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Boletín de la Sociedad Chilena de Química

versão impressa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.45 n.4 Concepción dez. 2000

http://dx.doi.org/10.4067/S0366-16442000000400017 

DIHYDROEUPARIN AS SUNSCREEN

Pedro A. Ortega*, Miguel E. Guzmán, Leonel R. Vera, Jorge A. Velázquez
Departamento de Química, U. Católica del Norte, Casilla 1280, Antofagasta-Chile.
Elsa B. Abuín
Departamento de Química, U. de Santiago de Chile, Santiago-Chile.
(Received: Yanuary 6, 2000 - Accepted: September 1, 2000)

ABSTRACT

Physicochemical parameters, e, l, solubility in water and Brij-35, pKa, Ka and photostability for dihydroeuparin, 5-acetyl-2, 3-dihydro-6-hydroxi-2-isopropenyl-benzo (b) furan (from Senecio Graveolens), were obtained and compared to those of o-hidroxyacetophenone. Its sunscreen capabilities, SPF and SSPF, were obtained for both compounds and compared to those of homomenthylsalicylate (homosalate). Results indicate that dihydroeuparin has sunscreen behavior at 270-340 nm zone, with high photostability, high solubility in miscellar system, apparently is a non-toxic substance and shows higher sunscreen capabilities than homosalate.

KEY WORDS: dihydroeuparin, miscellar system, solubility, photostability, SPF, SSPF.

RESUMEN

Parámetros fisicoquímicos, e, l, solubilidad en agua y Brij-35, pKa, Ka su fotoestabilidad para dihydroeuparin, 5-acetyl-2,3-dihydro-6-hydroxi-2-isopropenyl-benzo(b)furan ( de Senecio Graveolens), fueron obtenidos y comparados con los de o-hidroxyacetophenone. La capacidad como pantalla molecular, niveles de SPF y SSPF, fueron obtenidas para ambos compuestos y comparadas con aquellos de homomenthylsalicylate (homosalate). Los resultados indican que dihidroeuparin tiene un comportamiento como pantalla molecular en la zona de 270 - 340 nm, con alta fotoestabilidad, alta solubilidad en sistemas miscelares, aparentemente no es tóxico y muestra mejor comportamiento como pantalla molecular que el homosalato.

PALABRAS CLAVES: dihidroeuparin, sistema miscelar, solubilidad, fotoestabilidad, SPF, SSPF.

INTRODUCTION

The damage produced by the ultraviolet solar radiation (i.e. irreversible aged skin and dermis pathologies) is one of the major concerns of today’s world community. The use of sunscreens in areas under high solar radiation, as beaches and mountains, is an effective precaution to avoid these problems. The title’s compound seems to satisfy most of the tests for sunscreen molecules including toxicity, chemical stability, UV absorption and emission, solubility in miscellar solutions, pH behavior, etc. Descriptions of these tests on dihydroeuparin1), DHEU, and its comparison to o-hidroxyacetophenone, OHAP, is the main purpose of this contribution.

Dihydroeuparin, 5-acetyl-2, 3-dihydro-6-hydroxi-2-isopropenyl-benzo (b) furan, can be found in around 3% in the shrub Senecio Graveolens 2), "Chachacoma" (Quechua word meaning poor man). This species is endemic of the North part of Chile, over 3000 meters of altitude 3). Infusions of this plant are used to prevent the altitude illness (puna or soroche). DHEU was shown 4) to exhibit a strong hypotensive activity in rats at physiological concentration dosage.

It is well known that the UV component of the solar spectrum has three zones. The zone known as UV-A (400-320 nm) is the responsible for the skin toner and it may produce dermis erithemas, solar burns, upon its overexposure. The exposition to the UV-B zone (320-280 nm) may produce skin erithemas and indirectly, skin toner. Also, the overexposure to this zone may produce skin cancers, carcinoma, such as melanoma and others. Finally, the UV-C zone (280-200 nm) is mainly absorbed in the stratosphere by ozone.

