So my main area of research is molecular biology and biochemistry, but nuclear and particle physics frequently manages to find its way in it and it's always baffling to me. I've been participating in research on bacterial Cytochrome P450 enzymes; they catalyze the oxidation of organic substrates. There are a lot of redox partners, cofactors and sometimes secondary substrates involved, but the overall reaction is... RH + O2 + 2H+ + 2e-→ ROH + H2O They're identified by the change in their absorbance spectra when CO binds to them. The reason that this happens, according to a lot of research journals I've read, is because Cytochrome P450's contain a ferrous heme group (like blood hemoglobin). When the iron is unbound, it is in a "high spin state" and absorbs light at 420 nm. When CO is bound to the iron, it goes into a "low spin state" and absorbs light at 450 nm. I get the principles of absorbance spectra but "spin states" is beyond my understanding. Anyway, you can also identify the reaction products by their Proton Nuclear Magnetic Resonance spectra. You store the reaction product in extremely cold temperatures, apply a strong magnetic field and the protons adjust their "spin" either parallel or antiparellel. Then you apply radio waves and all the protons "spin-flip" and send back an echo of lower frequency. Each chemical has a unique profile for the signals you get back. I can read NMR spectra and interpret chemical structures from them, but I really don't understand the principles of "spin" and "spin flips." My orgo and biochem books don't really explain. They just say that "spin" has no real analogue in familiar, everyday mechanics or classical physics; that the closest concept is angular momentum, but that even that is a far cry. Can someone please explain what "spin" is? Why does an iron atom have a "high spin state" when it's unbound to CO but a "low spin state" when CO is bound? Why do protons' spins align parallel or antiparallel in a B-field but "spin-flip" and echo when radiowaves are applied?