The proton-transfer mechanism in hydrated 1-aminocyclopropane-1-carboxylic acid (ACC), which is a crucial precursor in the ethylene biosynthetic pathway in plants, was investigated by using an ab initio molecular orbital method to determine the importance of discrete water molecules in ethylene biosynthesis. Short-range local ACC-H2O interactions are treated with the ACC.(H2O)(6) cluster according to the analyses on the structures and stabilities of ACC.(H2O)(n) (n = 1-8) clusters. Long-range solvent effects were taken into account by using the continuum model (Onsager model and polarizable continuum model (PCM)) of water. The combined approach of both discrete and continuum models showed that the zwitterionic form of the ACC.(H2O)(6) cluster is 42.3 kJ mol(-1) (DeltaG) more stable than the neutral form at the level of B3LYP/6-31+G(d,p) with the PCM. The estimated energy barrier heights (DeltaG(double dagger)) for the neutral-to-zwitterion transition were 14.5 and 16.9 kJ mol(-1) for the direct and water-assisted proton-transfer mechanisms. These results indicate the efficiency of both ionization mechanisms in water and suggest that water molecules interacting directly with ACC may have significant roles in the reaction that forms ethylene from ACC in aqueous solution.