In the past, I’ve briefly touched on the subject that octinoxate degrades avobenzone. Specifically, in Are Inorganic Sunscreens Better Than Organic Ones? Part III: Toxicity and Spotlight On: Vitamin B3 (Niacinamide and Nicotinic Acid), readers have seen the phrase “2 + 2 addition of cinnamates and alkenes” repeatedly used to explain this phenomenon. And in subsequent weeks, I’ve received many comments and messages inquiring about what exactly does this math mean? Therefore, this post will document what happens when octinoxate and avobenzone come together.
Well, the “2 + 2 addition of cinnamates and alkenes” is a bit more complicated than adding two groups of fruits or vegetables.
But getting back on topic, this phrase contains two elements: “2 + 2 addition of cinnamates” and “2 + 2 addition of alkenes.” The first refers to when an octinoxate molecule forms a dimer with another octinoxate molecule. The resulting structure or dimer does not allow the octinoxate molecules to be photostable and therefore, meaningfully absorb UVB light. The top row shows the two main dimers that are formed from this “2 + 2 addition of cinnamates.”
The second element, the “2 + 2 addition of alkenes” is when an octinoxate molecule binds to the double bond of the dominant form of avobenzone, resulting in the formation of a cyclobutane, which then undergo ring opening to form structures that don’t allow either the octinoxate or avobenzone molecule to properly function. One of these structures can be seen in the bottom row of the picture shown.
As you can see, this degradation is caused by a structural transformation that’s precipitated by the presence of the UV filter octinoxate. Furthermore, note that these two reactions only occur in the presence of UV light. Therefore, if your sunscreen contains both octinoxate and avobenzone, you don't have worry about them interacting in their container, assuming that it's completely opaque. But, you do still have to worry about this interaction after you actually apply the sunscreen!
To answer this question, we need to understand how photostabilizers like octocrylene bring stability to avobenzone in the presence of UV light. When avobenzone absorbs a photon of UVA light, its electron goes into a triplet energy state. But avobenzone itself has no way of dissipating or quenching this excited electron. That’s where octocrylene comes in. It accepts this “excited” energy, which allows avobenzone to return to its previously ground or “un-excited” state, where it’s ready to receive another photon of UVA light. If there is no photostabilizer present, the excited electron will either destroy the avobenzone molecule, or avobenzone will pass it to whatever is nearby including the lipid bilayers of the skin, resulting in the generation of reactive oxygen species (free radicals), which leads to oxidative damage in the form of lipid peroxidation.
Now, as I stated in the last section, octinoxate degrades avobenzone by changing its very structure, which only occurs in the presence of UV light. Therefore, octocrylene would have some positive effect on the octinoxate-induced degradation of avobenzone because at times, it'd be able to accept the "excited" energy; other times, octinoxate accepts the energy first, leading to the inevitable "2 + 2 addition of alkenes."
A metaphor for all of this would be if there are hundreds of three-man squads--comprised of Privates Avobenzone, Octocrylene, and Octinoxate; trying to disarm a field of landmines (photons of UVA light). Only Pvt. Avobenzone knows how to disarm the mines. But by himself, Pvt. Avobenzone would blow up after a few successful disarmaments because he would tire and no longer move fast enough; land mines have timers, right? But with Pvt. Octocrylene at his side, the two would work as a team and disarm the mines without loss of life.
So how would Pvt. Octinoxate fit into the metaphor? He’d be the renegade soldier that binds Pvt. Avobenzone’s and his hands together. So what good would Pvt. Octocrylene do then? He has no one to assist anymore. The landmines would then explode. Across the hundreds of squads, sometimes Pvt. Octinoxate would succeed in sabotaging the disarming process, and sometimes he would fail.
While this metaphor is far from perfect, (I mean how can 1 out of every 3 soldiers be a renegade?) I hope it gives you a better idea of what I’m talking about.
Thank you for reading through this rather dense post!