I've been trying to create 3D structures of unionized tyrosine (and other amino acids, it's just tyrosine is the one I'm up to atm) using the structure of Zwitterions found within PDB files and to do this I need to know what C-OXT (which is found within the carboxylate functional groups) stands for in PDB files as this will help me to decide which oxygen in the carboxylate groups should be represented with the double bond from the associated carbon and which should have the single bond. The other oxygen in the carboxylate group is just called C-O in the PDB file.
The naming conventions for atoms in PDB file require that at the C-terminus of a protein one of the carboxylate oxygens is called
O and the other is called
OXT. But since it's a deprotonated carboxylate group they are both chemically equivalent, and you'd expect that bond lengths are equal.