Recently I came across the ligand thiamine pyrophosphate (TPP) in some RNA riboswitch structures. I was a bit surprised by the atom names adopted for the ligand by the PDB. See figures below for the chemical structure of TPP from the RCSB PDB website (first), and the three-dimensional structure of the ligand from the riboswitch 2gdi (second).
Specifically, the planar base-like moiety at the right has atom names ending with prime. To my knowledge, only sugar atom names of DNA and RNA nucleotides have the prime suffix, such as the 2′-hydroxyl group in RNA.
The RCSB webpage for TPP shows that currently there are 107 entries in the PDB, among which 100 are from proteins, 6 from RNA, and one in a RNA-protein complex. It is not clear to me whether the prime-bearing names in TPP are following any documented ‘standard’ or convention. DSSR is nevertheless taking a note of such ‘weird’ cases.