Chemical sunscreens are usually designed to protect the skin from the erithema zone (320-290 nm, with a maximum at 308 nm) through their absorption or its emission deactivation. However, they may allow the absorption of higher wavelength to produce skin toner. These compounds (i.e. salicilates, cinnamates, PABA derivatives, etc.) are known as UV-B type sunscreens. Type UV-A sunscreens allows the skin absorption of wavelength around 320 nm (i.e. benzophenones and antranilates). A level of skin protection by sunscreens have been rationalized 5) as "sun protection factor", SPF. The SPF is the ratio between the "minimal erythemal dose", MED6), experienced by a person using a sunscreen to the MED suffered by the same person without sunscreen protection. A novel and promising sun protection scale called "spectroscopic scale protection factor", SSPF, has been developed 7). This scale takes into account the absorbance of the sunscreen and the integrated area of the UV solar spectra. In spite of attempts to devise instrumental procedures, using diluted solutions to mimic the results of measurements on human patients, no laboratory substitute for testing with humans has emerged8. However, the SSPF scale may have a role in this challenge. Higher SPF and SSPF levels are valuable protection properties of sunscreens.

Appreciated 9) cosmetic properties of sunscreen molecules are it must absorb wavelength between 280-360 nm; it must exhibit a high absorption coefficient (e); its wavelength of maximum absorption (l max) must be coincident with the maximum of the radiation and without solvent effects. Also, it must show low water solubility; it must be non toxic or phototoxic; it must not produce skin effects such as doughtiness, irritation, itchiness, colors or bad odors and, it must not deposit crystals after application.

EXPERIMENTAL

Reactives were Merck or Aldrich. UV experiments were performed on a PERKIN-ELMER (Lambda 3 model) UV-VIS spectrophotometer; a HANNA pHmeter (model HP-103) was used for pH measurements. A Hannovia medium pressure 300 W Hg lamp was used as radiation source. Plant material was collected close to Toconce, a small town in the North part of Chile. Dried leaves were allowed to stand in petroleum ether for 10 h at 70-85 °C, this procedure was repeated three times with fresh solvent. Dihydroeuparin was obtained as a solid after solvent evaporation, then washed with cold petroleum ether and recrystalized from petroleum ether/ethyl acetate (70/30). TLC chromatography (petroleum ether/ethyl acetate 70/25) was used to check dihydroeuparin purity.

Three DHEU stock solutions were prepared, 5.54à10-4 M in 6% Brij-35, 5.73à10-4 M in 3% Brij-35 and, 4.17à10-4 M in water. On the other hand, stock solutions of OHAP were prepared at 5.6à10-4 M in 6% Brij-35, 5.2à10-4 M in 3% Brij-35 and, 4.2à10-4 M in water. Water solutions were allowed to stand 10 min in boiling water. Working solutions to obtain e, lmax, pKa, solubility, Ka (the partition constant in water/Brij-35 solutions) and photostability for DHEU at different solvent condition, were prepared from these stock solutions in 10 ml graduated flasks.

pH changes, to obtain 10) pKa, were obtained through the addition (chromatographic syringe) of concentrated HCl or NaOH solutions. Temperature was fixed at 22±1°C. Some of the parameters were compared with those of OHAP. Photostability were obtained through the irradiation of basic (pH = 11) solutions of DHEU in a 3 ml quartz cuvette at 15 cm from the UV lamp and their absorbencies recorded every 10 min. Azoxybenzene was used as actinometer in a parallel cuvette.

Absorption parameters e and lmax, were obtained through Lambert-Beer. Solubility and Ka were obtained by means of the following procedure, oversaturated solutions of OHAP in 3% Brij-35 and DHEU in water and 6% Brij-35 (0.05 M) were prepared and shaken for 24 h. The solutions were centrifuged and samples were collected with micropipetes from the upper layer solution. From these samples, several solutions were prepared and the concentration (and solubility) obtained through their absorbencies. Ka was obtained by means of the equations,

The protection levels, SPF and SSPF, for OHAP, DHEU and homosalate (at same concentration) were obtained through numerical integration of Morales´ protocol 7).

