 So what Christian Anfins sent it, he took one of those small proteins, the ribonuclease A shown here, and this is a protein that has a particular activity, you can show that it cuts RNA, that's why it's an ACE, it's an enzyme, and that means that we can measure that activity in a simple experiment. But what Anfins then did is that he added mercaptoethanol and urea, and they're both compounds that are known to destroy the protein structure, and then you have no function whatsoever. But in the second phase, he then removed these components from the test tube again, notabene, test tube, not cell, and under those conditions, the protein recovered its activity, and that leads to remarkable results, because what he has basically shown that we don't need any sort of magical life elixir. First, the structure of the protein is determined entirely by its amino acids. There is no magic knowledge in the cell here or anything, because again, it's just a string of amino acids when I've denatured it, and then it can spontaneously refold. The second part there means that if this happens spontaneously in a test tube, it must be governed entirely by physics, meaning thermodynamics. And in fact, this is something called the thermodynamic hypothesis, and in other times, we call it Anfins' dogma, and that means that the native state, the working state of a protein, should be the global minimum of free energy. And again, that's so obvious that today we take it for granted, but this was not granted in the 50s and the 60s. In fact, it was so non-obvious that he got the Nobel Prize in Chemistry for this in 1972. He did something else, too, that he tried removing only the merkeptoethanolactic it is, and leave the urea. And in that case, it showed that it has a tiny activity. It gets like 1%. So somehow, some of the protein managed to refold, but not everything, so kind of like it's gotten stuck. And it turns out that it did get stuck. There are some special bonds formed between these chains that I'll show you in a couple of slides, and we know now that in the cell, there are specific other proteins, residues, that are aiding these to fold by breaking those bonds and setting it off on the right path again. But remember the Anfins' dogma, because that's the basis of everything we do. If this was not true, we could not use physical concepts and in particular free energy to understand protein structure. So amino acid sequence determines everything. That's also why I showed you the previous slide, because the backbone, this N, C alpha C, that's the same for any protein chain. So it must be the amino acid side chains and the specific sequence of this that gives each protein its special properties and the way it works.