Degradation of triclosan in the presence of p-aminobenzoic acid under simulated sunlight irradiation.

This study aimed to investigate the degradation of triclosan (TCS) in the presence of p-aminobenzoic acid (PABA) under simulated sunlight irradiation (λ ≥ 290 nm). The effect of PABA concentration, pH, dissolved organic matter (DOM), and DOM-hydrolytic Fe(III) species complexes on the photodegradation of TCS in the presence of PABA (TCS-PABA) was also studied. The photolysis of TCS-PABA obeyed pseudo-first-order kinetics well, and the degradation of TCS-PABA enhanced with increasing solution pH (from 3.0 to 11.0). The presence of PABA inhibited the degradation of TCS-PABA, and the weakest inhibitory effect was observed while the concentration of PABA was 5 mg L-1. The addition of DOM (Suwannee River fulvic acid standard I [SRFA], Suwannee River HA standard II [SRHA], and Suwannee River natural organic matter [SRNOM]) showed different inhibition effects on TCS-PABA degradation. However, higher Fe(III) concentration at the DOM concentration of 5 mg L-1 could favor the formation of DOM-hydrolytic Fe(III) species complexes, further accelerating the degradation of TCS-PABA. In comparison with deionized water (DI water), TCS-PABA could be better photodegraded in river water nearby the effluent of a wastewater treatment plant. This study provides useful information for understanding the natural behavior of TCS in the presence of other organic contaminants.

Degradation of sunscreen agent p-aminobenzoic acid using a combination system of UV irradiation, persulphate and iron(II).

Increased usage and discharge of sunscreens have led to ecological safety crisis, and people are developing the advanced oxidation processes (AOPs) to treat them. The present study aimed to determine the degradation efficiency and mechanism of the sunscreen agent p-aminobenzoic acid (PABA) using the UV/Fe(2+)/persulphate (PS) method. A series of irradiation experiments were conducted to optimise the system conditions and to study the impacts of the natural anion. Free radicals and degradation products were identified in order to clarify the degradation mechanism. Initial PS and Fe(2+) concentrations showed significant impacts on PABA degradation. Natural anions, such as Cl(-), NO3 (-), H2PO4 (-) and HCO3 (-), impeded PABA degradation because of ion (Fe(2+)) capture, radical scavenging or pH effects. Hydroxyl (HO·) and sulphate (SO4 (·-)) radicals were two main radicals observed in the UV/Fe(2+)/PS system; of these, SO4 (·-) showed greater effects on PABA degradation. Over 99 % of the available PABA was completely degraded into carbon dioxide (CO2) and water (H2O) by the UV/Fe(2+)/PS system, and the remaining PABA participated in complex radical reactions. By-products were identified by total ion chromatography and mass spectrometry. Our research provides a treatment process for PABA with high degradation efficiency and environmental safety and introduces a new strategy for sunscreen degradation.

The effect of nitrate, bicarbonate and natural organic matter on the degradation of sunscreen agent p-aminobenzoic acid by simulated solar irradiation.

Our experiments revealed that a model sunscreen agent, p-aminobenzoic acid (PABA), can be effectively transformed through reactions that are mediated by simulated solar irradiation. We systematically explored the effects of nitrate ions, bicarbonate and different types of natural organic matter (NOM) on the degradation of PABA by simulated solar irradiation. Experimental data suggest that these components ubiquitous in nature water have different influence on the rates of the photoinduced removal of PABA. Products were extracted and analyzed using LC/MS and a total of four products probably resulting from OH and NO2 radicals attack were identified and the possible reaction pathways were proposed. The findings in this study provide useful information for understanding the environmental transformation of sunscreen agent in aquatic system.

Photobleaching of camphorquinone during polymerization of dimethacrylate-based resins.

