In a recent work, we reported that solution speciation of certain "tailor-made" impurities (viz. aspartic and glutamic acids) controlled their stereoselective interactions at the alpha-glycine crystal interfaces [S.K. Poornachary, P.S. Chow, R.B.H. Tan, R.J. Davey, Cryst. Growth Des. 7(2) (2007) 254]. In this contribution, we rationalize this mechanism using molecular modeling and atom-atom potential energy calculations. The simulation results show preferential interaction of the impurity zwitterions on a flat (0 10) surface, and of the impurity anions either at the kink site on the (0 10) step, or on the (0 11) surface. We corroborate these results with the growth inhibition observed along b- and c-axes of alpha-glycine crystals in the presence of these impurities. (c) 2008 Elsevier B.V. All rights reserved.