Formation Of Cyclobutane Pyrimidine Dimers Research Articles

Vitamin E inhibits the UVAI induction of “light” and “dark” cyclobutane pyrimidine dimers, and oxidatively generated DNA damage, in keratinocytes

Solar ultraviolet radiation (UVR)-induced DNA damage has acute, and long-term adverse effects in the skin. This damage arises directly by absorption of UVR, and indirectly via photosensitization reactions. The aim of the present study was to assess the effects of vitamin E on UVAI-induced DNA damage in keratinocytes in vitro. Incubation with vitamin E before UVAI exposure decreased the formation of oxidized purines (with a decrease in intracellular oxidizing species), and cyclobutane pyrimidine dimers (CPD). A possible sunscreening effect was excluded when similar results were obtained following vitamin E addition after UVAI exposure. Our data showed that DNA damage by UVA-induced photosensitization reactions can be inhibited by the introduction of vitamin E either pre- or post-irradiation, for both oxidized purines and CPD (including so-called “dark” CPDs). These data validate the evidence that some CPD are induced by UVAI initially via photosensitization, and some via chemoexcitation, and support the evidence that vitamin E can intervene in this pathway to prevent CPD formation in keratinocytes. We propose the inclusion of similar agents into topical sunscreens and aftersun preparations which, for the latter in particular, represents a means to mitigate on-going DNA damage formation, even after sun exposure has ended.

Oxidatively Generated Lesions as Internal Photosensitizers for Pyrimidine Dimerization in DNA.

In this work, the attention is focused on UVA-photosensitized reactions triggered by a DNA chromophore-containing lesion, namely 5-formyluracil. This is a major oxidatively generated lesion that exhibits an enhanced light absorption in the UVB-UVA region. The mechanistic study combining photochemical and photobiological techniques shows that irradiation of 5-formyluracil leads to a triplet excited state capable of sensitizing formation of cyclobutane pyrimidine dimers in DNA via a triplet-triplet energy transfer. This demonstrates for the first time that oxidatively generated DNA damage can behave as an intrinsic sensitizer and result in an important extension of the active fraction of the solar spectrum with photocarcinogenic potential. Overall, this raises the question of an aggravated photomutagenicity of the 5-formyluracil lesion.

Using Organic Synthesis and Chemical Analysis to Understand the Photochemistry of Spore Photoproduct and Other Pyrimidine Dimers

Pyrimidine dimerization is the dominant DNA photoreaction occurring in vitro and in vivo. Three types of dimers, cyclobutane pyrimidine dimers (CPDs), pyrimidine (6-4) pyrimidone photoproducts (6-4PPs), and the spore photoproduct (SP), are formed from the direct dimerization process; it is of significance to understand the photochemistry and photobiology of these dimers. Traditionally, pyrimidine dimerization was studied by using the natural pyrimidine residues thymine and cytosine, which share similar chemical structures and similar reactivity, making it sometimes less straightforward for one to identify the key pyrimidine residue that needs to be excited to trigger the photoreaction. We thus adopted synthetic chemistry to selectively modify the pyrimidine residues or to introduce pyrimidine analogs to the selected positions before UV irradiation is applied. By monitoring the subsequent outcomes from the photoreaction, we were able to gain unique mechanistic insights into the photochemistry of SP as well as of CPDs and 6-4PPs. Moreover, our approaches have resulted in several useful “tools” that can facilitate the understanding of lesion photobiology. Our results summarized in this account illustrate what organic synthesis/chemical analysis may allow us to achieve in future DNA lesion biology studies. 1 Introduction 2 Using the Deuterium Labeling Strategy to Understand SP Formation 3 Using Microcrystals to Reveal the Reaction Intermediates in SP Formation 4 Using a Phosphate Isostere to Understand the SP Structure 5 Synthesis of SP Phosphoramidite and SP Structural Studies 6 Using a Thymine Isostere to Understand CPD Formation 7 Using a Thymine Isostere to Understand 6-4PP Photoreaction 8 Understanding the Chemical Stability of SP 9 Understanding the Chemical Stability of 6-4PP10 Summary and Perspectives for Future Research

Multiple Electronic and Structural Factors Control Cyclobutane Pyrimidine Dimer and 6-4 Thymine-Thymine Photodimerization in a DNA Duplex.

