[Chimera-users] Charges for PTR+DT

Dear all, I have to prepare the pdb structure 1K4T to my study. When I do the dock prep process, it gives net charges as, PTR+DT -2 TGP +0 I added charges for standard resides by AMBER ff12SB. For other residues, I added charges by Gasteiger method. My problem is why PTR+DT gives -2 charge? I am not familiar with this one. <http://www.google.lk/intl/en/options/> Can anyone help me? Thank you.