OBJECTIVE: The aim of this study was to compare the photobleaching rate of CQ in different dental resins. METHODS: The photodecomposition rate of CQ/amine system in bis-GMA/TEGDMA, bis-EMA and UDMA polymerizing monomers was evaluated at different light intensities. The photobleaching of the CQ was studied by monitoring the decrease in light absorption as a function of continuous irradiation time. The absorption changes were assessed by recording the transmitted light that passed through samples of monomers containing CQ/amine. RESULTS: Complete photobleaching of CQ was observed in all the monomer tested and the rate constant for the photobleaching was proportional to the radiation intensity. Hydrogen abstraction from amines by the excited CQ state via electron transfer and direct hydrogen abstraction from monomer structures were involved in the CQ photoreduction. CQ was photobleached in the absence of coinitiator in a dimethacrylate monomer containing a carbamate functional group (UDMA). This behavior was attributed to the presence of labile hydrogen atoms in the UDMA monomer. The CQ photobleaching rate constant in UDMA containing CQ/amine was similar to that in UDMA in the absence of amine. Moreover, the efficiency of CQ to photoinitiate the polymerization of UDMA in the absence of amine demonstrated that the radicals derived from the UDMA monomer via hydrogen abstraction are highly reactive toward double bonds. SIGNIFICANCE: CQ photoinitiates the polymerization of the UDMA monomer in the absence of amine and the efficiency of this process is comparable to that of traditional bis-GMA and bis-EMA monomers activated with CQ/amine.

Influence of curing protocol on selected properties of light-curing polymers: degree of conversion, volume contraction, elastic modulus, and glass transition temperature.

OBJECTIVES: The purpose of this study was to investigate the effect of light-curing protocol on degree of conversion (DC), volume contraction (C), elastic modulus (E), and glass transition temperature (T(g)) as measured on a model polymer. It was a further aim to correlate the measured values with each other. METHODS: Different light-curing protocols were used in order to investigate the influence of energy density (ED), power density (PD), and mode of cure on the properties. The modes of cure were continuous, pulse-delay, and stepped irradiation. DC was measured by Raman micro-spectroscopy. C was determined by pycnometry and a density column. E was measured by a dynamic mechanical analyzer (DMA), and T(g) was measured by differential scanning calorimetry (DSC). Data were submitted to two- and three-way ANOVA, and linear regression analyses. RESULTS: ED, PD, and mode of cure influenced DC, C, E, and T(g) of the polymer. A significant positive correlation was found between ED and DC (r=0.58), ED and E (r=0.51), and ED and T(g) (r=0.44). Taken together, ED and PD were significantly related to DC and E. The regression coefficient was positive for ED and negative for PD. Significant positive correlations were detected between DC and C (r=0.54), DC and E (r=0.61), and DC and T(g) (r=0.53). Comparisons between continuous and pulse-delay modes of cure showed significant influence of mode of cure: pulse-delay curing resulted in decreased DC, decreased C, and decreased T(g). Influence of mode of cure, when comparing continuous and step modes of cure, was more ambiguous. SIGNIFICANCE: A complex relationship exists between curing protocol, microstructure of the resin and the investigated properties. The overall performance of a composite is thus indirectly affected by the curing protocol adopted, and the desired reduction of C may be in fact a consequence of the decrease in DC.

Influence of photoinitiator type on the rate of polymerization, degree of conversion, hardness and yellowing of dental resin composites.

OBJECTIVES: To evaluate the degree of conversion (DC), maximum rate of polymerization (Rpmax), Knoop hardness (KHN) and yellowing (b-value) of resin composites formulated with phenylpropanedione (PPD), camphorquinone (CQ), or CQ/PPD at different concentrations. The hypotheses tested were (i) PPD or CQ/PPD would produce less Rpmax and yellowing than CQ alone without affecting DC and KHN, and (ii) Rpmax, DC, and KHN would be directly related to the absorbed power density (PDabs). METHODS: CQ/amine, PPD/amine and CQ/PPD/amine were used at low, intermediate and high concentrations in experimental composites. Photoinitiator absorption and halogen-light emission were measured using a spectrophotometer, Rp with differential scanning calorimetry (DSC), DC with DSC and FTIR, KHN with Knoop indentation; and color with a chromameter. The results were analyzed with two-way analysis of variance (ANOVA)/Student-Newman-Keul's test (p<0.05). Correlation tests were carried out between PDabs and each of DC, Rpmax and KHN. RESULTS: The PDabs increased with photoinitiator concentration and PPD samples had the lowest values. In general, maximum DC was comparable at intermediate concentration, while Rpmax and KHN required higher concentrations. DC was similar for all photoinitiators, but Rpmax was lower with PPD and CQ/PPD. PPD produced the lowest KHN. Yellowing increased with photoinitiator concentration. PPD did not reduce yellowing at intermediate and/or high concentrations, compared to CQ-formulations. PDabs showed significant correlations with DC, Rpmax and KHN. CONCLUSION: PPD or CQ/PPD reduced Rpmax in experimental composites without affecting the DC. The use of PPD did not reduce yellowing, but reduced KHN. DC, Rpmax and KHN were dependent on PDabs.