The T-T photodimerization paths leading to the formation of cyclobutane pyrimidine dimer (CPD) and 6-4 pyrimidine pyrimidone (64-PP), the two main DNA photolesions, have been resolved for a T-T step in a DNA duplex by two complementary state-of-the-art quantum mechanical approaches: QM(CASPT2//CASSCF)/MM and TD-DFT/PCM. Based on the analysis of several different representative structures, we define a new-ensemble of cooperating geometrical and electronic factors (besides the distance between the reacting bonds) ruling T-T photodimerization in DNA. CPD is formed by a barrierless path on an exciton state delocalized over the two bases. Large interbase stacking and shift values, together with a small pseudorotation phase angle for T at the 3'-end, favor this reaction. The oxetane intermediate, leading to a 64-PP adduct, is formed on a singlet T→T charge-transfer state and is favored by a large interbase angle and slide values. A small energy barrier (<0.3 eV) is associated to this path, likely contributing to the smaller quantum yield observed for this process. Eventually, a clear directionality is always shown by the electronic excitation characterizing the singlet photoactive state driving the photodimerization process: an exciton that is more localized on T3 and a 5'-T→3'-T charge transfer for CPD and oxetane formation, respectively, thus calling for specific electronic constraints.

Persistence and Tolerance of DNA Damage Induced by Chronic UVB Irradiation of the Human Genome

Exposure to solar UVB radiation leads to the formation of the highly mutagenic cyclobutane pyrimidine dimers (CPDs), the DNA damage responsible for mutations found in skin cancer. The frequency of CPD formation and the repair rate of those lesions are two important parameters to determine the probability of UVR-induced mutations. Previous work has shown that chronic irradiation with sublethal doses of UVB radiation (chronic low-dose UVB radiation) leads to the accumulation of residual CPD that persists over time. We have thus investigated the persistence, localization, and consequences on genome stability of those chronic low-dose UVB radiation-induced residual CPDs. We show that chronic low-dose UVB radiation-induced residual CPDs persist on DNA and are diluted via semiconservative replication. They are overrepresented in the heterochromatin and at the TT dipyrimidine sites, and they catalyze the incidence of sister chromatin exchange. Our results shed some light on the impact of chronic UVB radiation exposure on DNA, with a focus on residual CPDs, their distribution, and consequences.

Apigenin prevents ultraviolet-B radiation induced cyclobutane pyrimidine dimers formation in human dermal fibroblasts

Exposure to solar ultraviolet-B (UVB) radiation leads to the formation of cyclobutane pyrimidine dimers (CPDs). We investigated the protective effect of apigenin against UVB-induced CPDs formation in human dermal fibroblasts cells (HDFa). For this purpose, HDFa cells were treated with apigenin (15μM) prior to UVB irradiation (20mJ/cm2); DNA damage and subsequent molecular end points were observed. Exposure to UVB radiation increased significant CPDs formation in HDFa cells and the frequencies of CPDs were reduced by treatment with apigenin (15μM). UVB-induced CPDs downregulates the expression of nucleotide excision repair (NER) genes such as xeroderma pigmentosum complementation group C, B, G and F (XPC, XPB, XPG and XPF), transcription factor II human (TFIIH) and excision repair cross-complementation group 1 (ERCC1) in HDFa cells. Conversely, apigenin treatment restored UVB-induced loss of NER proteins in HDFa cells, which indicates its preventive effect against CPDs formation. Besides, single low dose UVB-exposure induced nuclear fragmentation, apoptotic frequency and apoptotic proteins expression (Bax and Caspase-3) have been prevented by the apigenin pretreatment. Furthermore, apigenin exhibits strong UV absorbance property and showed 10.08 SPF value. Thus, apigenin can protect skin cells against UVB-induced CPDs formation probably through its sunscreen effect. Hence, apigenin can be considered as an effective protective agent against UV induced skin damages.

