Andre Deflandre

Photostability assessment of sunscreens. Benzylidene camphor and dibenzoylmethane derivatives

A rapid method has been developed in order to compare the photostability of several sunscreen agents incorporated in the same type of emulsion. Thin films of the different preparations were spread on quartz plates and irradiated with a solar simulator. Differences in energy distribution according to wavelengths observed with solar simulator and sun radiation were taken into account. A kinetic study of absorption properties was carried out under solar simulator irradiation. A simple calculation gave the results which would be obtained under sunlight.

Proposed protocol for determination of photostability Part I: cosmetic UV filters

The protocol described here has been developed to measure the stability of UV‐B filters; a modified version is recommended for UV‐A filters. It should be considered as a tool to predict the effectiveness remaining after exposure to UV‐A and UV‐B light. It is a simple and reliable in vitro model simulating conditions of actual use. The results show that each filter requires an appropriate choice and fine tuning of reproducible analytical conditions. While absolute values are directly influenced by uncertainties in irradiance (dosimetry), comparative measurements with respect to a known standard are very reliable.

A 13C solid-state NMR study of the structure and auto-oxidation process of natural and synthetic melanins.

This paper presents a 13C CP/MAS NMR study of the melanin pigments obtained through natural and synthetic origins: sepia-melanin from squid ink and three synthetic 5,6-dihydroxyindole-melanins prepared using different non-enzymatic oxidation pathways. The synthetic pigments can be distinguished from natural melanin by the absence of aliphatic carbons, thereby confirming the unreacted 3,4-dihydroxyphenylalanine and the proteinaceous origins of the aliphatic resonances in natural eumelanin. The spectra of selected non-protonated carbon resonances and those with only protonated carbon signals led to a quantitative analysis. An auto-oxidative experiment using a synthetic melanin, over a period of 130 h, has shown an unusually slow disappearance of hydrogen peroxide formed in situ. The 13C-NMR spectrum of the insoluble oxidized synthetic melanin compared to that before auto-oxidation clearly demonstrates that the oxidation process is associated with chemical changes within the pigment; i.e., carbonyl functional group formation and an increase of the non-protonated carbons fraction.

Femtosecond laser studies of excited state intramolecular proton transfer in an ultraviolet-filter molecule

Abstract Following excitation by a femtosecond laser pulse at 346 nm, phenol,2-(2H-benzotriazol-2-yl)-4-methyl-6-[2-methyl-3-[1,3,3,3-tetramethyl-1-[(trimethylsilyl)oxy]disiloxanyl]propyl] in n-heptane relaxes towards its ground state in a four-step mechanism. By excited state intramolecular proton transfer (50 fs) the enol S1 state produces the keto S1′ state responsible for a small gain spectrum above 600 nm. The latter relaxes to its keto ground state S0′ (130 fs) and the keto→enol back proton transfer (500 fs) yields the vibrationally hot enol ground state, the cooling of which is a relatively slow process (≫2 ps). We present kinetics and detailed transient absorption and gain spectra between 380 and 720 nm.

Study of the photochemical behaviour of sunscreens – benzylidene camphor and derivatives

Benzylidene camphor derivatives have been currently used as ultraviolet filters in sunscreen compositions. When solutions of these compounds are exposed to the light from a solar simulator, a photoinduced cis‐trans isomerization is observed. The initial quantum yields of this photochemical isomerization of studied molecules have been determined in several solvents. Their photochemical behaviour depends neither on the concentration nor on the presence or absence of oxygen or other inhibitors such as isopropanol or biacetyl. It means that the lifetime of the precursor states of isomerization is about 10−12s, that is to say, too short for them to react with neighbouring molecules.

ESR study of hair and melanin–keratin mixtures – the effects of temperature and light

The ESR spectrum of hair presents a symmetrical absorption band with the same characteristics as pure melanin (g= 2.0030; δH= 5 G). Between 250°K and 490°K, the intensity of the signal increases as for pure melanin. This variation may be explained by a change in the moisture content of hair as function of the temperature. Below 250° K, in a frozen medium, melanin‐water interactions remain constant and the number of radicals is no longer a function of temperature.

ELECTRON SPIN RESONANCE OF MELANIN FROM HAIR. EFFECTS OF TEMPERATURE, pH AND LIGHT IRRADIATION

Abstract— The variation with temperature, pH and light of the ESR signal of hydrated melanin powders from Japanese black hair has been studied. An explanation of the results is proposed on the basis of quinhydrone type complexes and of acid‐base equilibria of melanin and its semiquinone radicals. During exposure to light of wavelengths 254–600 nm, both stable and unstable radicals have been observed. The action spectrum for the formation of stable melanin radicals has been determined.

Study of the photochemical behaviour of sunscreens — benzylidene camphor and derivatives II: Photosensitized isomerization by aromatic ketones and deactivation of the 8-methoxypsoralen triplet state

Abstract The benzophenone- and acetophenone-photosensitized E—Z isomerization of benzylidene camphor (BC) and p-trimethylammoniobenzylidene camphor methylsulphate was studied quantitatively. Initial sensitized quantum yields and concentrations of both isomers in the photostationary state were measured in CCl4 for the systems (BC + benzophenone) and (BC + acetophenone). Simultaneously, the rate constants of the quenching process of the phosphorescent triplet state of the two sensitizers were determined in CCl4. A diffusion-controlled energy transfer between acetophenone and BC accounts for the results, the energy transfer between benzophenone and BC being somewhat slower. In the sensitized isomerization of the trimethyl-ammonio derivative in CHCl3, phosphorescence quenching experiments and the determination of the photostationary states show that the quaternized derivative is as efficient as BC in quenching the triplet state of ketones. A well-known skin photosensitizer 8-methoxypsoralen (8-MOP) is observed to be an efficient sensitizer for the isomerization of the quaternized derivative of BC. The latter compound quenches the photocycloaddition of 8-MOP on N-methylmaleimide. It is concluded that BC and the trimethylammonio derivative show deactivating properties in addition to the usual filter effect.

Photodecarboxylation of 2-(2′-carboxymethoxy-5′-methylphenyl)-benzotriazole

Abstract The photodecarboxylation of 2-(2′-carboxymethoxy-5′-methylphenyl)-benzotriazole was investigated under direct excitation in various solvents. This reaction leads cleanly to 2-(2′-methoxy-5′-methylphenyl)-benzotriazole which is phototransformed afterwards. The quantum yields of phototransformation were measured. The primary photoproduct was isolated, and its structure determined and compared with an authentic sample.

Photochemical study of substituted s-triazines

Abstract A study of the photochemical behaviour of three substituted s-triazines with low photoreactivity has been carried out. The photoproducts formed via irradiation at λ > 300 nm of solutions of s-triazines substituted by 4-amino benzylidene camphor (TABC), 4-amino diisobutyl benzal malonate (TBMA) or 4-amino-2-ethyl hexyl benzoate (TPAB) in methanol under aerated conditions have been identified. In methanol the direct irreversible phototransformation quantum yields are very low (10−4–10−6). The irradiation of the compound TABC leads to an aldehyde function attached to a position para to the aromatic ring by oxidative scission of the conjugated ethylenic bond. In the case of the compound TPAB the scission of an NH-Φ bond gives a photoproduct with a primary amino group attached to the s-triazine. The irradiation of the compound TBMA provokes the scission of the conjugated ethylenic bond, leading to an aldehyde function. In this case a tautomeric form of the initial molecule has also been proposed.