Diethylhexyl syringylidene malonate – a new cosmetic ingredient for product protection

Light is a form of energy. Therefore, molecules either able to absorb UV and visible light, such as sunscreens and organic colorants, or which are sensitive to oxidation by light induced reactive oxygen species, such as antioxidants, fragrances, and flavor ingredients, can be elevated to a higher energy level (excited state) or undergo a radical transformation. In these states, molecules are more reactive than in the ground state. They may react with other molecules or break down into lower energy degradation products resulting in the loss of product integrity, color loss, malodor, and viscosity changes etc. The probability of reaction or decomposition is directly related to the length of time molecules remain in the excited state.

Key words

UV filter, photostability, photostabilizer, singlet oxygen, color

Among potentially photosensitive materials are UV filters. For instance studies showed that avobenzone (INCI: butyl methoxydibenzoylmethane, BMDBM), globally the most important UV-A filter, requires photo-stabilization [1-5]. Further examples of photosensitive cosmetic compounds are antioxidants (eg, natural tocopherols, carotenoids), flavors (eg vanillin), fragrances (eg, vanillin), colored organic compounds (eg, guaiazulene, water-soluble dyes) etc. Currently available photostabilizers satisfy the need to a certain extent but are unable to quench free-radicals generated by photo-fragmentation under UV-exposure. Patent and regulatory considerations also prevent formulators from using some of the currently available photostabilizers.

The present study deals with a new and patent protected cosmetic ingredient, diethylhexyl syringylidene malonate (DESM) [6-8]. Specific examples of the compound’s photo-protecting properties are in sun care, day care, bath and shower, personal care, and household (including air fresheners) markets. In order to substantiate the benefits of DESM in these markets, we (i) demonstrate the effectiveness of DESM in stabilizing avobenzone; (ii) show the applicability of DESM in stabilizing photo-unstable dyes, and (iii) prove the protection of photosensitive compounds, like natural tocopherols and vanillin. We have recently submitted a paper [9, 10] describing in detail the photostabilization of avobenzone in broad-spectrum sunscreens and the mechanisms involved in photostabilization.

Experimental

Photostability tests

Avobenzone – The photo-induced degradation of avobenzone (Eusolex® 9020, Merck KGaA, Darmstadt, Germany) was tested using a method devised by Merck KGaA, Germany.

2.00 mg/cm2 ± 2% of the test formulations were transferred within 30 s to the frosted side of glass micro-slides (76 x 26 mm; Chance Propper Ltd. Spon Lane, England). Then emulsions were spread by naked finger (saturated with product) over the whole substrate. A full spectrum (290-800 nm) Solar Sun Simulator was used as the irradiation source (Suntest CPS, Atlas, Hanau, Germany), equipped with a cooling tray and optical filters (Atlas UV filter ‘Spezialglas’). The UV-radiation intensity supplied by the source at each stage is checked annually by an independent expert (Dr. Kockott UV-Technik, Hanau, Germany).

In order to administer radiation doses close to the DIN 67501 standard sun, irradiation of samples was carried out at 51.78 Wm2. The UV-A (320-400 nm) to UV-B (290-320 nm) ratio of the Suntest device at this dose was found to be 19.3. To reach an erythemal dose of 5MED of a standard sun an equivalent absolute UV irradiation dose of 250kJ/m2 was applied (irradiation time: 80.5 min). Sample plates were eluted with 2-propanol; all analyses were performed using HPLC.

Vanillin and tocopherols – The photo-induced degradation of vanillin and tocopherols was investigated photometrically in ethanolic solution. 0.1% tocopherols + 0.2% DESM in ethanol were measured at lmax 298 nm and 0.1% vanillin + 0.5% DESM in ethanol were measured at lmax 281 nm.

Photostabilization of water-soluble dyes – Test formulations (see Table 2) in 30 ml transparent borax glass bottles were placed either in the dark (dark control) or in a Suntest CPS (Atlas Sun Simulator, Hanau, Germany) equipped with a Solar Standard Filter that emitted 550 W/m2 between 300-800 nm at its maximum. The UV intensity (290-400 nm) was measured by an independent expert (Dr. Kockott UV-Technik, Hanau, Germany) to be 83 W/m2. A 1 h irradiation time corresponded to approximately 300 kJ/m2 (UV-Dose 300-400nm). Samples were irradiated for 2, 4, 6, 8, 10, 14, 16, 20, 24, 28, 30, 32, and 36 h. The temperature of the sample chamber was 35-40°C. Samples were evaluated visually and documented by photographs. DESM was used as a colour stabilizer at a 5-10% pre-mixture in 90-95% of PEG-40 hydrogenated castor oil or PEG-20 glyceryl laurate. The ingredients (see Table 2) were stirred slowly until a homogeneous clear solution was obtained. The pH was adjusted to 5.5 with citric acid solution. Tests were performed for each stabilizer with each one of the photo-unstable water-soluble colorants FD&C Blue No.1 (INCI: CI 42090); FD&C Yellow No. 5 (CI 19140); FD&C Red No. 40 (CI 16035); D&C Red No. 33 (CI 17200); D&C Orange No. 4 (CI 15510); FD&C Red No. 4 (CI 14700).