Protective effect of Arthrospira platensis extracts against ultraviolet B-induced cellular senescence through inhibition of DNA damage and matrix metalloproteinase-1 expression in human dermal fibroblasts

Ultraviolet (UV) light exposure causes skin photoaging, which is known to be preventable and controllable by application of UV-protective agents. In this study, we demonstrated, for the first time, that the extract of microalgae Arthrospira platensis has a reverse effect on UV-induced photodamage such as loss of cell viability, cellular senescence, DNA damage, and collagen destruction in dermal fibroblasts. Forty-eight extracts were prepared from the cell biomass by controlling culture light conditions, extract solvents, and disruption methods. Then, we analyzed their cytotoxicities using WST-1 assay and separated low and high cytotoxic extracts with normal human dermal fibroblasts (nHDFs). Using the low cytotoxic extracts, we performed UVB protection assay and selected the most effective extract demonstrating protective effect against UVB-induced nHDF damage. Flow cytometric analysis and senescence-associated (SA) β-galactosidase assay showed that pretreatment with the extract reversed UVB-induced G2/M phase cell cycle arrest and senescence in nHDFs. Furthermore, UVB-induced DNA damage in nHDFs, such as cyclobutane pyrimidine dimer formation, was significantly suppressed by the extract. Further, quantitative real-time PCR experiments revealed that the extract significantly inhibited UVB-induced upregulation of matrix metalloproteinase 1 (MMP1) and MMP3 expression in nHDFs. Therefore, we concluded that the microalgae extract can be a potential anti-photoaging agent.

Modulation of cyclobutane thymine photodimer formation in T11-tracts in rotationally phased nucleosome core particles and DNA minicircles.

Cyclobutane pyrimidine dimers (CPDs) are DNA photoproducts linked to skin cancer, whose mutagenicity depends in part on their frequency of formation and deamination. Nucleosomes modulate CPD formation, favoring outside facing sites and disfavoring inward facing sites. A similar pattern of CPD formation in protein-free DNA loops suggests that DNA bending causes the modulation in nucleosomes. To systematically study the cause and effect of nucleosome structure on CPD formation and deamination, we have developed a circular permutation synthesis strategy for positioning a target sequence at different superhelix locations (SHLs) across a nucleosome in which the DNA has been rotationally phased with respect to the histone octamer by TG motifs. We have used this system to show that the nucleosome dramatically modulates CPD formation in a T11-tract that covers one full turn of the nucleosome helix at seven different SHLs, and that the position of maximum CPD formation at all locations is shifted to the 5΄-side of that found in mixed-sequence nucleosomes. We also show that an 80-mer minicircle DNA using the same TG-motifs faithfully reproduces the CPD pattern in the nucleosome, indicating that it is a good model for protein-free rotationally phased bent DNA of the same curvature as in a nucleosome, and that bending is modulating CPD formation.

Searching for novel photolyases in UVC-resistant Antarctic bacteria

Ultraviolet (UV) light irradiation has serious consequences for cell survival, including DNA damage by formation of cyclobutane pyrimidine dimers (CPD) and pyrimidine (6,4) pyrimidone photoproducts. In general, the Nucleotide Excision Repair pathway repairs these lesions; however, all living forms, except placental mammals and some marsupials, produce a flavoprotein known as photolyase that directly reverses these lesions. The aim of this work was the isolation and identification of Antarctic UVC-resistant bacteria, and the search for novel photolyases. Two Antarctic water samples were UVC-irradiated (254nm; 50-200Jm- 2) and 12 UVC-resistant bacteria were isolated and identified by 16S rDNA amplification/analysis as members of the genera Pseudomonas, Janthinobacterium, Flavobacterium, Hymenobacter and Sphingomonas. The UVC 50% lethal dose and the photo-repair ability of isolates were analyzed. The occurrence of photolyase coding sequences in Pseudomonas, Hymenobacter and Sphingomonas isolates were searched by PCR or by searching in the draft DNA genome. Results suggest that Pseudomonas and Hymenobacter isolates produce CDP-photolyases, and Sphingomonas produces two CPD-photolyases and a 6,4-photolyase. Results suggest that the Antarctic environment is an important source of genetic material for the identification of novel photolyase genes with potential biotechnological applications.