A natural component as coinitiator for unfilled dental resin composites.

A natural component, 1,3-benzodioxole (BDO), was used for the purpose of replacing the conventional amine for dental composite. Camphorquinone (CQ)/BDO was used to initiate the photopolymerization of urethane dimethacrylate (UDMA)/triethylene glycol dimethacrylate (TEGDMA) (70/30 wt %). The kinetics was recorded by real-time Fourier transformation infrared spectroscopy (FTIR). The mechanical properties were measured by dynamic mechanical analyzer (DMA), and CQ/ethyl 4-N,N-dimethylaminobenzoate (EDMAB) mixture was used as control in the same photocuring condition. The results indicated that, the addition of BDO as coinitiator greatly improved the rate of polymerization and final double bond conversion (DC), when compared with the system initiated by CQ alone. BDO and EDMAB were found to reach almost the same final DC (75%), though the kinetics of two systems was different. Comparing with EDMAB, BDO brought approximately the same glass transition temperature (Tg), but slightly higher storage modulus around 37 degrees C. The water sorption and solubility for two mixtures were almost the same and within the range of the ISO 4049's standards. These results suggested that BDO was an effective alternative to conventional amine for coinitiator. And the human diet characteristics of BDO made it more promising than amine in the dental resin formulations.

Anionic clays for sunscreen agent safe use: photoprotection, photostability and prevention of their skin penetration.

The use of sunscreen preparations is recently growing and their efficacy and safety must be taken into account since they are applied on the skin frequently and for many hours. Exposition to sunlight, in fact, can cause sunscreen photodegradation and determine their decrease in UV protection often with the occurrence of allergic and/or toxic degradation products. A high photostability is hence very important for their effectiveness and safety. The aim of this work is to obtain new sunscreen formulations stabilized by intercalating PABA, within the lamellar structures of two kinds of hydrotalcite. PABA was chosen as model sunscreen because of its high photoinstability and photosensitizing properties that nowadays bar its utilization. Both intercalated products showed an increased protection range and, in one case, an improved sunscreen photostability. Sunscreen release from creams containing intercalated or free PABA was evaluated as well. The very low or negligible sunscreen release, obtained from the intercalated product loaded formulations, resulted in a lack of a close contact between skin and filter with the consequence that cutaneous reactions and allergy problems are eliminated. The use of these materials resulted in a good strategic technological approach in order to increase efficacy and safety of solar products.

Photosensitization of the sunscreen octyl p-dimethylaminobenzoate by UVA in human melanocytes but not in keratinocytes.

Sunscreens penetrate human epidermis and modify the biology of proliferating cells. This study addressed the question whether the UV response of cultured human cells is affected by direct treatment with nontoxic levels of sunscreens. Cell survival following exposure to UVC or unfiltered UBV was not altered by preincubation with 25 micrograms/mL of octyl p-dimethylaminobenzoate (o-PABA), 2-ethylhexyl p-methoxycinnamate (EHMC) or oxybenzone. However, UVA or UVB filtered to reproduce the solar UV spectrum penetrating to the basal layer of the epidermis, highly sensitized cells to killing by o-PABA but not by its hydrolysis product, 4-dimethylaminobenzoic acid. Sensitization was found in all cell types tested, except normal keratinocytes, and could be prevented by certain antioxidants particularly pyruvate and the hydroxyl radical scavenger mannitol. o-PABA and EHMC applied without UV reduced the adherence of cells. The results indicate that sunscreens may increase cell mobility and the combination of o-PABA with solar UV may selectively damage melanocytes in the skin.

