Transcript

INSTRUCTOR:

This is a model of the four-membered ring, referred to as a beta-lactam ring in the text, that's part of a penicillin molecule. And if you take a look at it, you can see that it's composed of three carbon atoms and one nitrogen atom. But even more importantly, note that as a four-membered ring, the bond angle now between the carbons-- instead of being like the tetrahedral carbon bond angle of 109 degrees, it's constrained to be just 90 degrees.

That introduces what we call ring strain into the molecule. And it's a way in which molecules are encouraged, therefore, to react. And indeed, the reaction of penicillin with bacteria and an enzyme in bacteria is to open up this four-membered ring and release that ring strain. I think you can see before I demonstrate that that the bonds are already in the four-membered ring quite bent.

But watch what happens when I break one of these bonds and simulate the reaction. You can see how it will spring apart to release that ring strain. So another important feature in the understanding of why chemical reactions take place is to release this ring strain that we're seeing in the four-membered ring and is demonstrated when the enzymes in bacteria react with penicillin.