Drug-DNA complexation as the key factor in photosensitized thymine dimerization.

The crucial role of photosensitizer@DNA complexation in the formation of cyclobutane pyrimidine dimers (CPDs) has been demonstrated using femtosecond and nanosecond transient absorption and emission measurements in combination with in vitro DNA damage assays. This finding opens the door to re-evaluate the mechanisms involved in CPDs photosensitized by other chemicals.

Preliminary study on antioxidant properties, phenolic contents, and effects of Aesculus hippocastanum (horse chestnut) seed shell extract on in vitro cyclobutane pyrimidine dimer repair

Aim: Human skin exposure to solar UV radiation induces cells to produce reactive oxygen species that directly cause DNA damage via the formation of cyclobutane pyrimidine dimers (CPDs), which are the primary UVB-induced DNA lesions. CPDs are responsible for cell death, mutation, and neoplastic transformation. Aesculus hippocastanum (horse chestnut) seed extract has traditionally been used for venotonic treatments and also as a raw material for cosmetics. This study aimed to characterize the antioxidant properties of the phenolic contents of extracts from A. hippocastanum seed shell and endosperm and to investigate their effects on CPD repair in UVB-exposed human dermal fibroblasts in vitro. Materials and Methods: Crude 60% aqueous ethanol extracts (v/v) were prepared, and their total polyphenol contents, antioxidant activities, and ORAC values were measured by Folin–Ciocalteu, 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging capacity, and ORAC assays, respectively. The levels of CPDs induced by UVB irradiation were measured by cell-based immunoassays after 6-h treatment with the extract of A. hippocastanum seed shell at a concentration of 100-μg/ml and 9-h treatment with it at concentrations of 50, 100, and 200- μg/ml. Result: A. hippocastanum seed shell extract had 602 ± 1.6-mg gallic acid equivalent per gram of extract and exhibited 4990 ± 70.9 and 7140 ± 835 μmol Trolox equivalents per gram of extract as DPPH radical-scavenging activity and ORAC value, respectively; these values were comparable to those of extracts from green tea and Mallotus japonicus leaves. However, these values of the seed endosperm extract were relatively low. A 9-h treatment of cells with seed shell extracts at concentrations of 50, 100, and 200-μg/ml after UVB exposure significantly reduced CPD levels compared with those in UVB-exposed cells without treatment. Conclusion: This study indicated that A. hippocastanum seed shell extract possesses a potential to limit the harmful effects of human skin exposure to UVB and that this could be the first step toward identifying novel benefits of this extract.

Bifurcating electron-transfer pathways in DNA photolyases determine the repair quantum yield.

Photolyase is a blue-light-activated enzyme that repairs ultraviolet-induced DNA damage that occurs in the form of cyclobutane pyrimidine dimers (CPDs) and pyrimidine-pyrimidone (6-4) photoproducts. Previous studies on microbial photolyases have revealed an electron-tunneling pathway that is critical for the repair mechanism. In this study, we used femtosecond spectroscopy to deconvolute seven electron-transfer reactions in 10 elementary steps in all classes of CPD photolyases. We report a unified electron-transfer pathway through a conserved structural configuration that bifurcates to favor direct tunneling in prokaryotes and a two-step hopping mechanism in eukaryotes. Both bifurcation routes are operative, but their relative contributions, dictated by the reduction potentials of the flavin cofactor and the substrate, determine the overall quantum yield of repair.

Chromosomal landscape of UV damage formation and repair at single-nucleotide resolution

