88 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS i I 1 I I I i 2_0 40 60 80 MINUTES Figure 4. Anaerobic photobleaching of malachite green in presence of 1 X 10-"M ascorbic acid. A, no additive B, 2% sodium decyl sulfate Table I Effect of Surface Active Agents on Rate of Photobleaching of Dyes- Surfactant k•urtactant/k 0 Malchite green-ascorbic acid (anaerobic photoreduction) None 1.0 1% Polysorbate 80 58 3% Polysorbate 80 200 5% Polysorbate 80 222 5% Triton X-100 8.6 2% Sodium decyl sulfate 7.8 5 % Benzalkonium chloride 2.3 Indigo carmine (photooxidation) None 1.0 2% Sodium decyl sulfate 0.51 5 % Benzalkonium chloride 0.20 • Rates determined from linear first-order plots of logarithm absorbance vs. time, 25øC The symbol k0 refers to rate of photobleaching without surfactants. molecules, by reagents which quench triplet states. Reagents for potential practical application in quenching excited states, and thus preventing photodecomposition, with particular application to cosmetic products were noted by Lewin (1). Another type of enhancement of photoreduction by polymers and surfactants has been observed even when there is no binding of the dye as determined by standard procedures such as solubility, equilibrium dialysis, partitioning between immiscible solvents, and ultraviolet and visible spectroscopy. Ribofiavin was found to exhibit an enhanced rate of aerobic photobleaching in presence of PVP and polysorbate 80, even

PHOTOCHEMICAL REACTIONS 89 though no binding to these macromolecules could be demonstrated by methods indicated above (2). However, it was observed that ribofiavin did bind to these macromolecules and to micellar sodium decyl sulfate during irradiation of the ribofiavin solution with visible light (2, 16). This photobinding was rapidly reversed when the light was turned off. In the absence ot/external reducing agent, the isoalloxazine nucleus of ribofiavin undergoes photoreduction through intramolecular hydrogen transfer from the ribityl side chain and it therefore is unnecessary to com- plicate the system by adding external reducing agent. The enhanced rate of aerobic photobleaching of ribofiavin in presence of PVP, polysorbate 80, and sodium decyl sulfate was attributed to binding of a ribofiavin triplet, or a reactive species arising from the triplet, to the macromole- cule. Since the enhanced rate of photobleaching could only be demon- strated in oxygen-containing solutions, it is proposed that the enhanced rate of photobleaching is not attributable to an increased rate of forma- tion of the triplet state. Rather, the enhanced photobleaching is at- tributed to either protection of the bound excited state from oxygen quenching, with an increased lifetime for the reactive triplet state, or to inhibition of the reaction of a partially reduced intermediate with oxygen to regenerate ribofiavin or a species with an absorption spectrum similar to ribofiavin. The proposed interaction of photoexcited ribofiavin with a macromolecule can be represented as: R • R* • R' 4- macromolecule products R'-- macromolecule The photobinding phenomenon is not limited to ribofiavin, but can be demonstrated for a number of other dyes and for drugs such as menadione and hydrocortisone. Binding to appropriate macromolecules might also lead to the stabili- zation of dyes to photoreduction. For example, in the presence of the polynucleotide nucleic acid, ribofiavin is relatively stable to light (2). The stabilization of ribofiavin to photoreduction by nucleic acid has been attributed to interaction of ribofiavin with the adenine moiety of nucleic acid (17). Ribofiavin complexes not only with adenine, but with caffeine (18), theophylline (18), phenols (19), and many other compounds (20). It has been shown, however, that the complexation of gTound state ribofiavin with these molecules cannot account for the magnitude of the fluorescence quenching (18, 19), nor for the even more dramatic increase