A hydrogen bond is a polar interaction between two electronegative atoms, a donor and an acceptor. In proteins hydrogen bonds involving the main chain oxygen and amide are critical in forming the secondary structures. The polar side chains also form hydrogen bonds. Energetically it is important to satisfy all hydrogen bond donors and acceptors. In proteins more than 90% of side chain atoms are 'satisfied' by forming hydrogen bonds to protein atoms or solvent.
McDonald IK, Thornton JM
J Mol Biol238p777-93(1994 May 20)