UV-induced DNA lesions are important contributors to mutagenesis and cancer, but it is not fully understood how the chromosomal landscape influences UV lesion formation and repair. Genome-wide profiling of repair activity in UV irradiated cells has revealed significant variations in repair kinetics across the genome, not only among large chromatin domains, but also at individual transcription factor binding sites. Here we report that there is also a striking but predictable variation in initial UV damage levels across a eukaryotic genome. We used a new high-throughput sequencing method, known as CPD-seq, to precisely map UV-induced cyclobutane pyrimidine dimers (CPDs) at single-nucleotide resolution throughout the yeast genome. This analysis revealed that individual nucleosomes significantly alter CPD formation, protecting nucleosomal DNA with an inward rotational setting, even though such DNA is, on average, more intrinsically prone to form CPD lesions. CPD formation is also inhibited by DNA-bound transcription factors, in effect shielding important DNA elements from UV damage. Analysis of CPD repair revealed that initial differences in CPD damage formation often persist, even at later repair time points. Furthermore, our high-resolution data demonstrate, to our knowledge for the first time, that CPD repair is significantly less efficient at translational positions near the dyad of strongly positioned nucleosomes in the yeast genome. These findings define the global roles of nucleosomes and transcription factors in both UV damage formation and repair, and have important implications for our understanding of UV-induced mutagenesis in human cancers.

Photoexcitation Dynamics of Thymine in Acetonitrile and an Ionic Liquid Probed by Time‐resolved Infrared Spectroscopy

Femtosecond transient IR absorption spectroscopy was used to probe the decay mechanism of electronically excited thymine (a naturally occurring pyrimidine base in DNA) dissolved in an ionic liquid ([Bmim][ BF4 ]) or CD3CN after the absorption of UV light (267 nm). In both solvents, an absorption band grew on a picosecond timescale, along with decaying bleach and evolving red‐shifted absorption signals. A population analysis of the observed kinetic data suggested that most of the photoexcited thymine underwent a sub‐picosecond non‐radiative relaxation to the vibrationally hot ground electronic state. About 4% (16%) of the excited thymine in the ionic liquid (CD3CN) relaxed to an intermediate electronic state, which relaxed into a low‐lying triplet state by intersystem crossing (ISC) (ISC did not relax to the ground electronic state within the experimental period (1 ns)). The low ISC yield for thymine in an ionic liquid was correlated with molecular properties of the solvent. This observation is significant because the ISC to triplet state transition for excited thymine has been considered as a precursor to cyclobutane–pyrimidine dimer formation, which led to functional damage of the base after UV absorption. This finding may shed light on the photostability of DNA in ionic liquids.

Ultraviolet absorbing compounds provide a rapid response mechanism for UV protection in some reef fish

The external mucus surface of reef fish contains ultraviolet absorbing compounds (UVAC), most prominently Mycosporine-like Amino Acids (MAAs). MAAs in the external mucus of reef fish are thought to act as sunscreens by preventing the damaging effects of ultraviolet radiation (UVR), however, direct evidence for their protective role has been missing. We tested the protective function of UVAC's by exposing fish with naturally low, Pomacentrus amboinensis, and high, Thalassoma lunare, mucus absorption properties to a high dose of UVR (UVB: 13.4W∗m−2, UVA: 6.1W∗m−2) and measuring the resulting DNA damage in the form of cyclobutane pyrimidine dimers (CPDs). For both species, the amount of UV induced DNA damage sustained following the exposure to a 1h pulse of high UVR was negatively correlated with mucus absorbance, a proxy for MAA concentration. Furthermore, a rapid and significant increase in UVAC concentration was observed in P. amboinensis following UV exposure, directly after capture and after ten days in captivity. No such increase was observed in T. lunare, which maintained relatively high levels of UV absorbance at all times. P. amboinensis, in contrast to T. lunare, uses UV communication and thus must maintain UV transparent mucus to be able to display its UV patterns. The ability to rapidly alter the transparency of mucus could be an important adaptation in the trade off between protection from harmful UVR and UV communication.

289 Oral administration of table grapes has a photoprotective effect on the sunburn response and on cyclobutane pyrimidine dimers in humans in vivo