Results and discussion

DESM was designed as a photostabilizer by careful consideration of the principle of atom economy [9,10]. This means that the rôle of each and every atom in this target molecule is critical in achieving the objective. DESM has a hindered phenolic functionality to give highly potent antioxidant activity (proven in 1O2-Quenching-assay, TEAC-assay, lipid-assay; data not shown here), and a benzylidene malonate functionality required for the photostability of the molecule itself. Both functions support the stabilization of photo-unstable molecules as shown in this section.

Photostability of avobenzone in formulations

Avobenzone photostability studies were performed using DESM in formulated products (Table 1). Excellent stabilization effects were achieved using 2% avobenzone and 1.6% DESM, or DESM further combined with 1.6% homosalate. A 7-fold increase in the photostability (at 5 MED exposure) of avobenzone was observed over the control (base with 2% BMDBM without stabilizer) when DESM was used in 2:1.6 (w/w) ratios with avobenzone (Figure 1). This increased to an 8-fold stabilizing effect with the addition of 1.6% HMS.

The stabilizing efficiency of DESM offers new opportunities to formulators for the development of photostable formulations containing BMDBM. DESM alone was found to be photo-stable at 5 MED (data not shown).

The underlying mechanism of the stabilizing effect is probably a combination of DESM’s ability to quench singlet oxygen and to act as triplet energy quencher. Both effects play an important role in BMDBM photostability [9, 10, 11]. Triplet energy state calculations showed that the triplet energies of the counterparts avobenzone and DESM differ only slightly (few Kcal/mol less for DESM, data not shown) which is essential to enable energy transfer between both molecules [9, 11]. Therefore, the triplet energy quenching effect of DESM also contributes to the stabilization of avobenzone.

Stabilization of water-soluble dyes

It is striking to see that DESM has provided 4 to 10-fold improvement in reducing photo-fading of water-soluble dyes over many leading stabilizers. Figure 2 clearly demonstrates a significant improvement in the photostability of all types of dyes. A test formulation with 0.001% of each of the dyes FD&C Blue No.1, D&C Orange No. 4, D&C Red No. 33 showed a complete color-fading of the irradiated control (without stabilizer) compared to the un-irradiated dark control. This fading can be completely inhibited by adding either 0.05 or 0.1% of DESM while standard stabilizers such as benzophenone-3 or benzophenone-4 failed.

The time dependency of fading of different dyes is indicated in Figure 3. Benzophenone-4 (BP-4) failed to prevent fading of FD&C Blue No.1 and D&C Red No.33 dyes when exposed to irradiation for only four hours. Prevention of fading of D&C Orange No.4 dye using BP-4 was found to be almost ineffective. DESM is superior (4-10 fold) at both active use level concentrations (0.05% and 0.1%). Only for D&C Red No.33 we could observe some fading starting after 34 hours with 0.05% of DESM while we could not detect any fading at the 0.1% level even at 36 hours.

Further studies have been performed with the FD&C Yellow No. 5, FD&C Red No. 40, and FD&C Red No. 4 dyes. Data were very similar to the data shown in Figure 2 and Figure 3 but the details are not included here.

Broad applicability of diethylhexyl

syringylidene malonate

Remarkably, further protecting effects of DESM can be demonstrated by carrying out photostability studies using natural tocopherols, b-carotenes, guaiazulene, flavor and fragrances, e.g. vanillin and methyl anthranilate. Data clearly demonstrate a significant improvement in the photostability of all types of compounds. Here we selected two of them, vanillin and natural tocopherols, as shown in Figures 4 and 5. The stabilization of fragrance ingredients may offer opportunities to improve the odor of Eau de Toilettes by using DESM.

Conclusion and outlook

Photostability data in sunscreen formulations showed that DESM is photostable and improves the photostability of avobenzone significantly when compared to a control without photostabilizer. With appropriate selection of UV-B sunscreens such as homosalate and ethylhexyl salicylate or other combinations, photostable broad-spectrum sunscreen formulations with high SPF can easily be achieved using DESM [9, 10]. It has been demonstrated that, (a) triplet state energy transfer from avobenzone to DESM, and (b) scavenging of reactive species are responsible for the observed stabilization of avobenzone [9, 10]. Excellent singlet oxygen quenching ability as well as photostabilization effect of DESM can contribute to reduced risk of UV-induced free-radical damage to human skin [5, 9, 10, 12, 13].

DESM has wide applicability in improving stabilization of photosensitive compounds, namely natural tocopherols, b-carotene, vanillin, guaiazulene, and a range of water-soluble FD&C and D&C dyes. In short, the multiplicity of effects and formulation benefits seen with DESM makes it an ideal choice as a unique antioxidant-photostabilizer for cosmetic products targeting young and matured skin alike. DESM is applicable in many applications areas such as sun care, day care, bath & Shower, Personal Care, and Household including air fresheners.

At room temperature DESM is a viscous yellowish liquid or low-melting solid and has a very good miscibility with various cosmetic emollients and UV filters. DESM will be available as a blend of about 90% DESM and about 10% Caprylic/Capric Triglycerides, which will be trade named as Oxynex® ST Liquid.