Epoxides are substances containing three-membered rings in which one of the ring atoms is oxygen. Three-membered rings pull the carbon-carbon and carbon-oxygen bond angles away from their preferred angles, generating a large amount of strain which can be relieved most easily by cleavage of one of the carbon-oxygen bonds of the ring. The relative electronegativities of carbon and oxygen make the carbon atoms susceptible to nucleophilic attack. Reaction of a nucleophile with one of the carbon atoms of an epoxide (in most cases, the least sterically hindered carbon atom) forms a new bond to the carbon atom and an alcohol (after protonation of the oxygen atom). The strain of the three-membered ring is large enough to render the reverse reaction (reclosure of the epoxide and expulsion of the nucleophile) difficult under physiological conditions, rendering epoxide opening (and enzyme inhibition) irreversible.