288 Dietary green tea catechins protect dermal elasic fibers from UV-induced remodeling N Charoenchon, LE Rhodes, A Nicolaou, G Williamson, MD Farrar and RE Watson 1 The University of Manchester, Manchester, United Kingdom and 2 The University of Leeds, Leeds, United Kingdom Cutaneous photoaging is associated with dermal elastic fiber remodeling; daily photoprotection is therefore essential to prevent ultraviolet (UV)-induced skin damage. Green tea (C sinensis), rich in polyphenol catechins (GTC), have been reported to have significant skin benefits in vitro and in vivo. We have performed a double-blind randomised controlled trial to assess whether systemic GTC supplementation can protect dermal elastic fibers from acute UV exposure. Healthy white Caucasians (n1⁄450; 18-65 years) were randomized (1:1) to 12 weeks daily oral supplement (1080mg GTC with 100mg viatmin C) or placebo (maltodextrin). Biopsies were taken from skin exposed to 3xMED of solar simulated radiation (SSR) and unexposed skin at baseline and post-supplementation; compliance was confirmed by urinary GTC metabolite analysis. Dermal elastic fibers were identified via histology and immunohistochemistry, their distribution assessed in the papillary dermis (from the dermal-epidermal junction to a depth of 100mm) by image analysis. Differences between UV-exposed and unexposed skin were analysed by paired t-test. A total of 44 subjects completed the study, being compliant with supplementation and providing all skin samples. At baseline, SSR induced a significant overall loss of elastic fiber components (P1⁄4 0.001), and specifically fibulins-2 and -5. Post-supplementation, UV-mediated loss of fibulin-5 remained significant in the placebo group with mean (SD) % fiber area 19.4 (3.8) and 17.7 (4.1) in unexposed and UV-exposed skin resectively (P1⁄40.01), whilst in the active group fibulin-5 was protected with % fiber area 18.1 (4.0) and 17.1 (2.7) in unexposed and UV-exposed skin respectively (P1⁄4 0.30). Hence, acute SSR exposure results in elastic fiber remodeling similar to that observed in chronically photoaged skin. Furthermore, in a randomized control trial, dietary GTC protected fibulin-5 microfibrils in the papillary dermis from UV-mediated degradation. 289 Oral administration of table grapes has a photoprotective effect on the sunburn response and on cyclobutane pyrimidine dimers in humans in vivo AS Oak, R Shafi, S Katiyar, AT Slominski, W Cantrell, C Wang, RK Srivastava, C Li, M Athar and C Elmets Dermatology, University of Alabama at Birmingham, Birmingham, AL Table grapes contain proanthocyanidins and resveratrol which have been shown to have photoprotective properties in animal models. The purpose of this study was to determine whether oral administration of grapes had a similar effect in humans. Eleven healthy human volunteers (Fitzpatrick skin phototypes I-III, aged 19-46 and without any significant comorbidities) were recruited for the study (uncontrolled longitudinal study). The minimal erythema dose (MED) on the inner aspect of the forearm was determined using a broadband UVB light source. Skin biopsies were also performed at baseline from a sun protected area on the hip and from the 1 MED treated site on the forearm. Volunteers were then instructed to take 25 grams of grape powder three times per day for 2 weeks. At the end of the treatment period, the MED was again performed. In addition, skin biopsies were repeated at the end of two weeks in 9 of the 11 subjects after taking the grape powder. The biopsies were processed and stained for cyclobutane pyrimidine dimers (CPDs). The mean MED SEM increased in subjects from 205 30 J/m to 346 59 J/m, after taking the grape powder, which was statistically significant (paired t-test, p1⁄40.05). Oral administration of grape powder also inhibited CPD formation. There was a 32% decline in CPDs in pretreatment vs. post-grape treatment biopsies from the MED skin site (pretreatment: 32.6 5.2%, posttreatment: 23.1 4.1%, p1⁄40.005). Oral administration of grape powder did not have a significant effect on the number of CPDs in the unirradiated skin sites (pretreatment: 2.7 .9%, posttreatment: 3.8 .9%, p1⁄4.15). These studies demonstrate that oral administration of grapes has a photoprotective effect on the MED and in attenuating CPDs in human skin, and validate the findings from preclinical studies.

Nucleotide Excision Repair and Vitamin D--Relevance for Skin Cancer Therapy.

