2. Molecular interactions Non-covalent interactions are key to understanding the behavior of biological molecules. So just by way of introduction, I wanted to show you this picture which shows the interaction of an antibody with its antigen. Antibodies circulate in the blood stream and are found on the surfaces of many cells in the blood and they interact with foreign bodies and destroy them in one way or another. Over here on the right, is actually an antibody molecule. It is actually not a complete antibody molecule; it's a part of an antibody molecule. You can see down here what happens when they actually bind together. You can see how closely they fit together to bring many different atoms together in this structure. This structure was determined by X-ray diffraction crystallography and I think it makes very clearly the point that if you have many of these non-covalent bonds working together you can get very nice, strong interaction which is critical for the working of antibodies. And so this is just my little summary that non-covalent interactions are a key to understanding the behavior of biological molecules. And here on the left is the antigen, in this case the protein lysozyme. And the point I want to make is that there are never any covalent bonds between the antibody and the antigen. The interaction between them is just due to non- covalent bonds, non-covalent interactions.

3. Molecular interactions Non-covalent interactions are key to understanding the behavior of biological molecules. These interactions include:- electrostatic interactions - - like charges repel + + opposite charges attract electrostatic interactions Both molecules must be charged. hydrogen bonding hydrophobic interactions Van der Waals interactions steric hindrance. electrostatic interactions This is just the same thing; it's the key to understanding. So let's look and see what these major interactions are. So let's start with electrostatic interactions. Here are two atoms or two chemical groups and they both are negatively charged and in these circumstances they are going to repel one another. And so in this situation we have like charges repelling each other, they are both negative. And the second one is the opposite one: that is that opposite charges attract. So the positives and negatives will attract one another while the like charges will repel one another. So that is the basis of electrostatic interactions. Essentially, these interactions can be quite strong and they are major interactions within the body. We'll talk a little bit about how to calculate these charges and determine how those depend on pH in the body. So these are electrostatic interactions. The other thing that you'll notice about this is that So the first one is electrostatic interactions, which we'll talk about in a moment. Then we'll talk about hydrogen bonding, hydrophobic interactions, van der Waals interactions and steric hindrance, which is a complicated name for a simple concept. Same thing happens when they are both positive, they repel each other. So that's the first principle for electrostatic interactions. both of the molecules must be charged. So that if you have a charged molecule or charged group and an uncharged molecule or group there will be no electrostatic interaction. They must both be charged.