The adsorption of lysine (Lys) on hydroxyapatite (HA) (1 0 0) surface was studied using density functional theory calculations within the generalized gradient approximation framework. The optimal adsorption pattern was identified from a number of interactive Lys-HA structures. The carbonyl-O, amine-H and C-epsilon-H atoms are active sites in Lys that interact directly with the Ca ion and phosphate-O atoms on the HA (1 0 0) surface, respectively. Multiple interactions were noted between Lys and HA surface. The adsorption is mainly stabilized by the Ca-O ionic bonding, followed by hydrogen bond and/or van der Waals interaction between phosphate-O and Lys-HA. Both the Lys and HA surface structures are subjected to some changes upon adsorption. The zwitterionic form of Lys was found more stable on the HA (1 0 0) surface than its neutral form. Our calculations reveal the structural characteristics and their variations for the Lys-HA systems, which are essential for understanding the interaction mechanism of biomolecules with inorganic biomaterials. (C) 2012 Elsevier B.V. All rights reserved.