Ultraviolet (UV) radiation is involved in almost all skin cancer cases, but on the other hand, it stimulates the production of pre-vitamin D3, whose active metabolite, 1,25-dihydroxyvitamin D3 (1,25VD3), plays important physiological functions on binding with its receptor (vitamin D receptor, VDR). UV-induced DNA damages in the form of cyclobutane pyrimidine dimers or (6-4)-pyrimidine-pyrimidone photoproducts are frequently found in skin cancer and its precursors. Therefore, removing these lesions is essential for the prevention of skin cancer. As UV-induced DNA damages are repaired by nucleotide excision repair (NER), the interaction of 1,25VD3 with NER components can be important for skin cancer transformation. Several studies show that 1,25VD3 protects DNA against damage induced by UV, but the exact mechanism of this protection is not completely clear. 1,25VD3 was also shown to affect cell cycle regulation and apoptosis in several signaling pathways, so it can be considered as a potential modulator of the cellular DNA damage response, which is crucial for mutagenesis and cancer transformation. 1,25VD3 was shown to affect DNA repair and potentially NER through decreasing nitrosylation of DNA repair enzymes by NO overproduction by UV, but other mechanisms of the interaction between 1,25VD3 and NER machinery also are suggested. Therefore, the array of NER gene functioning could be analyzed and an appropriate amount of 1.25VD3 could be recommended to decrease UV-induced DNA damage important for skin cancer transformation.

Quantum Yield of Cyclobutane Pyrimidine Dimer Formation Via the Triplet Channel Determined by Photosensitization.

UV-induced formation of the cyclobutane pyrimidine dimer (CPD) lesion is investigated by stationary and time-resolved photosensitization experiments. The photosensitizer 2'-methoxyacetophenone with high intersystem crossing efficiency and large absorption cross-section in the UV-A range was used. A diffusion controlled reaction model is presented. Time-resolved experiments confirmed the validity of the reaction model and provided information on the dynamics of the triplet sensitization process. With a series of concentration dependent stationary illumination experiments, we determined the quantum efficiency for CPD formation from the triplet state of the thymine dinucleotide TpT to be 4 ± 0.2%.

The Protection Effect of Topical Cocoa Extract on Cyclobutane Pyrimidine Dimer (CPD) Formation and Tissue Neutrophil in Mice Induced by Ultraviolet B

Ultraviolet (UV) radiation has dangerous effects on the skin that may cause multiple clinical and cellular effects, which will produce DNA damage characterized by formation of photoproducts such as cyclobutane pyrimidine dimer (CPD), and acute inflammation characterized by neutrophil infiltration in the skin tissues. Cocoa flavanols are natural anti inflammatory molecules that has a role in preventing cutaneous UV damage. The aims of this study are to assess protection effects of topical cocoa extract against formation of CPD and tissue neutrophils caused by ultraviolet B radiation. This study is a true experimental study using mice as animal model, conducted at animal laboratory of Hasanuddin University and Sentra Diagnostic Patologia Laboratory from April to Mei 2016. Total of samples were 30. Protection effects of CPD formation occur with application of 200 ppm cocoa extract, with frequency of negative and weak CPD expression were 60% and 40%, respectively. There was an increased in protection with application of 400 ppm cocoa extract, with frequency of negative and weak CPD expression were 80% and 20%, respectively, and there was decrease of protection with application of 800 ppm cocoa extract, with frequency of negative and weak CPD expression were 40% and 60% respectively. The group which only exposed with UVB has higher mean of tissue neutrophils (mean=10,4), and groups with application of 400 ppm and 800 ppm cocoa extract have lowest mean of tissue neutrophils (mean=1,4). There was no significant differences between groups with application of 400 ppm and 800 ppm cocoa extract. The best protection effects on CPD formation and tissue neutrophil achieved with application 400 ppm of cocoa extract.

Blocking cyclobutane pyrimidine dimer formation by steric hindrance.

The efficiency of thymine (Thy) and uracil (Ura) to form cyclobutane pyrimidine dimers (CPDs) in solution, upon UV irradiation differs by one order of magnitude. This could to be partially related to the steric hindrance induced by the methyl at C5 in thymine. The aim of the present work is to establish the influence of a bulky moiety at this position on the photoreactivity of pyrimidines. With this purpose, photosensitization with benzophenone and acetone of a 5-tert-butyl uracil derivative () and the equivalent Thy () has been compared. Introduction of the tert-butyl group completely blocks CPD formation. Moreover, the mechanistic insight obtained by laser flash photolysis is in accordance with the observed photoreactivity.