Oxygen is made in the heart of stars, so am I breathing sunbeams?
Oxygen is one of those things, like the parasympathetic nervous system, that is so ubiquitous and so essential to our general survival that we often neglect to pay it proper attention. Perhaps, we even forget to acknowledge the severe, even deadly, damage that it is doing to our body. If Bunsen and Kirchhoff showed that chemistry is universal, then let’s say that oxygen is the third most abundant element in the universe. While our knowledge of oxygen began in the labs of Joseph Priestley and Antoine Lavoisier, its role in giving those men the cognitive capacity to conceptualize its existence began long before that.
Oxygen is thought to have made its first appearance on earth from early Cyanobacteria - the first creatures to do good ol’ photosynthesis (ironic considering their potential use as an alternative to sunscreen now). Not all was well with this new talent; this imposed deadly stress on most other creatures on earth. Those that did survive found ways to burry themselves and get away from oxygen. Like those early ancestors, seeking shelter from the nasty oxygen, the methane producing bacteria in our gut have actually buried themselves there for that same purpose. So why not blame oxygen for that pesky flatulence? However, with the introduction of photosynthesis and perhaps the outlandish, statistically unlikely, symbiotic splicing of different cell types, more complex life was able to manifest in the long run (while simultaneously de-rusting all the iron on earth!). So, in fact, oxygen is not vital for life, but it is vital for complex life; as its presence allowed for multi-celled creatures, animals, brains, etc. Equally relevant to the “life” topic is the impact that oxygen has on the unique properties of water (abnormally high boiling point, dissolution properties, liquid range, etc.), including the fascinating chemistry of ice, which would not be possible without the charge separation created when molecular oxygen binds with hydrogen.
Why was the newly found presence of oxygen imposing such deadly stress on the other planetary creatures of the time? Molecular oxygen is extremely reactive - reactions which typically involve intermediate molecules known as radicals (ones which have unpaired electrons) – it’s hungry for electrons. Oxygen is unusual in that the electrons don't pair up in the manner which most atoms do when pairing in stable molecular form. This can manifest visually, for the experimentally curious. If you were to pour liquid oxygen between the poles of a magnet, the liquid would form a cylinder between the poles. In other words, the liquid becomes magnetized (its para-magnetic). This happens because, even though oxygen has two unpaired electrons, it is a stable molecule. The underlying mechanism here involves quantum mechanics (Pauli exclusion principle and Hund's rule) and is typically explained using molecular orbital theory.
Oxygen and it’s 16 Electrons - Molecular Orbital Theory
But what is the impact of oxygen on other aspects of life? When you exercise, you are increasing the workload on the skeletal muscle and, thus, the energetic needs of your muscle cells. The mitochondria found in each of these cells kick into gear in order to help meet this demand and start sucking in the oxygen found in your blood in order to produce new energy in the form of ATP. This process is called oxidative phosphorylation. A by-product of this process, however, is the generation of oxygen free radicals, such as superoxide and, eventually, hydrogen peroxide; this is more generally referred to simply as “oxidative stress”. An overproduction of hydrogen peroxide, and low catalase activity, results in an overabundance of harmful oxidation reaction in our cells. This is likely the reason for one’s hairs transforming into a nice silver colour, the telomeres shortening at the end of one’s chromosomes, and, more noticeable perhaps, the negative effect imparted on our beers' flavour stability.
Oxidation is the brewers’ worst enemy. The pathway to stale flavours is a complex one. Reduction of the ground state oxygen diradical, from biochemical reactions involving thiols (from beer raw materials) results in the production of superoxide radical ions. These ions are subsequently converted to hydrogen peroxide, typically, enzymatically by superoxide dismutase, or spontaneously in an acidic medium. If catalase is not around in high enough quantities, hydrogen peroxide can accumulate, or become oxygen species which are even more reactive (i.e. hydroxyl radical) via reaction with iron ions (Fenton reaction) or via the Haber-Weiss reaction; eventually leading to the autoxidation of unsaturated fatty acids (which will give you stale beer flavours of wet cardboard). Beer has plenty of unsaturated fatty acids, thiols, and even some iron! Introduce too much oxygen and, like the grey hairs increasing on one’s head over time, you may be tasting wet cardboard in your beer after some time on your shelf!
However, with proper brewing techniques, this may be undetectable to you! Or better yet, if you leave it on your shelf for three or more years, through a whole swarm of complicating chemical reactions, you might be left with a sweet sherry-like beer, with hints of toffee. The nice calming, yet arousing, euphoria that one can receive from a small dose of perfectly brewed beer may surely reduced ones cortisol levels, as well as ones oxidative stress, well beyond those imposed by the metabolism of the alcohol itself, and possibly subsequently ones hydrogen peroxide production! Is properly aged beer thus, with the proper amount of oxidation, the proper defence for one’s remaining pigment-containing hair? Or should I turn to anti-oxidant powerhouse super-foods, such as Nikita Chaga?
Read more on how anti-oxidants can help to subdue the effects of oxidative stress in the post Plant Phenolic Compounds, Part I: Their Relevance, Diversity, and Biosynthesis.