The substructure R represents one of {HEAD ~ a halogen}, —SH, —SZ, —S—S M L C2, wherein {TAIL ~ Z} represents a thiol protecting group 
The substructure R represents one of { HEAD ~ a halogen } , — SH , — SZ , — S — S M L C2 , wherein { TAIL ~ Z } represents a thiol protecting group
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH