CCL: What does PHE(OL) mean?



 Sent to CCL by: Andrew DeYoung [andrewdaviddeyoung=gmail.com]
 Hi,
 This might be more of a biochemistry question rather than a
 computational chemistry question, so I apologize in advance...
 In the "top" directory (e.g., on my installation, it's
 /usr/local/gromacs/share/gromacs/top/) of the Gromacs installation,
 several force fields are included.  I'm looking at the file
 "ffbonded.itp" in the "oplsaa.ff" directory.  I see, for
 example, this
 entry under [ angles ]:
   CA     CA     CA      1   120.000    527.184   ; PHE(OL)
 PHE, I'm sure, refers to phenylalanine, where CA is an aromatic
 carbon.  But to what does (OL) refer?
 I see other (OL) entries in the topology: for example, TYR(OL),
 TRP(OL), and ARG(OL)?  Does (OL) somehow refer to the protonation
 state of the amino acid?
 Thanks so much for your time!
 Andrew DeYoung
 Carnegie Mellon University