Four interaction models for the binding of Torpedo californica acetylcholinesterase (TcAChE) with Loop1 of Fasciculin2 are investigated at the B3LYP/6-311G(d,p) level of theory. The total binding energy of three fragments (P1-P3) which belong to the omega loop Cys67-Cys94 of TcAChE contributes almost 67% of the entire binding, suggesting the domination of this omega loop on the interaction between AChE and Loop1 of Fas2. The energy decomposition illustrates that the interactions mainly consist of electrostatic components. The polar solvent which reduces the binding energies of the studied models implies the significant impact of the solvent on the binding of Fas2 and AChE. (c) 2006 Elsevier B.V. All rights reserved.