Dear Sir, I had a query regarding charge assignment to any ligand in chimera. While minimizing the energy of a particular ligand (Tools --> Structure Editing --> Minimize Structure) and also during Dock Prep, I found that for most of the aliphatic and alicyclic amines, they are getting protonated with a residual positive charge; while the aromatic amines remain unprotonated (and hence no residual charge). I understand that this is because of their differential pKb values (basic strengths). On the other side, deprotonation never occurs with the carboxylic acid (-COOH) group. Does the Chimera (force fields) control this issue of protonation and deprotonation considering the physiological pH ? I will be extremely grateful if some one may kindly clarify my doubts. Thanking you in advance, With Best Regards, *Dr. Prabuddha Bhattacharya * *Dept. Of Chemistry* *Adamas University* *Kolkata 700126* *India*