Satisfying Hydrogen Bonding Potential in Proteins

We have analysed the frequency with which potential hydrogen bond donors and acceptors are satisfied in protein molecules. There are a small percentage of nitrogen or oxygen atoms that do not form hydrogen bonds with either solvent or protein atoms, when standard criteria are used. For high resolution structures 9·5% and 5·1% of buried main-chain nitrogen and oxygen atoms, respectively, fail to hydrogen bond under our standard criteria, representing 5·8% and 2·1% of all main-chain nitrogen and oxygen atoms. We find that as the resolution of the data improves, the percentages fall. If the hydrogen bond criteria are relaxed many of these unsatisfied atoms form weak hydrogen bonds. However, there remain some buried atoms (1·3% NH and 1·8% CO) that fail to hydrogen bond without any immediately obvious compensating interactions.