What's wrong with peroxides?

In my last post I mentioned that antioxidants are helpful because they assist in dealing with peroxides and other Reactive Oxygen Species (no, science nerds are not immune to the Capitalize-Everything-for-Dramatic-Effect Syndrome). In fact, every organism I'm aware of has some mechanism in place for dealing with peroxides. That raises the question of why peroxides need to be dealt with so badly, and that's what I'm going to answer today.

Let's begin by explaining what a peroxide is: a molecule in which two oxygen atoms are single-bonded to each other. This is a fairly weak bond, prone to breaking. Additionally, oxygen has one of the strongest affinity for electrons in the known universe, so when oxygen bonds with other atoms it takes on a slight negative charge. In a peroxide, these oxygen atoms are arranged so their partially-negative-charged selves are right next to each other. The resulting repulsion makes the bond even more prone to breaking.

Now, to explain what happens when the bond actually breaks, there's something you need to understand about electrons: they "like" to be in pairs. The most stable electron configurations have even numbers of electrons, so often when a bond breaks, either one atom takes both the electrons and creates two charged species, or both atoms find something else to bond with. Either option keeps the electrons neatly paired up. When the bond between oxygen atoms in a peroxide breaks, neither oxygen atom wants to let go of the electrons; their electron affinity is too strong. What often happens instead is each oxygen leaves the bond with one electron, creating two very unstable molecules called radicals.

Between oxygen's strong affinity for electrons and having an unpaired electron hanging around, these molecules will react with just about anything - including all your biological molecules whose functions depend on their molecular structure being exactly the way it is. To make matters worse, most of these biological molecules have a nice, stable, paired-electron configuration. Reacting with a radical can turn them into a radical, which then reacts with another biological molecule and produces a different radical ... and the only way to break this cycle once and for all is to react with another radical and pair their odd electrons.

So there you have it; peroxides are not only very unstable, but they can initiate a chain reaction that will seriously fuck up your shit if they're not dealt with (e.g. by antioxidants) as soon as possible!

7/17/14 addendum: It has been pointed out to me that reactive oxygen species (ROS) may play a role in the body's signaling mechanisms, and that large doses of antioxidants may actually inhibit those mechanisms. However, that doesn't change many aspects of my message. Even when ROS are used for signaling, the body still needs to have mechanisms in place to dispose of them in a timely manner, so antioxidants still have an important role to play in our bodies. Also, since people in the industrialized world tend not to eat enough vegetables and eat too many high-calorie foods, it's probably not unreasonable to suggest that we include antioxidants in our diet.