High-performance liquid chromatographic method for the comparison of the photostability of five sunscreen agents.

Sunscreen agents are commonly used in cosmetic products to filter out noxious radiation in sunlight. A convenient high-performance liquid chromatographic (HPLC) method for the quantification of five sunscreens after irradiation has been selected. We used this analytical method to compare the photostability of benzophenone-3, PEG-25 PABA, octyl dimethyl PABA, 4-methylbenzylidene camphor and butyl methoxydibenzoylmethane, at levels in the range of 25-60 microM. The assay was carried out, using a C8 column with a methanol--water mobile phase. The detector was set at a wavelength of 300 nm. The assay was linear with the following limits: 0.2 microgram ml-1 for benzophenone-3, 1 microgram ml-1 for PEG-25 PABA, 0.15 microgram ml-1 for octyl dimethyl PABA, 0.1 microgram ml-1 for methylbenzylidene camphor and 0.05 microgram ml-1 for butyl methoxydibenzoylmethane. The half-lives calculated indicate a very good photostability of the sunscreens studied and permit to classify amongst themselves.

Inclusion complexation of the sunscreen agent 2-ethylhexyl-p-dimethylaminobenzoate with hydroxypropyl-beta-cyclodextrin: effect on photostability.

The interaction between the UV filter, 2-ethylhexyl-p-dimethylaminobenzoate, and unmodified and modified alpha-, beta- or gamma-cyclodextrins was studied in water by phase-solubility analysis. Of the cyclodextrins available, only hydroxypropyl-beta-cyclodextrin caused a marked increase in the aqueous solubility of 2-ethylhexyl-p-dimethylaminobenzoate. The data from the solubility study indicated the formation of a 1:1 (sunscreen-cyclodextrin) complex. The inclusion of the sunscreen agent into the hydroxypropyl-beta-cyclodextrin cavity was confirmed by thermal analysis and by nuclear magnetic resonance spectroscopy. Irradiation-induced degradation of 2-ethylhexyl-p-dimethylaminobenzoate was reduced by complexation with hydroxypropyl-beta-cyclodextrin, this effect being more pronounced in solution (the extent of degradation was 25.5% for the complex compared with 54.6% for free 2-ethylhexyl-p-dimethylaminobenzoate) than in the emulsion vehicle (the extent of degradation was 25.1% for the complex compared with 33.4% for free 2-ethylhexyl-p-dimethylaminobenzoate). Although photodegradation of the sunscreen agent is significantly reduced by formation of the inclusion complex it is important to design a suitable vehicle. Inclusion of 2-ethylhexyl-p-dimethylaminobenzoate-DMAB into the hydroxypropyl-beta-cyclodextrin cavity limits interaction of the UV filter with the skin reducing the side-effects of the formulation.

Photochemical formation of singlet molecular oxygen (1O2) in illuminated aqueous solutions of p-aminobenzoic acid (PABA).

Evidence is presented for the photochemical formation of singlet molecular oxygen (1O2) in air-saturated buffered aqueous solutions of p-aminobenzoic acid (PABA) using sunlight-range illumination. This is significant because PABA is widely used as an active ingredient in sunscreen preparations that are applied to the surface of the skin and 1O2 is known to cause oxidative damage to cells via the formation and subsequent reactions of lipid peroxides. Furfuryl alcohol (FFA), a well known chemical trap for 1O2, was added to aqueous PABA solutions prior to illumination. The FFA was consumed when the solution was illuminated, but no loss of FFA occurred in the dark and loss by direct photolysis was negligibly slow. Further evidence for the formation of 1O2 in illuminated aqueous PABA solutions is provided by the results of experiments in which individual solutions containing PABA and FFA that were diluted with D2O exhibited an increased rate of FFA consumption due to the increased lifetime and concentration of 1O2 in this solvent.