RESULTS

Absorption spectra. Figure 1 shows the absorption spectra of DHEU and OHAP in water at pH = 2-3 and pH = 12-13. The lmax and e in water and Brij-35 solutions of these compounds at different pH are presented in Table I.


pKa determination. Figure 2 shows the plot of pH against wavelength in nm for OHAP in 3% Brij 35 and DHEU in water, as example. pKa values were obtained from inflexion points. The complete data set is show in Table I.



Solubility and Ka. Concentration of DHEU in different phases and condition was obtained through their absorption spectra. Table I shows relevant values of solubility and Ka for DHEU and OHAP.

Photostability. Figures 3 shows the DHEU behavior. Light intensity was obtained from irradiation of aqueous solutions of azoxybenzene 11) (analytical parameters, lmax = 458; e = 4700), resulting Io = 2.1610-4 Einstein/min after application of equation (5)


In this equation Ao is the initial concentration of azoxybenzene, 1.20x10-4 M, P is the product concentration after irradiation time t, Io is the light intensity, f is the product quantum yield, 0.02.

Quantum yield of disappearance of DHEU, fD, was obtained from equation (6),

In this equation Vi is the irradiated volume, other symbols have the same meaning as in equation (5). The quantum yield obtained for decomposition of DHEU, fD , was 0.0075.

TABLE II shows UV-A and UV-B SSPF and SPF values for OHAP and DHEU at different pH´s. Also, these values are compared to those of homosalate.


DISCUSSION

Electronic transitions for OHAP and DHEU are exhibited in Table I. Those at lower wavelength (239-275 nm, So ® 1p,p*) are characteristic of aromatic and conjugated system. Transitions at higher wavelength (322-359 nm, So ® 1np*) are originated on the protonation-deprotonation of the hidroxy group at different pH and explain their wavelength displacement and intensity change with pH, Figure 1. This effect is the basis to obtain pKa values for both compounds. In this case, the high pKa values indicate that these compounds are difficult to protonate or deprotonate, consequently they will show little variation in absorption or wavelength displacement in the pH zone 6-10. No dependence of wavelength on solvent composition (Table I) is observed for the So ® 1np* transition. However, solvent polarity plays an important role in the photophysical behavior of these compounds through the interaction of the hidroxy group with the solvent 12), 13). Structures are formed depending the medium polarity as show,

Under the UV protection perspective, DHEU can be considered as a sunscreen that effectively can protect the skin in the 270-340 nm zone and whose lmax and e can be compared with other sunscreens taken from literature 14), Figure 4.

Solubility on creams was simulated with Brij-35, which forms a miscellar system with water. Distribution of DHEU depends on the pH. At low pH values the compound is found mainly, 87%, in the miscelles and the rest in water. At higher pH it is almost evenly distributed in Brij-35 and water, Table I. Possible lost of DHEU from miscelles can be anticipated as low, according with these results.

Decomposition quantum yield, fD, for DHEU was obtained through irradiation with an intense source, c.a. 300 times more intense than sunlight. Data shows that even under this condition, the compound is photostable. This stability may be explained as a consequence of three non-radiative paths that mainly deactivated the compound. These paths were discovered during photophysical studies on OHAP 12.


The estimation of the sun protection property of a sunscreen is much better described by the SSPF parameter, than the usual comparison of absorption at single wavelengths because the former is related directly to the solar irradiance.

CONCLUSIONS

Photochemical and photophysical results indicate that DHEU can be considered as a sunscreen for the 270-340 nm zone, with high photostability, low water solubility and high solubility in miscellar system and, apparently, is a non toxic substance. It can be obtained in high yield in plants, very simple to extract and no structure modification is needed to be used as sunscreen.

ACKNOWLEDGEMENTS

We would like to thanks U. Católica del Norte for financial support and U. de Santiago de Chile for the use of laboratory facilities.

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