J. Cosmet. Sci., 69, 233–243 ( July/August 2018) 233 Antioxidant Ability and Stability Studies of 3-O-Ethyl Ascorbic Acid, a Cosmetic Tyrosinase Inhibitor WAYNE C. LIAO, YU-TING HUANG, LI-PING LU, and WEN-YING HUANG, Department of Nursing, Chang Gung University of Science and Technology, Chia-Yi Campus, Taiwan (W.C.L.), Department of Applied Cosmetology and Graduate Institute of Cosmetic Science, Hung-Kuang University, Taiwan (Y.-T.H., L.-P.L., W.-Y.H.) Accepted for publication June 10, 2018. Synopsis 3-O-ethyl ascorbic acid may be a good whitening ingredient in cosmetics. However, before it can be successfully used in cosmetics, its biofunctionality and stability need to be comprehensively investigated. The reduction and 2,2-Diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging ability of this compound were analyzed to assess its antioxidant potential. In addition, the tyrosinase inhibitory ability was analyzed to show the whitening capacity of 3-O-ethyl ascorbic acid. Response surface methodology (RSM) was used to determine the optimal conditions for the ascorbic acid derivative in cosmetics. Based on the DPPH radical scavenging ability results, the half-inhibitory concentration (IC50) value of 3-O-ethyl ascorbic acid was 0.032 g/L. It also showed a good reducing ability at 1.5 g/L concentration. Based on the tyrosinase inhibition analysis, the IC50 value was 7.5 g/L. The optimal conditions to achieve the best stability were determined from the RSM as 36.3°C and pH 5.46. INTRODUCTION Ascorbic acid is known as a good whitening ingredient in cosmetics (1). It has also been used to protect the skin from damage caused by ultra violet (UV) rays (2,3). However, the degradation of ascorbic acid reduces the biofunctionality of cosmetics containing it. Therefore, a more stable derivative such as 3-O-ethyl ascorbic acid would be preferred. The chemical structures of these compounds are shown in Figure 1. Although a resonance structure is formed between two hydroxyl groups at the ortho position of ascorbic acid, ascorbic acid is a relatively unstable compound. It can be easily oxidized when oxygen is present. Because the ethyl group prevents 3-O-ethyl ascorbic acid from being oxidized, 3-O-ethyl ascorbic acid is more stable than ascorbic acid (4,5). Because of their free radical scavenging ability, ascorbic acid and its derivatives are known as good antioxidants (6). In addition, Yen et al. reported that ascorbic acid has a powerful reducing ability (7). Because 3-O-ethyl ascorbic acid is proposed for use in cosmetics, its antioxidant ability has been evaluated in this study. Address all correspondence to beca690420@hk.edu.tw.

JOURNAL OF COSMETIC SCIENCE 234 Tyrosinase catalyzes the oxidation of L-tyrosine to 3,4-dihydroxyphenylalanine (DOPA), which can in turn transform into DOPAchrome (8). These catalyzed reactions result in the formation of melanin, which is responsible for skin pigmentation (9). The inhibitory activity of tyrosinase (EC 1.14.18.1) has been extensively studied (10-13). 3-O-ethyl ascorbic acid successfully inhibits the synthesis of melanin however, its high water solu- bility hampers its permeation across the skin. In addition, the instability of this water- soluble compound leads to complications for formulation chemists (14). Before 3-O-ethyl ascorbic acid can be successfully used in cosmetics, its stability needs to be comprehensively investigated. Storage temperature and pH are two factors that generally affect the stability of active ingredients in cosmetics. Changes in these two fac- tors may cause the degradation of ingredients. Thus, optimal conditions need to be deter- mined when using 3-O-ethyl ascorbic acid in cosmetics. In this study, response surface methodology (RSM) was used to study the stability of 3-O-ethyl ascorbic. RSM is a good technique to determine the optimal conditions for many applications. Huang et al. suc- cessfully used RSM models to obtain the optimal conditions for using ascorbic acid 2-glucoside, which is also used in cosmetics (15). Central composite design was used to establish a second-order RSM model to predict the stability of 3-O-ethyl ascorbic acid. The second-order regression model (equation 1) included linear, quadratic, and interac- tive components. 2 0 =1 =1 =1 = + + + + C C C F, œ œ œœC k k k k i i ii i ij i j i i i i j Y X X X X (1 ) where Y is the response value Xi and Xj are the input variables β0 is the intercept βi is the linear coeffi cients βii is the square coeffi cients βij is the interaction coeffi cients and ε is an error term. This study aims to assess the antioxidant and reducing abilities of 3-O-ethyl ascorbic acid. The DPPH free radical scavenging ability, which is commonly used to represent the antioxidant ability, was evaluated. The tyrosinase inhibitory activity of 3-O-ethyl ascorbic acid was also investigated. The stability of this compound was studied using RSM. The optimal conditions to retain the best stability were determined. F igure 1. Chemical structures of (A) ascorbic acid and (B) 3-O-ethyl ascorbic acid.