Rapid reaction of singlet molecular oxygen (1O2) with p-aminobenzoic acid (PABA) in aqueous solution.

The reaction of singlet molecular oxygen (1O2) with p-aminobenzoic acid (PABA) is quite rapid. The rate constant for reaction of 1O2 with PABA was measured in buffered aqueous solution by the method of competition kinetics and found to be 8.9 x 10(8) M-1 sec-1. The results of a Stern-Volmer analysis indicate that additional loss of 1O2 does not occur through physical quenching by PABA. Although illuminated PABA solutions are known to form 1O2, it appears that PABA is also a very efficient sink for 1O2.

Sunlight-induced mutagenicity of a common sunscreen ingredient.

We have tested the mutagenicity of a UV-B sunscreen ingredient called Padimate-O or octyl dimethyl PABA, which, chemically speaking, is identical to an industrial chemical that generates free radicals when illuminated. It is harmless in the dark but mutagenic in sunlight, attacking DNA directly. A commercial sunscreen containing Padimate-O behaves in the same way. UV-A in sunlight also excites Padimate-O, although less than UV-B. Some related compounds, including a known carcinogen, behave similarly. As mutagens may be carcinogenic, our results suggest that some sunscreens could, while preventing sunburn, contribute to sunlight-related cancers.

The photochemistry of p-aminobenzoic acid.

We have studied the photoreactions occurring when p-aminobenzoic acid (PABA), a component of some sunscreens, is irradiated in aqueous solution. These studies were carried out in the presence and absence of oxygen, using light of lambda = 254 nm as well as light of wavelengths greater than 290 nm. In deoxygenated solution between pH 7.5 and 11.0, we found two photoproducts that were identified as 4-(4'-aminophenyl)aminobenzoic acid (I) and 4-(2'-amino-5'-carboxyphenyl)aminobenzoic acid (V); we used 1H and 13C NMR, electron impact mass spectrometry and synthesis by an independent route to identify each of these compounds. Rapid discoloration of the photolyzed sample was observed when PABA was irradiated in aerated solution. Although a number of products were detected under these conditions, the three most abundant stable compounds have been isolated and identified as 4-amino-3-hydroxybenzoic acid, 4-aminophenol and 4-(4'-hydroxyphenyl)aminobenzoic acid (IV). The latter compound was shown to result from rapid photo-induced oxidation of I in the presence of oxygen. Even in the presence of trace amounts of oxygen, the yield of I was significantly reduced in favor of IV. Studies of the thermal oxidation of I, coupled with evidence gathered from studies of the photochemistry of incompletely deoxygenated PABA solutions, indicate that 4-(2,5-cyclohexadien-4-one)iminobenzoic acid (III) is an intermediate on the pathway between I and IV. Qualitatively, we found that the photochemical reactions resulting from irradiation of PABA solutions with lambda = 254 nm light and light with lambda greater than 290 nm were the same. The quantum yields for formation of I and V are highly pH dependent, both being less than 10(-4) at pH 7 and rising steadily to values greater than 10(-3) at pH 11. The detailed pH dependence suggests that the deprotonated PABA radical cation may be an important intermediate entering into the reactions forming I and IV.

Photoaddition of p-aminobenzoic acid to thymine and thymidine.

Several studies in the literature have shown that DNA is damaged after UV irradiation in the presence of the sunscreen agent p-aminobenzoic acid (PABA), both in vivo and in vitro. One type of damage has been shown to be the result of increased yields of pyrimidine cyclobutane dimer formation. However, it has been suggested that other types of lesions are produced as well. We have studied the photochemistry of the thymine-PABA and thymidine-PABA systems and report here the isolation and characterization of thymine-PABA and thymidine-PABA photoadducts. These products have been identified, respectively, as 5-(2-amino-5-carboxyphenyl)-5,6-dihydrothymine and isomeric forms of 5-(2-amino-5-carboxyphenyl)-5,6-dihydrothymine. The quantum yields for the formation of these adducts in deaerated aqueous solutions at pH 7.0 have been determined to be 9.5 x 10(-4) and 4.3 x 10(-3) for the thymine and thymidine based adducts respectively. A pH profile for the thymine-PABA system indicated a maximum quantum yield for adduct formation at pH 6.5, although it could be detected over the whole pH range studied (pH 3.5-11.0).

