater than zero, both in the presence and absence of UV-B. No evidence was found to support the notion that the genomic mutation rate is increased by exposure to natural levels of UV-B.

Shallow-dwelling scleractinian corals live in high irradiance environments where they are exposed to large fluxes of ultraviolet radiation (UVR, 280 - 400 nm). A suite of UV-aborbing compounds, know as mycosporine-like amino acids, is found within the tissues of coral-algal symbioses and may perform a sun-screening role. The seasonal variation in MAA concentration was investigated for two corals in Kaneohe Bay, Hawai'i, Porites compressa and Montipora verrucosa. Regressions of MAA concentration and the amount of UVR measured prior to collection date were not significant for total MAA concentration of either species. However, individual MAAs, shinorine in P. compressa and palythene in M. verrucosa, did show significant correlation with UVR. The effects of UVR and water motion on Porites compressa were investigated in a flume and in the field. Exposure to ambient UVR was the most important factor tested in determining the concentration of MAAs in the tissues of P. compressa. Water motion also positively affected the concentration of MAAs, but only in the presence of UVR. When UVR was screened from the corals' environment, the tissue concentration of MAAs slowly decreased over time (approximately 2.5 to 5% per week) regardless of water motion. The effect of UVR on coral planulae was investigated in field experiments with Pocillopora damicornis. Larvae were taken from four different source adults: those from <0.5 m, those from 3 m, those incubated in the absence of UVR for two months, and those incubated in ambient UVR for two months. Deep larvae and larvae from adults incubated in the absence of UVR had roughly half the amount ofMAAs found in the shallow larvae and the larvae from adults in ambient UVR. Origin of larvae was not a significant factor in determining larval survival or recruitment success. UVR, however, was important in determining recruitment rate. Larvae were less likely to recruit to the settlement tile in the presence of ambient UVR than in treatments where the UVR was screened out. / Thesis (Ph. D.)--University of Hawaii at Manoa, 1999. Includes bibliographical references (leaves 151-164).

Thesis (Ph.D.) -- McMaster University, 1998. / Includes bibliographical references (leaves 167-182). Also available via World Wide Web.

Thesis (Ph.D.) -- McMaster University, 2002. / Includes bibliographical references (leaves 145-157). Also available via World Wide Web.

Thesis (M.S.)--University of Missouri-Columbia, 2006. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on April 21, 2009) Includes bibliographical references.

Mémoire (M.Sc.)- Université du Québec à Chicoutimi, 1975. / "Thèse présentée dans le cadre du programme UQ d'études avancées en énergie pour l'obtention du grade de la maîtrise es science (énergie)". CaQCU Document électronique également accessible en format PDF. CaQCU

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2004. / Includes bibliographical references (leaves 106-124). Also available in electronic version. Access restricted to campus users.