Pictures of the Day CH320M328M

Click on the pictures and you will see two animated GIFs of proteins that contain disulfide (-S-S-) bonds. In the above structures, the disulfide bonds are shown in green as space filling models, the rest of the protein structure is shown in a cartoon format in which the chain of the amino acids is shown as a helix (pink), loop reagion (thin white) or beta sheet (burnt organe). Both of these proteins also have a substrate analog bound to them (purple) that is shown for reference. The grenn covalent disulfide bonds provide a link between distant parts of the protein structure and thus serve to stablize the three-dimensional folded structure of the protein.

Epoxides react with nucleophiles because this allows the opening of the three-membered ring and relief of ring strain (thus providing motive). In an epoxide such as the one shown above with different number of substituents on the carbons of the ring, strong nucleophiles like HO- will react predominantly on the backside of the less hindered carbon atom. In acid, a positively-charged intermediate analogous to a bromonium or mercurinium ion is produced. Reaction with nucleophiles such as H2O will occur at the more substituted carbon atom, because this is the one with more partial postive charge (blue color). Thus the regiochemistry can be controlled by choosing whether to use acid. Note that the sterochemistry of addition is anti in both cases (there is backside attack in both cases), leading to trans products.