Development of assays for the detection of photomutagenicity of chemicals during exposure to UV light--I. Assay development.

Two complementary assay systems have been adapted for the detection of compounds which may form mutagenic photoproducts during exposure to UV light from an Osram Ultra-Vitalux sunlamp as used in the evaluation of the effectiveness of sun filters. The effects of UVA and of UVB were evaluated. A bacterial plate test using Escherichia coli strain WP2 allowed the bacteria, co-plated with test chemical in soft agar, to be irradiated with various doses of UV light. Mutagenesis was assessed by scoring numbers of tryptophan-independent colonies. The chosen reference compound was 8-methoxypsoralen (8-MOP) which was non-mutagenic alone at the highest dose tested (1000 micrograms/plate). Following simultaneous exposure of bacteria to 8-MOP and doses of UV light which alone had little effect, large numbers of revertants were scored. Numbers of mutants were dependent upon the doses of both 8-MOP and of UV light. The second test system involved the exposure of Chinese hamster ovary cells to UV light in the presence of test chemical to determine the clastogenic effects of photoproducts. Treatment with 8-MOP alone up to 50 micrograms/ml was not clastogenic but concomitant exposure to non-damaging doses of UV light caused large increases in the incidence of chromosome aberrations of all types. Damage was again dependent on the doses of both components. Two additional photoactive compounds, para-aminobenzoic acid and chlorpromazine both show photoclastogenic but not photomutagenic properties. These two complementary assay systems take advantage of using no-effect levels of UV light as a baseline against which photomutagenicity readily can be compared.(ABSTRACT TRUNCATED AT 250 WORDS)

UV-irradiation potentiates the antimutagenicity of p-aminobenzoic and p-aminosalicylic acids in Salmonella typhimurium.

UV-irradiation (254 nm, 10 or 20 J/cm2) of p-aminobenzoic acid (PABA) and p-aminosalicylic acid (NaPAS) potentiated their antimutagenicity towards N-methyl-N'-nitro-N-nitrosoguanidine mutagenesis in Salmonella typhimurium. Their inhibitory action towards the formation of the mutagen N-methyl-N-nitrosourea from the nitrosation mixture of N-methylurea and nitrite was also increased by UV-irradiation. In contrast, UV-irradiated PABA exhibited no inhibitory effects towards the mutagenicity of sodium azide or 3-azidoglycerol. Neither PABA nor NaPAS nor their UV-irradiation products were themselves mutagenic in the Ames assay.

Photocarcinogenesis is retarded by a partly photodegraded solution of para-aminobenzoic acid.

A solution of para-aminobenzoic acid (PABA) was exposed to ultraviolet (UV) radiation emitted from a Philips TL 40 W/12 sunlamp and the degree of photodegradation following an exposure of 27 J/cm2 was estimated to be approximately 40%. The formation of the photoproducts was confirmed by mass spectroscopy and UV spectroscopy. The solution was painted on the backs of hairless light-pigmented mice prior to daily UV irradiation by the above sunlamp, and this procedure was continued for 30 weeks. The preirradiated solution of PABA significantly retarded the tumor induction time and reduced significantly the number of squamous cell carcinomas compared with nonprotected controls. This tumor-retarding ability did not differ significantly from the effect achieved when using nonirradiated PABA.

Stability of PABA after UV irradiation in vivo and in vitro.

Sunscreens are widely used for the prevention of acute and chronic sun damage. One of the most widely used sunscreens is para-aminobenzoic acid (PABA). It has been reported that PABA decomposes on exposure to air and light as well as exposure to high doses of UVB. In the present study we found that PABA was stable during long-term storage. PABA seems to be stable after irradiation of physiological doses of UVA and UVB in vitro. However, the in vivo studies demonstrate that significantly less PABA could be extracted from UVA-irradiated sites compared to controls.