I was blithely throwing around the term “chemical space” in yesterday’s post. So, what am I talking about, and how much room is in there, anyway?
Let's narrow it down to organic compounds, to start with, or at least compounds that are mostly organic. A working definition, as far as people interested in biology and medicine go, might then be “the domain of chemical compounds compatible with living systems”. That excludes the red-hot reactive stuff and the unstable exploders, but leaves most everything else. Let’s also ignore macromolecules of various kinds and cut back to “drug-like” sizes – say, molecular weight 500 or less. That way we don’t have infinite numbers of polymers going off in all directions; that should help. And that leaves us with. . .?
A ridiculously large set of compounds, still. You can see how things get out of control pretty quickly if you just consider a building-block problem. Imagine breaking compounds down into simple units - an aryl ring, an ether, a tertiary amine, and so on. What sorts of numbers do you get when you start mixing and matching them? Well, there are an awful lot of possible building blocks. You could quickly fill out a hundred different examples of each of those three subunits, so there's one hundred to the third, or a million possible compounds without even exerting yourself very much.
This sort of thought experiment has been done several times. One estimate done by this fragment approach and considering only stable structures came in between 10 to the twentieth and ten to the twenty-fourth compounds that could potentially be prepared using known synthetic methods. (See here for another "how many compounds are possible?" paper, from a different angle - the group that did that work has followed it up recently, which will be the subject of another post sometime). Needless to say, that is considerably larger than the total number of organic compounds ever described in reality. There's not enough carbon, oxygen, and nitrogen on earth to prepare a vial of each of these, and where would you put the vials? The terrifying thing is that this is actually one of the lower estimates, and thus perhaps a very reasonable and conservative one. You can find ten-to-the-sixtieth estimates out there, which is a figure that cannot be dealt with by human efforts.
These sorts of numbers are why some people doubt the utility of just cranking out neat structures. But looked at from the other direction, the number of compounds we have available isn't nearly so impressive, so making new ones, especially long lists of new ones, makes a difference in what we actually have in hand. But is it a difference akin to buying a thousand lottery tickets rather than buying one?